A Byte of Python, v1.92 (for python 3.0)

PDF generated using the open source mwlib toolkit see http://code.pediapress.com/ for more information A Byte of Python v1.92 (for Python 3.0) Python 2 Python Choose your Python version: If you want to learn the current Python 2.x, read here [1] or download the PDF [2] If you want to learn the new Python 3.0, read here or download the PDF [3] You can also buy a printed hardcopy. [4] Introduction 'A Byte of Python' is a book on programming using the Python language. It serves as a tutorial or guide to the Python language for a beginner audience. If all you know about computers is how to save text files, then this is the book for you. This book is updated for the new Python 3.0 language. If you are looking for a tutorial on the current Python 2.x version, please download the previous revision of the book [5]. On the same note, if you're wondering whether to learn Python 2.x or 3.x, then read this article by James Bennett [6]. Who Reads 'A Byte of Python'? Here are what people are saying about the book: Feedback From Readers This is the best beginner's tutorial I've ever seen! Thank you for your effort. - Walt Michalik (wmich50-at-theramp-dot-net) You've made the best Python tutorial I've found on the Net. Great work. Thanks! - Joshua Robin (joshrob-at-poczta-dot-onet-dot-pl) Hi, I'm from Dominican Republic. My name is Pavel, recently I read your book 'A Byte of Python' and I consider it excellent!! :). I learnt much from all the examples. Your book is of great help for newbies like me... - Pavel Simo (pavel-dot-simo-at-gmail-dot-com) I recently finished reading Byte of Python, and I thought I really ought to thank you. I was very sad to reach the final pages as I now have to go back to dull, tedious oreilly or etc. manuals for learning about python. Anyway, I really appreciate your book - Samuel Young (sy-one-three-seven-at-gmail-dot-com) Dear Swaroop, I am taking a class from an instructor that has no interest in teaching. We are using Learning Python, second edition, by O'Reilly. It is not a text for beginner without any programming knowledge, and an instructor that should be working in another field. Thank you very much for your book, without it I would be cluless about Python and programming. Thanks a million, you are able to 'break the message down' to a level that beginners can understand and not everyone can. - Joseph Duarte (jduarte1-at-cfl-dot-rr-dot-com) I love your book! It is the greatest Python tutorial ever, and a very useful reference. Brilliant, a true masterpiece! Keep up the good work! - Chris-André Sommerseth Python 3 I'm just e-mailing you to thank you for writing Byte of Python online. I had been attempting Python for a few months prior to stumbling across your book, and although I made limited success with pyGame, I never completed a program. Thanks to your simplification of the categories, Python actually seems a reachable goal. It seems like I have finally learned the foundations and I can continue into my real goal, game development. ... Once again, thanks VERY much for placing such a structured and helpful guide to basic programming on the web. It shoved me into and out of OOP with an understanding where two text books had failed. - Matt Gallivan (m-underscore-gallivan12-at-hotmail-dot-com) I would like to thank you for your book 'A byte of python' which i myself find the best way to learn python. I am a 15 year old i live in egypt my name is Ahmed. Python was my second programming language i learn visual basic 6 at school but didn't enjoy it, however i really enjoyed learning python. I made the addressbook program and i was sucessful. i will try to start make more programs and read python programs (if you could tell me source that would be helpful). I will also start on learning java and if you can tell me where to find a tutorial as good as yours for java that would help me a lot. Thanx. - Ahmed Mohammed (sedo-underscore-91-at-hotmail-dot-com) A wonderful resource for beginners wanting to learn more about Python is the 110-page PDF tutorial A Byte of Python by Swaroop C H. It is well-written, easy to follow, and may be the best introduction to Python programming available. - Drew Ames in an article on Scripting Scribus [7] published on Linux.com Yesterday I got through most of Byte of Python on my Nokia N800 and it's the easiest and most concise introduction to Python I have yet encountered. Highly recommended as a starting point for learning Python. - Jason Delport on his weblog [8] Academic Courses This book is being used as instructional material in various educational institutions: 1.  'Principles of Programming Languages' course at Vrije Universiteit, Amsterdam [9] 2.  'Basic Concepts of Computing' course at University of California, Davis [10] 3.  'Programming With Python' course at Harvard University [11] 4.  'Introduction to Programming' course at University of Leeds [12] 5.  'Introduction to Application Programming' course at Boston University [13] 6.  'Information Technology Skills for Meteorology' course at University of Oklahoma [14] 7.  'Geoprocessing' course at Michigan State University [15] 8.  'Multi Agent Semantic Web Systems' course at the University of Edinburgh [16] Python 4 Even NASA The book is even used by NASA! It is being used in their Jet Propulsion Laboratory [17] with their Deep Space Network project. Official Recommendation This book has been listed on the official website for Python in the Full Tutorials [18] section, next to the official documentation. License 1.  This book is licensed under the Creative Commons Attribution-Share Alike 3.0 Unported [19] license. •  This means: •  You are free to Share i.e. to copy, distribute and transmit this book •  You are free to Remix i.e. to adapt this book •  Under the following conditions: •  Attribution. You must attribute the work in the manner specified by the author or licensor (but not in any way that suggests that they endorse you or your use of this book). •  Share Alike. If you alter, transform, or build upon this work, you may distribute the resulting work only under the same or similar license to this one. •  For any reuse or distribution, you must make clear to others the license terms of this book. •  Any of the above conditions can be waived if you get permission from the copyright holder. •  Nothing in this license impairs or restricts the author's moral rights. 2.  Attribution must be shown by linking back to http:/ / www. swaroopch. com/ notes/ Python and clearly indicating that the original text can be fetched from this location. 3.  All the code/scripts provided in this book is licensed under the 3-clause BSD License [20] unless otherwise noted. 4.  Volunteer contributions to this original book must be under this same license and the copyright must be assigned to the main author of this book. Read Now You can read the book online at Python_en:Table of Contents. Buy the Book A printed hardcopy of the book can be purchased [21] for your offline reading pleasure, and to support the continued development and improvement of this book. Download •  PDF (631KB) [22] •  Mediawiki XML dump (276KB) [23] (for advanced users only) If you wish to support the continued development of this book, please consider making a donation [24] or buy a printed hardcopy [25]. Python 5 Translations If you are interested in reading or contributing translations of this book to other human languages, please see Translations. Next References [1]  http:/ / www. ibiblio. org/ swaroopch/ byteofpython/ read/ [2]  http:/ / www. ibiblio. org/ swaroopch/ byteofpython/ files/ 120/ byteofpython_120. pdf [3]  http:/ / www. swaroopch. com/ files/ byteofpython/ byte_of_python_v191. pdf [4]  http:/ / www. swaroopch. com/ buybook [5]  http:/ / www. ibiblio. org/ swaroopch/ byteofpython/ files/ 120/ [6]  http:/ / www. b-list. org/ weblog/ 2008/ dec/ 05/ python-3000/ [7]  http:/ / www. linux. com/ feature/ 126522 [8]  http:/ / paxmodept. com/ telesto/ blogitem. htm?id=627 [9]  http:/ / www. few. vu. nl/ ~nsilvis/ PPL/ 2007/ index. html [10]  http:/ / www. cs. ucdavis. edu/ courses/ exp_course_desc/ 10. html [11]  http:/ / www. people. fas. harvard. edu/ ~preshman/ python_winter. html [12]  http:/ / www. comp. leeds. ac. uk/ acom1900/ [13]  http:/ / www. cs. bu. edu/ courses/ cs108/ materials. html [14]  http:/ / gentry. metr. ou. edu/ byteofpython/ [15]  http:/ / www. msu. edu/ ~ashton/ classes/ 825/ index. html [16]  http:/ / homepages. inf. ed. ac. uk/ ewan/ masws/ [17]  http:/ / dsnra. jpl. nasa. gov/ software/ Python/ byte-of-python/ output/ byteofpython_html/ [18]  http:/ / www. python. org/ doc/ intros/ [19]  http:/ / creativecommons. org/ licenses/ by-sa/ 3. 0/ [20]  http:/ / www. opensource. org/ licenses/ bsd-license. php [21]  http:/ / www. swaroopch. com/ buybook [22]  http:/ / www. swaroopch. com/ files/ byteofpython/ byte_of_python_v191. pdf [23]  http:/ / www. swaroopch. com/ files/ byteofpython/ byte_of_python_v191. xml [24]  https:/ / www. paypal. com/ cgi-bin/ webscr?cmd=_donations& business=swaroop%40swaroopch%2ecom& item_name=A%20Byte%20of%20Python& no_shipping=0& no_note=1& tax=0& currency_code=USD& lc=IN& bn=PP%2dDonationsBF& charset=UTF%2d8 [25]  http:/ / www. swaroopch. com/ buybook Source: http:/ / www. swaroopch. com/ mediawiki/ index. php? oldid=1391 Contributors: Swaroop, 1 anonymous edits Python en:Table of Contents 6 Python en:Table of Contents •  → Front Page 1.  → Translations 2.  → Preface 3.  → Introduction 4.  → Installation 5.  → First Steps 6.  → Basics 7.  → Operators and Expressions 8.  → Control Flow 9.  → Functions 10.  → Modules 11.  → Data Structures 12.  → Problem Solving 13.  → Object Oriented Programming 14.  → Input Output 15.  → Exceptions 16.  → Standard Library 17.  → More 18.  → What Next 19.  → Appendix: FLOSS 20.  → Appendix: About 21.  → Appendix: Revision History → Previous → Next Source: http:/ / www. swaroopch. com/ mediawiki/ index. php? oldid=1343 Contributors: Swaroop, Waterox888, 3 anonymous edits Python en:Translations 7 Python en:Translations There are many translations of the book available in different human languages, thanks to many tireless volunteers! If you want to help these translations, please see the list of volunteers and languages below and decide if you want to start a new translation or help in existing translation projects. If you plan to start a new translation, please read the Translation Howto. Chinese Juan Shen (orion-underscore-val-at-163-dot-com) has volunteered to translate the book to Chinese. what - I am a postgraduate at Wireless Telecommunication Graduate School, Beijing University of Technology, China PR. My current research interest is on the synchronization, channel estimation and multi-user detection of multicarrier CDMA system. Python is my major programming language for daily simulation and research job, with the help of Python Numeric, actually. I learned Python just half a year before, but as you can see, it's really easy-understanding, easy-to-use and productive. Just as what is ensured in Swaroop's book, 'It's my favorite programming language now'. 'A Byte of Python' is my tutorial to learn Python. It's clear and effective to lead you into a world of Python in the shortest time. It's not too long, but efficiently covers almost all important things in Python. I think 'A Byte of Python' should be strongly recommendable for newbies as their first Python tutorial. Just dedicate my translation to the potential millions of Python users in China. Chinese Traditional Fred Lin (gasolin-at-gmail-dot-com) has volunteered to translate the book to Chinese Traditional. It is available at http:/ / code. google. com/ p/ zhpy/ wiki/ ByteOfZhpy (http:/ / code. google. com/ p/ zhpy/ wiki/ ByteOfZhpy). An exciting feature of this translation is that it also contains the executable chinese python sources side by side with the original python sources. Fred Lin - I'm working as a network firmware engineer at Delta Network, and I'm also a contributor of TurboGears web framework. As a python evangelist (:-p), I need some material to promote python language. I found 'A Byte of Python' hit the sweet point for both newbies and experienced programmers. 'A Byte of Python' elaborates the python essentials with affordable size. The translation are originally based on simplified chinese version, and soon a lot of rewrite were made to fit the current wiki version and the quality of reading. The recent chinese traditional version also featured with executable chinese python sources, which are achieved by my new 'zhpy' (python in chinese) project (launch from Aug 07). zhpy(pronounce (Z.H.?, or zippy) build a layer upon python to translate or interact with python in chinese(Traditional or Simplified). This project is mainly aimed for education. Python en:Translations 8 Italian Enrico Morelli (mr-dot-mlucci-at-gmail-dot-com) and Massimo Lucci (morelli-at-cerm-dot-unifi-dot-it) have volunteered to translate the book to Italian. The Italian translation is present at www.gentoo.it/Programmazione/byteofpython (http:/ / www. gentoo. it/ Programmazione/ byteofpython). The new translation is in progress and start with "Prefazione". Massimo Lucci and Enrico Morelli - we are working at the University of Florence (Italy) - Chemistry Department. I (Massimo) as service engineer and system administrator for Nuclear Magnetic Resonance Spectrometers; Enrico as service engineer and system administrator for our CED and parallel / clustered systems. We are programming on python since about seven years, we had experience working with Linux platforms since ten years. In Italy we are responsible and administrator for www.gentoo.it web site for Gentoo/Linux distrubution and www.nmr.it (now under construction) for Nuclear Magnetic Resonance applications and Congress Organization and Managements. That's all! We are impressed by the smart language used on your Book and we think this is essential for approaching the Python to new users (we are thinking about hundred of students and researcher working on our labs). German Lutz Horn (lutz-dot-horn-at-gmx-dot-de), Bernd Hengelein (bernd-dot-hengelein-at-gmail-dot-com) and Christoph Zwerschke (cito-at-online-dot-de) have volunteered to translate the book to German. Their translation is located at http:/ / abop-german. berlios. de (http:/ / abop-german. berlios. de). Lutz Horn : I'm 32 years old and have a degree of Mathematics from University of Heidelberg, Germany. Currently I'm working as a software engineer on a publicly funded project to build a web portal for all things related to computer science in Germany. The main language I use as a professional is Java, but I try to do as much as possible with Python behind the scenes. Especially text analysis and conversion is very easy with Python. I'm not very familiar with GUI toolkits, since most of my programming is about web applications, where the user interface is build using Java frameworks like Struts. Currently I try to make more use of the functional programming features of Python and of generators. After taking a short look into Ruby, I was very impressed with the use of blocks in this language. Generally I like the dynamic nature of languages like Python and Ruby since it allows me to do things not possible in more static languages like Java. I've searched for some kind of introduction to programming, suitable to teach a complete non-programmer. I've found the book 'How to Think Like a Computer Scientist: Learning with Python', and 'Dive into Python'. The first is good for beginners but to long to translate. The second is not suitable for beginners. I think 'A Byte of Python' falls nicely between these, since it is not too long, written to the point, and at the same time verbose enough to teach a newbie. Besides this, I like the simple DocBook structure, which makes translating the text a generation the output in various formats a charm. Python en:Translations 9 Bernd Hengelein : Lutz and me are going to do the german translation together. We just started with the intro and preface but we will keep you informed about the progress we make. Ok, now some personal things about me. I am 34 years old and playing with computers since the 1980's, when the "Commodore C64" ruled the nurseries. After studying computer science I started working as a software engineer. Currently I am working in the field of medical imaging for a major german company. Although C++ is the main language I (have to) use for my daily work, I am constantly looking for new things to learn. Last year I fell in love with Python, which is a wonderful language, both for its possibilities and its beauty. I read somewhere in the net about a guy who said that he likes python, because the code looks so beautiful. In my opinion he's absolutly right. At the time I decided to learn python, I noticed that there is very little good documentation in german available. When I came across your book the spontaneous idea of a german translation crossed my mind. Luckily, Lutz had the same idea and we can now divide the work. I am looking forward to a good cooperation! Norwegian (bokmål) Eirik Vågeskar (or Vages) is a high school student at Sandvika videregående skole (http:/ / no. wikipedia. org/ wiki/ Sandvika_videregÃ¥ende_skole) in Norway, a blogger (http:/ / forbedre. blogspot. com/ ) and currently translating the book to Norwegian (bokmål). The translation is in progress, and you can check the table of contents for more details. Eirik Vågeskar: I have always wanted to program, but because I speak a small language, the learning process was much harder. Most tutorials and books are written in very technical English, so most high school graduates will not even have the vocabulary to understand what the tutorial is about. When I discovered this book, all my problems were solved. "A Byte of Python" used simple non-technical language to explain a programming language that is just as simple, and these two things make learning Python fun. After reading half of the book, I decided that the book was worth translating. I hope the translation will help people who have found themself in the same situation as me (especially young people), and maybe help spread interest for the language among people with less technical knowledge. Indonesian Daniel (daniel-dot-mirror-at-gmail-dot-com) is translating the book to Indonesian at http:/ / python. or. id/ moin. cgi/ ByteofPython Polish Dominik Kozaczko (dkozaczko-at-gmail-dot-com) has volunteered to translate the book to Polish. Translation is in progress and it's main page is available here: Ukąś Pythona (http:/ / wiki. mercury. lo5. bielsko. pl/ index. php/ UkÄ Å›_Pythona). Dominik Kozaczko - I'm a Computer Science and Information Technology teacher. Python en:Translations 10 Catalan Moises Gomez (moisesgomezgiron-at-gmail-dot-com) has volunteered to translate the book to Catalan. The translation is in progress, and starts with the chapter "Taula de continguts". Moisès Gómez - I am a developer and also a teacher of programming (normally for people without any previous experience). Some time ago I needed to learn how to program in Python, and Swaroop's work was really helpful. Clear, concise, and complete enough. Just what I needed. After this experience, I thought some other people in my country could take benefit from it too. But English language can be a barrier. So, why not try to translate it? And I did for a previous version of BoP. I my country there are two official languages. I selected the Catalan language assuming that others will translate it to the more widespread Spanish. Portuguese Fidel Viegas (fidel-dot-viegas-at-gmail-dot-com) has volunteered to translate the book to Portuguese. Romanian Paul-Sebastian Manole (brokenthorn-at-gmail-dot-com) has volunteered to translate this book to Romanian. Paul-Sebastian Manole - I'm a second year Computer Science student at Spiru Haret University, here in Romania. I'm more of a self-taught programmer and decided to learn a new language, Python. The web told me there was no better way to do so but read A Byte of Python. That's how popular this book is (congratulations to the author for writing such an easy to read book). I started liking Python so I decided to help translate the latest version of Swaroop's book in Romanian. Although I could be the one with the first initiative, I'm just one volunteer so if you can help, please join me. The translation is being done here (http:/ / www. swaroopch. com/ notes/ Python_ro). Brazilian Portuguese Rodrigo Amaral (http:/ / rodrigoamaral. net) (rodrigoamaral-at-gmail-dot-com) has volunteered to translate the book to Brazilian Portuguese... French Gregory (coulix-at-ozforces-dot-com-dot-au) has volunteered to translate the book to French. Danish Lars Petersen (lars-at-ioflux-dot-net) has volunteered to translate the book to Danish. Spanish Alfonso de la Guarda Reyes (alfonsodg-at-ictechperu-dot-net) and Gustavo Echeverria (gustavo-dot-echeverria-at-gmail-dot-com) have volunteered to translate the book to Python en:Translations 11 Spanish. The translation is in progress, you can read the spanish (argentinian) translation starting by the table of contents (tabla de contenidos). Gustavo Echeverria: I work as a software engineer in Argentina. I use mostly C# and .Net technologies at work but strictly Python or Ruby in my personal projects. I knew Python many years ago and I got stuck inmediately. Not so long after knowing Python I discovered this book and it helped me to learn the language. Then I volunteered to translate the book to Spanish. Now, after receiving some requests, I've begun to translate "A Byte of Python" with the help of Maximiliano Soler. Arabic Alaa Abadi (alaanassir-at-gmail-dot-com) has volunteered to translate the book to Arabic. ISA . Swedish Mikael Jacobsson (leochingkwake-at-gmail-dot-com) has volunteered to translate the book to Swedish. Russian and Ukranian Averkiev Andrey (averkiyev-at-ukr-dot-net) has volunteered to translate the book to Russian, and perhaps Ukranian (time permitting). Turkish Türker SEZER (tsezer-at-btturk-dot-net) and Bugra Cakir (bugracakir-at-gmail-dot-com) have volunteered to translate the book to Turkish. Mongolian Ariunsanaa Tunjin (tariunsanaa-at-yahoo-dot-com) has volunteered to translate the book to Mongolian. Note Replace '-at-' with '@' , '-dot-' with '.' and '-underscore-' with '_' in the email addresses mentioned on this page. Dashes in other places in the email address remain as-is. Python en:Translations 12 Previous Next Source: http:/ / www. swaroopch. com/ mediawiki/ index. php? oldid=2278 Contributors: Geopop65, Gustavo.echeverria, Leochingkwake, Moises, Morellik, Rodrigoamaral, Swaroop, Thorns, Vages, Waterox888, 20 anonymous edits Python en:Preface Python is probably one of the few programming languages which is both simple and powerful. This is good for both and beginners as well as experts, and more importantly, is fun to program with. This book aims to help you learn this wonderful language and show how to get things done quickly and painlessly - in effect 'The Perfect Anti-venom to your programming problems'. Who This Book Is For This book serves as a guide or tutorial to the Python programming language. It is mainly targeted at newbies. It is useful for experienced programmers as well. The aim is that if all you know about computers is how to save text files, then you can learn Python from this book. If you have previous programming experience, then you can also learn Python from this book. If you do have previous programming experience, you will be interested in the differences between Python and your favorite programming language - I have highlighted many such differences. A little warning though, Python is soon going to become your favorite programming language! History Lesson I first started with Python when I needed to write an installer for a software I had written called 'Diamond' so that I could make the installation easy. I had to choose between Python and Perl bindings for the Qt library. I did some research on the web and I came across an article where Eric S. Raymond, the famous and respected hacker, talked about how Python has become his favorite programming language. I also found out that the PyQt bindings were more mature compared to Perl-Qt. So, I decided that Python was the language for me. Then, I started searching for a good book on Python. I couldn't find any! I did find some O'Reilly books but they were either too expensive or were more like a reference manual than a guide. So, I settled for the documentation that came with Python. However, it was too brief and small. It did give a good idea about Python but was not complete. I managed with it since I had previous programming experience, but it was unsuitable for newbies. About six months after my first brush with Python, I installed the (then) latest Red Hat 9.0 Linux and I was playing around with KWord. I got excited about it and suddenly got the idea of writing some stuff on Python. I started writing a few pages but it quickly became 30 pages long. Then, I became serious about making it more useful in a book form. After a lot of rewrites, it has reached a stage where it has become a useful guide to learning the Python language. I consider this book to be my contribution and tribute to the open source community. Python en:Preface 13 This book started out as my personal notes on Python and I still consider it in the same way, although I've taken a lot of effort to make it more palatable to others :) In the true spirit of open source, I have received lots of constructive suggestions, criticisms and feedback from enthusiastic readers which has helped me improve this book a lot. Status Of The Book Changes since the last major revision in March 2005 is updating for the Python 3.0 release (expected in August/September 2008). Since the Python 3.0 language itself is still not finalized/released, this book is constantly undergoing changes. However, in the spirit of the open source philosophy of "Release Early, Release Often", the updated book has been released and is constantly being updated. The book needs the help of its readers such as yourselves to point out any parts of the book which are not good, not comprehensible or are simply wrong. Please write to the main author (http:/ / www. swaroopch. com/ contact/ ) or the respective translators with your comments and suggestions. It's a constant tussle to balance this book between a beginner's needs and the tendency towards 'completeness' of information. It would be helpful if readers also gave feedback on how much depth this book should go into. Official Website The official website of the book is http:/ / www. swaroopch. com/ notes/ Python where you can read the whole book online, download the latest versions of the book, buy a printed hard copy (http:/ / www. swaroopch. com/ buybook), and also send me feedback. License 1.  This book is licensed under the Creative Commons Attribution-Noncommercial-Share Alike 3.0 Unported (http:/ / creativecommons. org/ licenses/ by-nc-sa/ 3. 0/ ) license. •  This means: •  You are free to Share i.e. to copy, distribute and transmit this book •  You are free to Remix i.e. to adapt this book •  Under the following conditions: •  Attribution. You must attribute the work in the manner specified by the author or licensor (but not in any way that suggests that they endorse you or your use of this book). •  Share Alike. If you alter, transform, or build upon this work, you may distribute the resulting work only under the same or similar license to this one. •  For any reuse or distribution, you must make clear to others the license terms of this book. •  Any of the above conditions can be waived if you get permission from the copyright holder. •  Nothing in this license impairs or restricts the author's moral rights. 2.  Attribution must be shown by linking back to http:/ / www. swaroopch. com/ notes/ Python and clearly indicating that the original text can be fetched from this location. Python en:Preface 14 3.  All the code/scripts provided in this book is licensed under the 3-clause BSD License (http:/ / www. opensource. org/ licenses/ bsd-license. php) unless otherwise noted. 4.  Volunteer contributions to this original book must be under this same license and the copyright must be assigned to the main author of this book. Feedback I have put in a lot of effort to make this book as interesting and as accurate as possible. However, if you find some material to be inconsistent or incorrect, or simply needs improvement, then please do inform me, so that I can make suitable improvements. You can reach me via my user page. Buy the Book If you wish to support the continued development of this book, please consider purchasing a printed copy (http:/ / www. swaroopch. com/ buybook) or making a donation. Something To Think About There are two ways of constructing a software design: one way is to make it so simple that there are obviously no deficiencies; the other is to make it so complicated that there are no obvious deficiencies. -- C. A. R. Hoare Success in life is a matter not so much of talent and opportunity as of concentration and perseverance. -- C. W. Wendte Previous Next Source: http:/ / www. swaroopch. com/ mediawiki/ index. php? oldid=987 Contributors: Gasolin, Swaroop, 2 anonymous edits Python en:Introduction 15 Python en:Introduction Introduction Python is one of those rare languages which can claim to be both simple and powerful. You will find that you will be pleasantly surprised on how easy it is to concentrate on the solution to the problem rather than the syntax and structure of the language you are programming in. The official introduction to Python is: Python is an easy to learn, powerful programming language. It has efficient high-level data structures and a simple but effective approach to object-oriented programming. Python's elegant syntax and dynamic typing, together with its interpreted nature, make it an ideal language for scripting and rapid application development in many areas on most platforms. I will discuss most of these features in more detail in the next section. Note Guido van Rossum, the creator of the Python language, named the language after the BBC show "Monty Python's Flying Circus". He doesn't particularly like snakes that kill animals for food by winding their long bodies around them and crushing them. Features of Python Simple Python is a simple and minimalistic language. Reading a good Python program feels almost like reading English, although very strict English! This pseudo-code nature of Python is one of its greatest strengths. It allows you to concentrate on the solution to the problem rather than the language itself. Easy to Learn As you will see, Python is extremely easy to get started with. Python has an extraordinarily simple syntax, as already mentioned. Free and Open Source Python is an example of a FLOSS (Free/Libré and Open Source Software). In simple terms, you can freely distribute copies of this software, read its source code, make changes to it, and use pieces of it in new free programs. FLOSS is based on the concept of a community which shares knowledge. This is one of the reasons why Python is so good - it has been created and is constantly improved by a community who just want to see a better Python. High-level Language When you write programs in Python, you never need to bother about the low-level details such as managing the memory used by your program, etc. Portable Due to its open-source nature, Python has been ported to (i.e. changed to make it work on) many platforms. All your Python programs can work on any of these platforms without requiring any changes at all if you are careful enough to avoid any Python en:Introduction 16 system-dependent features. You can use Python on Linux, Windows, FreeBSD, Macintosh, Solaris, OS/2, Amiga, AROS, AS/400, BeOS, OS/390, z/OS, Palm OS, QNX, VMS, Psion, Acorn RISC OS, VxWorks, PlayStation, Sharp Zaurus, Windows CE and even PocketPC ! Interpreted This requires a bit of explanation. A program written in a compiled language like C or C++ is converted from the source language i.e. C or C++ into a language that is spoken by your computer (binary code i.e. 0s and 1s) using a compiler with various flags and options. When you run the program, the linker/loader software copies the program from hard disk to memory and starts running it. Python, on the other hand, does not need compilation to binary. You just run the program directly from the source code. Internally, Python converts the source code into an intermediate form called bytecodes and then translates this into the native language of your computer and then runs it. All this, actually, makes using Python much easier since you don't have to worry about compiling the program, making sure that the proper libraries are linked and loaded, etc, etc. This also makes your Python programs much more portable, since you can just copy your Python program onto another computer and it just works! Object Oriented Python supports procedure-oriented programming as well as object-oriented programming. In procedure-oriented languages, the program is built around procedures or functions which are nothing but reusable pieces of programs. In object-oriented languages, the program is built around objects which combine data and functionality. Python has a very powerful but simplistic way of doing OOP, especially when compared to big languages like C++ or Java. Extensible If you need a critical piece of code to run very fast or want to have some piece of algorithm not to be open, you can code that part of your program in C or C++ and then use it from your Python program. Embeddable You can embed Python within your C/C++ programs to give 'scripting' capabilities for your program's users. Extensive Libraries The Python Standard Library is huge indeed. It can help you do various things involving regular expressions, documentation generation, unit testing, threading, databases, web browsers, CGI, FTP, email, XML, XML-RPC, HTML, WAV files, cryptography, GUI (graphical user interfaces), Tk, and other system-dependent stuff. Remember, all this is always available wherever Python is installed. This is called the 'Batteries Included' philosophy of Python. Besides, the standard library, there are various other high-quality libraries such as wxPython (http:/ / www. wxpython. org) , Twisted (http:/ / www. twistedmatrix. com/ products/ twisted), Python Imaging Library (http:/ / www. pythonware. com/ products/ pil/ index. htm) and many more. Python en:Introduction 17 Python is indeed an exciting and powerful language. It has the right combination of performance and features that make writing programs in Python both fun and easy. Why not Perl? If you didn't know already, Perl is another extremely popular open source interpreted programming language. If you have ever tried writing a large program in Perl, you would have answered this question yourself! In other words, Perl programs are easy when they are small and it excels at small hacks and scripts to 'get work done'. However, they quickly become unwieldy once you start writing bigger programs and I am speaking this out of my experience writing large Perl programs at Yahoo! When compared to Perl, Python programs are definitely simpler, clearer, easier to write and hence more understandable and maintainable. I do admire Perl and I do use it on a daily basis for various things but whenever I write a program, I always start thinking in terms of Python because it has become so natural for me. Perl has undergone so many hacks and changes, that it feels like it is one big (but one hell of a) hack. Sadly, the upcoming Perl 6 does not seem to be making any improvements regarding this. The only and very significant advantage that I feel Perl has, is its huge CPAN (http:/ / cpan. perl. org) library - the Comprehensive Perl Archive Network. As the name suggests, this is a humongous collection of Perl modules and it is simply mind-boggling because of its sheer size and depth - you can do virtually anything you can do with a computer using these modules. One of the reasons that Perl has more libraries than Python is that it has been around for a much longer time than Python. However this seems to be changing with the growing Python Package Index (http:/ / pypi. python. org/ pypi). Why not Ruby? If you didn't know already, Ruby is another popular open source interpreted programming language. If you already like and use Ruby, then I would definitely recommend you to continue using it. For other people who have not used it and are trying to judge whether to learn Python or to learn Ruby, then I would recommend Python, purely from an ease-of-learning perspective. I personally found it hard to grok the Ruby language, but for people who understand Ruby, they all praise the beauty of the language. Unfortunately, I am not as lucky. What Programmers Say You may find it interesting to read what great hackers like ESR have to say about Python: •  Eric S. Raymond is the author of "The Cathedral and the Bazaar" and is also the person who coined the term Open Source. He says that Python has become his favorite programming language (http:/ / www. linuxjournal. com/ article. php?sid=3882). This article was the real inspiration for my first brush with Python. •  Bruce Eckel is the author of the famous Thinking in Java and Thinking in C++ books. He says that no language has made him more productive than Python. He says that Python is perhaps the only language that focuses on making things easier for the Python en:Introduction 18 programmer. Read the complete interview (http:/ / www. artima. com/ intv/ aboutme. html) for more details. •  Peter Norvig is a well-known Lisp author and Director of Search Quality at Google (thanks to Guido van Rossum for pointing that out). He says that Python has always been an integral part of Google. You can actually verify this statement by looking at the Google Jobs (http:/ / www. google. com/ jobs/ index. html) page which lists Python knowledge as a requirement for software engineers. About Python 3. 0 Python 3.0 is the new version of the language. It is sometimes referred to as Python 3000 or Py3K. The main reason for a major new version of Python is to remove all the small problems and nitpicks that have accumulated over the years and to make the language even more clean. If you already have a lot of Python 2.x code, then there is a utility to assist you to convert 2.x to 3.x source (http:/ / docs. python. org/ dev/ 3. 0/ library/ 2to3. html). More details are at: •  Guido van Rossum's introduction (http:/ / www. artima. com/ weblogs/ viewpost. jsp?thread=208549) •  What's New in Python 2.6 (http:/ / docs. python. org/ dev/ whatsnew/ 2. 6. html) (features significantly different from previous Python 2.x versions and most likely will be included in Python 3.0) •  What's New in Python 3.0 (http:/ / docs. python. org/ dev/ 3. 0/ whatsnew/ 3. 0. html) •  Python 2.6 and 3.0 Release Schedule (http:/ / www. python. org/ dev/ peps/ pep-0361/ ) •  Python 3000 (the official authoritative list of proposed changes) (http:/ / www. python. org/ dev/ peps/ pep-3000/ ) •  Miscellaneous Python 3.0 Plans (http:/ / www. python. org/ dev/ peps/ pep-3100/ ) •  Python News (detailed list of changes) (http:/ / www. python. org/ download/ releases/ 3. 0/ NEWS. txt) Previous Next Source: http:/ / www. swaroopch. com/ mediawiki/ index. php? oldid=1789 Contributors: JeremyBicha, Swaroop, 2 anonymous edits Python en:Installation 19 Python en:Installation If you have Python 2.x installed already, you do not have to remove it to install Python 3.0. You can have both installed at the same time. For Linux and BSD users If you are using a Linux distribution such as Ubuntu, Fedora, OpenSUSE or {put your choice here}, or a BSD system such as FreeBSD, then it is most likely you already have Python installed on your system. To test if you have Python already installed on your Linux box, open a shell program (like konsole or gnome-terminal) and enter the command python -V as shown below. $ python -V Python 3.0b1 Note $ is the prompt of the shell. It will be different for you depending on the settings of your OS, hence I will indicate the prompt by just the $ symbol. If you see some version information like the one shown above, then you have Python installed already. However, if you get a message like this one: $ python -V bash: Python: command not found Then you don't have Python installed. This is highly unlikely but possible. Note If you have Python 2.x already installed, then try python3 -V. In this case, you have two ways of installing Python on your system. •  You can compile Python from the source code (http:/ / www. python. org/ download/ releases/ 3. 0/ ) and install it. The compilation instructions are provided at the website. •  [This option will be available after the final release of Python 3.0] Install the binary packages using the package management software that comes with your OS, such as apt-get in Ubuntu/Debian and other Debian-based Linux, yum in Fedora Linux, pkg_add in FreeBSD, etc. Note that you will need an internet connection to use this method. Alternatively, you can download the binaries from somewhere else and then copy to your PC and install it. Python en:Installation 20 For Windows Users Visit http:/ / www. python. org/ download/ releases/ 3. 0/ and download the latest version from this website, which was 3.0 beta 1 (http:/ / www. python. org/ ftp/ python/ 3. 0/ python-3. 0b1. msi) as of this writing. This is just 12.8 MB which is very compact compared to most other languages or software. The installation is just like any other Windows-based software. Caution When you are given the option of unchecking any "optional" components, don't uncheck any! Some of these components can be useful for you, especially IDLE. An interesting fact is that majority of Python downloads are by Windows users. Of course, this doesn't give the complete picture since almost all Linux users will have Python installed already on their systems by default. DOS Prompt If you want to be able to use Python from the Windows command line i.e. the DOS prompt, then you need to set the PATH variable appropriately. For Windows 2000, XP, 2003 , click on Control Panel -> System -> Advanced -> Environment Variables. Click on the variable named PATH in the 'System Variables' section, then select Edit and add ;C:\Python30 to the end of what is already there. Of course, use the appropriate directory name. For older versions of Windows, add the following line to the file C:\AUTOEXEC.BAT : 'PATH=%PATH%;C:\Python30' (without the quotes) and restart the system. For Windows NT, use the AUTOEXEC.NT file. For Mac OS X Users Mac OS X Users will find Python already installed on their system. Open the Terminal.app and run python -V and follow the advice in the above Linux section. Summary For a Linux system, you most probably already have Python installed on your system. Otherwise, you can install it using the package management software that comes with your distribution. For a Windows system, installing Python is as easy as downloading the installer and double-clicking on it. From now on, we will assume that you have Python installed on your system. Next, we will write our first Python program. Python en:Installation 21 Previous Next Source: http:/ / www. swaroopch. com/ mediawiki/ index. php? oldid=1746 Contributors: Swaroop, 1 anonymous edits Python en:First Steps Introduction We will now see how to run a traditional 'Hello World' program in Python. This will teach you how to write, save and run Python programs. There are two ways of using Python to run your program - using the interactive interpreter prompt or using a source file. We will now see how to use both of these methods Using The Interpreter Prompt Start the interpreter on the command line by entering python at the shell prompt. For Windows users, you can run the interpreter in the command line if you have set the PATH variable appropriately. If you are using IDLE, click on Start → Programs → Python 3.0 → IDLE (Python GUI). Now enter print('Hello World') followed by the Enter key. You should see the words Hello World as output. $ python Python 3.0b2 (r30b2:65106, Jul 18 2008, 18:44:17) [MSC v.1500 32 bit (Intel)] on win32 Type "help", "copyright", "credits" or "license" for more information. >>> print('Hello World') Hello World >>> Notice that Python gives you the output of the line immediately! What you just entered is a single Python statement. We use print to (unsurprisingly) print any value that you supply to it. Here, we are supplying the text Hello World and this is promptly printed to the screen. How to Quit the Interpreter Prompt To exit the prompt, press ctrl-d if you are using IDLE or are using a Linux/BSD shell. In case of the Windows command prompt, press ctrl-z followed by enter key. Choosing An Editor Before we move on to writing Python programs in source files, we need an editor to write the source files. The choice of an editor is crucial indeed. You have to choose an editor as you would choose a car you would buy. A good editor will help you write Python programs easily, making your journey more comfortable and helps you reach your destination (achieve your goal) in a much faster and safer way. Python en:First Steps 22 One of the very basic requirements is syntax highlighting where all the different parts of your Python program are colorized so that you can see your program and visualize its running. If you are using Windows, then I suggest that you use IDLE. IDLE does syntax highlighting and a lot more such as allowing you to run your programs within IDLE among other things. A special note: Do not use Notepad - it is a bad choice because it does not do syntax highlighting and also importantly it does not support indentation of the text which is very important in our case as we will see later. Good editors such as IDLE (and also VIM) will automatically help you do this. If you are using Linux/FreeBSD, then you have a lot of choices for an editor. If you are just beginning to program, you might want to use geany. It has a graphical user interface and has buttons to compile and run your python program without a fuss. If you are an experienced programmer, then you must be already using Vim or Emacs. Needless to say, these are two of the most powerful editors and you will be benefitted by using them to write your Python programs. I personally use Vim for most of my programs. If you are a beginner programmer, then you can use Kate which is one of my favorites. In case you are willing to take the time to learn Vim or Emacs, then I highly recommend that you do learn to use either of them as it will be very useful for you in the long run. In this book, we will use IDLE, our IDE and editor of choice. IDLE is installed by default with the Windows and Mac OS X Python installers. It is also available for installation for Linux (http:/ / love-python. blogspot. com/ 2008/ 03/ install-idle-in-linux. html) and BSDs in their respective repositories. We will explore how to use IDLE in the next section. For further details, please refer the IDLE documentation (http:/ / www. python. org/ idle/ doc/ idlemain. html). If you still want to explore other choices of an editor, see the comprehensive list of Python editors (http:/ / www. python. org/ cgi-bin/ moinmoin/ PythonEditors) and make your choice. You can also choose an IDE (Integrated Development Environment) for Python. See the comprehensive list of IDEs that support Python (http:/ / www. python. org/ cgi-bin/ moinmoin/ IntegratedDevelopmentEnvironments) for more details. Once you start writing large Python programs, IDEs can be very useful indeed. I repeat once again, please choose a proper editor - it can make writing Python programs more fun and easy. For Vim users There is a good introduction on how to make Vim a powerful Python IDE by John M Anderson (http:/ / blog. sontek. net/ 2008/ 05/ 11/ python-with-a-modular-ide-vim/ ). For Emacs users There is a good introduction on how to make Emacs a powerful Python IDE by Ryan McGuire (http:/ / www. enigmacurry. com/ 2008/ 05/ 09/ emacs-as-a-powerful-python-ide/ ). Python en:First Steps 23 Using A Source File Now let's get back to programming. There is a tradition that whenever you learn a new programming language, the first program that you write and run is the 'Hello World' program - all it does is just say 'Hello World' when you run it. As Simon Cozens [1] puts it, it is the 'traditional incantation to the programming gods to help you learn the language better' :) . Start your choice of editor, enter the following program and save it as helloworld.py If you are using IDLE, click on File → New Window and enter the following program. Then click on File → Save. #!/usr/bin/python #Filename: helloworld.py print('Hello World') Run this program by opening a shell (Linux terminal or DOS prompt) and entering the command python helloworld.py. If you are using IDLE, use the menu Run → Run Module or the keyboard shortcut F5. The output is as shown below. $ python helloworld.py Hello World If you got the output as shown above, congratulations! - you have successfully run your first Python program. In case you got an error, please type the above program exactly as shown and above and run the program again. Note that Python is case-sensitive i.e. print is not the same as Print - note the lowercase p in the former and the uppercase P in the latter. Also, ensure there are no spaces or tabs before the first character in each line - we will see why this is important later. How It Works Let us consider the first two lines of the program. These are called comments - anything to the right of the # symbol is a comment and is mainly useful as notes for the reader of the program. Python does not use comments except for the special case of the first line here. It is called the shebang line - whenever the first two characters of the source file are #! followed by the location of a program, this tells your Linux/Unix system that this program should be run with this interpreter when you execute the program. This is explained in detail in the next section. Note that you can always run the program on any platform by specifying the interpreter directly on the command line such as the command python helloworld.py . Important Use comments sensibly in your program to explain some important details of your program - this is useful for readers of your program so that they can easily understand what the program is doing. Remember, that person can be yourself after six months! The comments are followed by a Python statement. Here we call the print function this just prints the text 'Hello World'. We will learn about functions in a → later chapter, what Python en:First Steps 24 you should understand now is that whatever you supply in the parentheses will be printed back to the screen. In this case, we supply 'Hello World' which is referred to as a string - don't worry, we will explore these terminologies in detail later. Executable Python Programs This applies only to Linux/Unix users but Windows users might be curious as well about the first line of the program. First, we have to give the program executable permission using the chmod command then run the source program. $ chmod a+x helloworld.py $ ./helloworld.py Hello World The chmod command is used here to change the mode of the file by giving execute permission to all users of the system. Then, we execute the program directly by specifying the location of the source file. We use the ./ to indicate that the program is located in the current directory. To make things more fun, you can rename the file to just helloworld and run it as ./helloworld and it will still work since the system knows that it has to run the program using the interpreter whose location is specified in the first line in the source file. What if you don't know where Python is located? Then, you can use the special env program on Linux/Unix systems. Just change the first line of the program to the following: #!/usr/bin/env python The env program will in turn look for the Python interpreter which will run the program. So far, we have been able to run our program as long as we know the exact path. What if we wanted to be able to run the program from anywhere? You can do this by storing the program in one of the directories listed in the PATH environment variable. Whenever you run any program, the system looks for that program in each of the directories listed in the PATH environment variable and then runs that program. We can make this program available everywhere by simply copying this source file to one of the directories listed in PATH. $ echo $PATH /usr/local/bin:/usr/bin:/bin:/usr/X11R6/bin:/home/swaroop/bin $ cp helloworld.py /home/swaroop/bin/helloworld $ helloworld Hello World We can display the PATH variable using the echo command and prefixing the variable name by $ to indicate to the shell that we need the value of this variable. We see that /home/swaroop/bin is one of the directories in the PATH variable where swaroop is the username I am using in my system. There will usually be a similar directory for your username on your system. Alternatively, you can add a directory of your choice to the PATH variable - this can be done by running PATH=$PATH:/home/swaroop/mydir where '/home/swaroop/mydir' is the directory I want to add to the PATH variable. This method is very useful if you want to write useful scripts that you want to run the program anytime, anywhere. It is like creating your own commands just like cd or any Python en:First Steps 25 other commands that you use in the Linux terminal or DOS prompt. Caution W.r.t. Python, a program or a script or software all mean the same thing. Getting Help If you need quick information about any function or statement in Python, then you can use the built-in help functionality. This is very useful especially when using the interpreter prompt. For example, run help(print) - this displays the help for the print function which is used to print things to the screen. Note Press q to exit the help. Similarly, you can obtain information about almost anything in Python. Use help() to learn more about using help itself! In case you need to get help for operators like return, then you need to put those inside quotes such as help('return') so that Python doesn't get confused on what we're trying to do. Summary You should now be able to write, save and run Python programs at ease. Now that you are a Python user, let's learn some more Python concepts. References: [1]  The author of the amazing 'Beginning Perl' book → Previous → Next Source: http:/ / www. swaroopch. com/ mediawiki/ index. php? oldid=2332 Contributors: Swaroop, 9 anonymous edits Python en:Basics 26 Python en:Basics Just printing 'Hello World' is not enough, is it? You want to do more than that - you want to take some input, manipulate it and get something out of it. We can achieve this in Python using constants and variables. Literal Constants An example of a literal constant is a number like 5, 1.23, 9.25e-3 or a string like 'This is a string' or "It's a string!". It is called a literal because it is literal - you use its value literally. The number 2 always represents itself and nothing else - it is a constant because its value cannot be changed. Hence, all these are referred to as literal constants. Numbers Numbers in Python are of three types - integers, floating point and complex numbers. •  An examples of an integer is 2 which is just a whole number. •  Examples of floating point numbers (or floats for short) are 3.23 and 52.3E-4. The E notation indicates powers of 10. In this case, 52.3E-4 means 52.3 * 10-4. •  Examples of complex numbers are (-5+4j) and (2.3 - 4.6j) Note for Experienced Programmers There is no separate 'long int' type. The default integer type can be any large value. Strings A string is a sequence of characters. Strings are basically just a bunch of words. The words can be in English or any other language that is supported in the Unicode standard, which means almost any language in the world (http:/ / www. unicode. org/ faq/ basic_q. html#16). Note for Experienced Programmers There are no "ASCII-only" strings because Unicode is a superset of ASCII. If a strictly ASCII-encoded byte-stream is needed, then use str.encode("ascii"). For more details, please see the related discussion at StackOverflow (http:/ / stackoverflow. com/ questions/ 175240/ how-do-i-convert-a-files-format-from-unicode-to-ascii-using-python#175270). By default, all strings are in Unicode. I can almost guarantee that you will be using strings in almost every Python program that you write, so pay attention to the following part on how to use strings in Python. Python en:Basics 27 Single Quotes You can specify strings using single quotes such as 'Quote me on this'. All white space i.e. spaces and tabs are preserved as-is. Double Quotes Strings in double quotes work exactly the same way as strings in single quotes. An example is "What's your name?" Triple Quotes You can specify multi-line strings using triple quotes - (""" or '''). You can use single quotes and double quotes freely within the triple quotes. An example is: '''This is a multi-line string. This is the first line. This is the second line. "What's your name?," I asked. He said "Bond, James Bond." ''' Escape Sequences Suppose, you want to have a string which contains a single quote ('), how will you specify this string? For example, the string is What's your name?. You cannot specify 'What's your name?' because Python will be confused as to where the string starts and ends. So, you will have to specify that this single quote does not indicate the end of the string. This can be done with the help of what is called an escape sequence. You specify the single quote as \' - notice the backslash. Now, you can specify the string as 'What\'s your name?'. Another way of specifying this specific string would be "What's your name?" i.e. using double quotes. Similarly, you have to use an escape sequence for using a double quote itself in a double quoted string. Also, you have to indicate the backslash itself using the escape sequence \\. What if you wanted to specify a two-line string? One way is to use a triple-quoted string as shown previously or you can use an escape sequence for the newline character - \n to indicate the start of a new line. An example is This is the first line\nThis is the second line. Another useful escape sequence to know is the tab - \t. There are many more escape sequences but I have mentioned only the most useful ones here. One thing to note is that in a string, a single backslash at the end of the line indicates that the string is continued in the next line, but no newline is added. For example: "This is the first sentence.\ This is the second sentence." is equivalent to "This is the first sentence. This is the second sentence.". Python en:Basics 28 Raw Strings If you need to specify some strings where no special processing such as escape sequences are handled, then what you need is to specify a raw string by prefixing r or R to the string. An example is r"Newlines are indicated by \n". Strings Are Immutable This means that once you have created a string, you cannot change it. Although this might seem like a bad thing, it really isn't. We will see why this is not a limitation in the various programs that we see later on. String Literal Concatenation If you place two string literals side by side, they are automatically concatenated by Python. For example, 'What\'s ' 'your name?' is automatically converted in to "What's your name?". Note for C/C++ Programmers There is no separate char data type in Python. There is no real need for it and I am sure you won't miss it. Note for Perl/PHP Programmers Remember that single-quoted strings and double-quoted strings are the same - they do not differ in any way. Note for Regular Expression Users Always use raw strings when dealing with regular expressions. Otherwise, a lot of backwhacking may be required. For example, backreferences can be referred to as '\\1' or r'\1'. The format Method Sometimes we may want to construct strings from other information. This is where the format() method is useful. #!/usr/bin/python # Filename: str_format.py age = 25 name = 'Swaroop' print('{0} is {1} years old'.format(name, age)) print('Why is {0} playing with that python?'.format(name)) Output: $ python str_format.py Swaroop is 25 years old Why is Swaroop playing with that python? How It Works: A string can use certain specifications and subsequently, the format method can be called to substitute those specifications with corresponding arguments to the format method. Python en:Basics 29 Observe the first usage where we use {0} and this corresponds to the variable name which is the first argument to the format method. Similarly, the second specification is {1} corresponding to age which is the second argument to the format method. Notice that we could achieved the same using string concatenation: name + ' is ' + str(age) + ' years old' but notice how much uglier and error-prone this is. Second, the conversion to string would be done automatically by the format method instead of the explicit conversion here. Third, when using the format method, we can change the message without having to deal with the variables used and vice-versa. What Python does in the format method is that it substitutes each argument value into the place of the specification. There can be more detailed specifications such as: >>> '{0:.3}'.format(1/3) # decimal (.) precision of 3 for float '0.333' >>> '{0:_^11}'.format('hello') # fill with underscores (_) with the text centered (^) to 11 width '___hello___' >>> '{name} wrote {book}'.format(name='Swaroop', book='A Byte of Python') # keyword-based 'Swaroop wrote A Byte of Python' Details of this formatting specification is explained in the Python Enhancement Proposal No. 3101 (http:/ / www. python. org/ dev/ peps/ pep-3101/ ). Variables Using just literal constants can soon become boring - we need some way of storing any information and manipulate them as well. This is where variables come into the picture. Variables are exactly what the name implies - their value can vary, i.e., you can store anything using a variable. Variables are just parts of your computer's memory where you store some information. Unlike literal constants, you need some method of accessing these variables and hence you give them names. Identifier Naming Variables are examples of identifiers. Identifiers are names given to identify something. There are some rules you have to follow for naming identifiers: •  The first character of the identifier must be a letter of the alphabet (uppercase ASCII or lowercase ASCII or Unicode character) or an underscore ('_'). •  The rest of the identifier name can consist of letters (uppercase ASCII or lowercase ASCII or Unicode character), underscores ('_') or digits (0-9). •  Identifier names are case-sensitive. For example, myname and myName are not the same. Note the lowercase n in the former and the uppercase N in the latter. •  Examples of valid identifier names are i, __my_name, name_23, a1b2_c3 and resumé_count. •  Examples of invalid identifier names are 2things, this is spaced out, my-name, and "this_is_in_quotes". Python en:Basics 30 Data Types Variables can hold values of different types called data types. The basic types are numbers and strings, which we have already discussed. In later chapters, we will see how to create our own types using classes. Objects Remember, Python refers to anything used in a program as an object. This is meant in the generic sense. Instead of saying 'the something', we say 'the object'. Note for Object Oriented Programming users Python is strongly object-oriented in the sense that everything is an object including numbers, strings and functions. We will now see how to use variables along with literal constants. Save the following example and run the program. How to write Python programs Henceforth, the standard procedure to save and run a Python program is as follows: 1.  Open your favorite editor. 1.  Enter the program code given in the example. 1.  Save it as a file with the filename mentioned in the comment. I follow the convention of having all Python programs saved with the extension .py. 1.  Run the interpreter with the command python program.py or use IDLE to run the programs. You can also use the executable method as explained earlier. Example: Using Variables And Literal Constants # Filename : var.py i = 5 print(i) i = i + 1 print(i) s = '''This is a multi-line string. This is the second line.''' print(s) Output: $ python var.py 5 6 This is a multi-line string. This is the second line. How It Works: Here's how this program works. First, we assign the literal constant value 5 to the variable i using the assignment operator (=). This line is called a statement because it states that Python en:Basics 31 something should be done and in this case, we connect the variable name i to the value 5. Next, we print the value of i using the print statement which, unsurprisingly, just prints the value of the variable to the screen. Then we add 1 to the value stored in i and store it back. We then print it and expectedly, we get the value 6. Similarly, we assign the literal string to the variable s and then print it. Note for static language programmers Variables are used by just assigning them a value. No declaration or data type definition is needed/used. Logical And Physical Lines A physical line is what you see when you write the program. A logical line is what Python sees as a single statement. Python implicitly assumes that each physical line corresponds to a logical line. An example of a logical line is a statement like print('Hello World') - if this was on a line by itself (as you see it in an editor), then this also corresponds to a physical line. Implicitly, Python encourages the use of a single statement per line which makes code more readable. If you want to specify more than one logical line on a single physical line, then you have to explicitly specify this using a semicolon (;) which indicates the end of a logical line/statement. For example, i = 5 print(i) is effectively same as i = 5; print(i); and the same can be written as i = 5; print(i); or even i = 5; print(i) However, I strongly recommend that you stick to writing a single logical line in a single physical line only. Use more than one physical line for a single logical line only if the logical line is really long. The idea is to avoid the semicolon as much as possible since it leads to more readable code. In fact, I have never used or even seen a semicolon in a Python program. An example of writing a logical line spanning many physical lines follows. This is referred to as explicit line joining. s = 'This is a string. \ This continues the string.' print(s) Python en:Basics 32 This gives the output: This is a string. This continues the string. Similarly, print\ (i) is the same as print(i) Sometimes, there is an implicit assumption where you don't need to use a backslash. This is the case where the logical line uses parentheses, square brackets or curly braces. This is is called implicit line joining. You can see this in action when we write programs using lists in later chapters. Indentation Whitespace is important in Python. Actually, whitespace at the beginning of the line is important. This is called indentation. Leading whitespace (spaces and tabs) at the beginning of the logical line is used to determine the indentation level of the logical line, which in turn is used to determine the grouping of statements. This means that statements which go together must have the same indentation. Each such set of statements is called a block. We will see examples of how blocks are important in later chapters. One thing you should remember is that wrong indentation can give rise to errors. For example: i = 5 print('Value is ', i) # Error! Notice a single space at the start of the line print('I repeat, the value is ', i) When you run this, you get the following error: File "whitespace.py", line 4 print('Value is ', i) # Error! Notice a single space at the start of the line ^ IndentationError: unexpected indent Notice that there is a single space at the beginning of the second line. The error indicated by Python tells us that the syntax of the program is invalid i.e. the program was not properly written. What this means to you is that you cannot arbitrarily start new blocks of statements (except for the default main block which you have been using all along, of course). Cases where you can use new blocks will be detailed in later chapters such as the control flow chapter. How to indent Do not use a mixture of tabs and spaces for the indentation as it does not work across different platforms properly. I strongly recommend that you use a single tab or four Python en:Basics 33 spaces for each indentation level. Choose either of these two indentation styles. More importantly, choose one and use it consistently i.e. use that indentation style only. Note to static language programmers Python will always use indentation for blocks and will never use braces. Run from __future__ import braces to learn more. Summary Now that we have gone through many nitty-gritty details, we can move on to more interesting stuff such as control flow statements. Be sure to become comfortable with what you have read in this chapter. Previous Next Source: http:/ / www. swaroopch. com/ mediawiki/ index. php? oldid=2376 Contributors: Swaroop, Vages, 10 anonymous edits Python en:Operators and Expressions Introduction Most statements (logical lines) that you write will contain expressions. A simple example of an expression is 2 + 3. An expression can be broken down into operators and operands. Operators are functionality that do something and can be represented by symbols such as + or by special keywords. Operators require some data to operate on and such data is called operands. In this case, 2 and 3 are the operands. Operators We will briefly take a look at the operators and their usage: Note that you can evaluate the expressions given in the examples using the interpreter interactively. For example, to test the expression 2 + 3, use the interactive Python interpreter prompt: >>> 2 + 3 5 >>> 3 * 5 15 >>> Operator Name Explanation Examples + Plus Adds the two objects 3 + 5 gives 8. 'a' + 'b' gives 'ab'. - Minus Either gives a negative number or gives the subtraction of one number from the other -5.2 gives a negative number. 50 - 24 gives 26. Python en:Operators and Expressions 34 * Multiply Gives the multiplication of the two numbers or returns the string repeated that many times. 2 * 3 gives 6. 'la' * 3 gives 'lalala'. ** Power Returns x to the power of y 3 ** 4 gives 81 (i.e. 3 * 3 * 3 * 3) / Divide Divide x by y 4 / 3 gives 1.3333333333333333. // Floor Division Returns the floor of the quotient 4 // 3 gives 1. % Modulo Returns the remainder of the division 8 % 3 gives 2. -25.5 % 2.25 gives 1.5. << Left Shift Shifts the bits of the number to the left by the number of bits specified. (Each number is represented in memory by bits or binary digits i.e. 0 and 1) 2 << 2 gives 8. 2 is represented by 10 in bits. Left shifting by 2 bits gives 1000 which represents the decimal 8. >> Right Shift Shifts the bits of the number to the right by the number of bits specified. 11 >> 1 gives 5. 11 is represented in bits by 1011 which when right shifted by 1 bit gives 101 which is the decimal 5. & Bitwise AND Bitwise AND of the numbers 5 & 3 gives 1. | Bit-wise OR Bitwise OR of the numbers 5 | 3 gives 7 ^ Bit-wise XOR Bitwise XOR of the numbers 5 ^ 3 gives 6 ~ Bit-wise invert The bit-wise inversion of x is -(x+1) ~5 gives -6. < Less Than Returns whether x is less than y. All comparison operators return True or False. Note the capitalization of these names. 5 < 3 gives False and 3 < 5 gives True. Comparisons can be chained arbitrarily: 3 < 5 < 7 gives True. > Greater Than Returns whether x is greater than y 5 > 3 returns True. If both operands are numbers, they are first converted to a common type. Otherwise, it always returns False. <= Less Than or Equal To Returns whether x is less than or equal to y x = 3; y = 6; x <= y returns True. >= Greater Than or Equal To Returns whether x is greater than or equal to y x = 4; y = 3; x >= 3 returns True. == Equal To Compares if the objects are equal x = 2; y = 2; x == y returns True. x = 'str'; y = 'stR'; x == y returns False. x = 'str'; y = 'str'; x == y returns True. != Not Equal To Compares if the objects are not equal x = 2; y = 3; x != y returns True. not Boolean NOT If x is True, it returns False. If x is False, it returns True. x = True; not x returns False. and Boolean AND x and y returns False if x is False, else it returns evaluation of y x = False; y = True; x and y returns False since x is False. In this case, Python will not evaluate y since it knows that the left hand side of the 'and' expression is False which implies that the whole expression will be False irrespective of the other values. This is called short-circuit evaluation. Python en:Operators and Expressions 35 or Boolean OR If x is True, it returns True, else it returns evaluation of y x = True; y = False; x or y returns True. Short-circuit evaluation applies here as well. Shortcut for math operation and assignment It is common to run a math operation on a variable and then assign the result of the operation back to the variable, hence there is a shortcut for such expressions: You can write: a = 2; a = a * 3 as: a = 2; a *= 3 Notice that var = var operation expression becomes var operation= expression. Evaluation Order If you had an expression such as 2 + 3 * 4, is the addition done first or the multiplication? Our high school maths tells us that the multiplication should be done first. This means that the multiplication operator has higher precedence than the addition operator. The following table gives the precedence table for Python, from the lowest precedence (least binding) to the highest precedence (most binding). This means that in a given expression, Python will first evaluate the operators and expressions lower in the table before the ones listed higher in the table. The following table, taken from the Python reference manual (http:/ / docs. python. org/ dev/ 3. 0/ reference/ expressions. html#evaluation-order), is provided for the sake of completeness. It is far better to use parentheses to group operators and operands appropriately in order to explicitly specify the precedence. This makes the program more readable. See Changing the Order of Evaluation below for details. Operator Description lambda Lambda Expression or Boolean OR and Boolean AND not x Boolean NOT in, not in Membership tests is, is not Identity tests <, <=, >, >=, !=, == Comparisons | Bitwise OR ^ Bitwise XOR & Bitwise AND <<, >> Shifts +, - Addition and subtraction *, /, //, % Multiplication, Division, Floor Division and Remainder +x, -x Positive, Negative Python en:Operators and Expressions 36 ~x Bitwise NOT ** Exponentiation x.attribute Attribute reference x[index] Subscription x[index1:index2] Slicing f(arguments ...) Function call (expressions, ...) Binding or tuple display [expressions, ...] List display {key:datum, ...} Dictionary display The operators which we have not already come across will be explained in later chapters. Operators with the same precedence are listed in the same row in the above table. For example, + and - have the same precedence. Changing the Order Of Evaluation To make the expressions more readable, we can use parentheses. For example, 2 + (3 * 4) is definitely easier to understand than 2 + 3 * 4 which requires knowledge of the operator precedences. As with everything else, the parentheses should be used reasonably (do not overdo it) and should not be redundant (as in 2 + (3 + 4)). There is an additional advantage to using parentheses - it helps us to change the order of evaluation. For example, if you want addition to be evaluated before multiplication in an expression, then you can write something like (2 + 3) * 4. Associativity Operators are usually associated from left to right i.e. operators with same precedence are evaluated in a left to right manner. For example, 2 + 3 + 4 is evaluated as (2 + 3) + 4. Some operators like assignment operators have right to left associativity i.e. a = b = c is treated as a = (b = c). Expressions Example: #!/usr/bin/python # Filename: expression.py length = 5 breadth = 2 area = length * breadth print('Area is', area) print('Perimeter is', 2 * (length + breadth)) Output: Python en:Operators and Expressions 37 $ python expression.py Area is 10 Perimeter is 14 How It Works: The length and breadth of the rectangle are stored in variables by the same name. We use these to calculate the area and perimeter of the rectangle with the help of expressions. We store the result of the expression length * breadth in the variable area and then print it using the print function. In the second case, we directly use the value of the expression 2 * (length + breadth) in the print function. Also, notice how Python 'pretty-prints' the output. Even though we have not specified a space between 'Area is' and the variable area, Python puts it for us so that we get a clean nice output and the program is much more readable this way (since we don't need to worry about spacing in the strings we use for output). This is an example of how Python makes life easy for the programmer. Summary We have seen how to use operators, operands and expressions - these are the basic building blocks of any program. Next, we will see how to make use of these in our programs using statements. Previous Next Source: http:/ / www. swaroopch. com/ mediawiki/ index. php? oldid=1579 Contributors: Swaroop, 4 anonymous edits Python en:Control Flow 38 Python en:Control Flow Introduction In the programs we have seen till now, there has always been a series of statements and Python faithfully executes them in the same order. What if you wanted to change the flow of how it works? For example, you want the program to take some decisions and do different things depending on different situations such as printing 'Good Morning' or 'Good Evening' depending on the time of the day? As you might have guessed, this is achieved using control flow statements. There are three control flow statements in Python - if, for and while. The if statement The if statement is used to check a condition and if the condition is true, we run a block of statements (called the if-block), else we process another block of statements (called the else-block). The else clause is optional. Example: #!/usr/bin/python # Filename: if.py number = 23 guess = int(input('Enter an integer : ')) if guess == number: print('Congratulations, you guessed it.') # New block starts here print('(but you do not win any prizes!)') # New block ends here elif guess < number: print('No, it is a little higher than that') # Another block # You can do whatever you want in a block ... else: print('No, it is a little lower than that') # you must have guess > number to reach here print('Done') # This last statement is always executed, after the if statement is executed Output: $ python if.py Enter an integer : 50 No, it is a little lower than that Done $ python if.py Enter an integer : 22 Python en:Control Flow 39 No, it is a little higher than that Done $ python if.py Enter an integer : 23 Congratulations, you guessed it. (but you do not win any prizes!) Done How It Works: In this program, we take guesses from the user and check if it is the number that we have. We set the variable number to any integer we want, say 23. Then, we take the user's guess using the input() function. Functions are just reusable pieces of programs. We'll read more about them in the next chapter. We supply a string to the built-in input function which prints it to the screen and waits for input from the user. Once we enter something and press enter key, the input() function returns what we entered, as a string. We then convert this string to an integer using int and then store it in the variable guess. Actually, the int is a class but all you need to know right now is that you can use it to convert a string to an integer (assuming the string contains a valid integer in the text). Next, we compare the guess of the user with the number we have chosen. If they are equal, we print a success message. Notice that we use indentation levels to tell Python which statements belong to which block. This is why indentation is so important in Python. I hope you are sticking to the "consistent indentation" rule. Are you? Notice how the if statement contains a colon at the end - we are indicating to Python that a block of statements follows. Then, we check if the guess is less than the number, and if so, we inform the user to guess a little higher than that. What we have used here is the elif clause which actually combines two related if else-if else statements into one combined if-elif-else statement. This makes the program easier and reduces the amount of indentation required. The elif and else statements must also have a colon at the end of the logical line followed by their corresponding block of statements (with proper indentation, of course) You can have another if statement inside the if-block of an if statement and so on - this is called a nested if statement. Remember that the elif and else parts are optional. A minimal valid if statement is: if True: print('Yes, it is true') After Python has finished executing the complete if statement along with the associated elif and else clauses, it moves on to the next statement in the block containing the if statement. In this case, it is the main block where execution of the program starts and the next statement is the print('Done') statement. After this, Python sees the ends of the program and simply finishes up. Although this is a very simple program, I have been pointing out a lot of things that you should notice even in this simple program. All these are pretty straightforward (and surprisingly simple for those of you from C/C++ backgrounds) and requires you to become Python en:Control Flow 40 aware of all these initially, but after that, you will become comfortable with it and it'll feel 'natural' to you. Note for C/C++ Programmers There is no switch statement in Python. You can use an if..elif..else statement to do the same thing (and in some cases, use a dictionary to do it quickly) The while Statement The while statement allows you to repeatedly execute a block of statements as long as a condition is true. A while statement is an example of what is called a looping statement. A while statement can have an optional else clause. Example: #!/usr/bin/python # Filename: while.py number = 23 running = True while running: guess = int(input('Enter an integer : ')) if guess == number: print('Congratulations, you guessed it.') running = False # this causes the while loop to stop elif guess < number: print('No, it is a little higher than that.') else: print('No, it is a little lower than that.') else: print('The while loop is over.') # Do anything else you want to do here print('Done') Output: $ python while.py Enter an integer : 50 No, it is a little lower than that. Enter an integer : 22 No, it is a little higher than that. Enter an integer : 23 Congratulations, you guessed it. The while loop is over. Done How It Works: Python en:Control Flow 41 In this program, we are still playing the guessing game, but the advantage is that the user is allowed to keep guessing until he guesses correctly - there is no need to repeatedly run the program for each guess, as we have done in the previous section. This aptly demonstrates the use of the while statement. We move the input and if statements to inside the while loop and set the variable running to True before the while loop. First, we check if the variable running is True and then proceed to execute the corresponding while-block. After this block is executed, the condition is again checked which in this case is the running variable. If it is true, we execute the while-block again, else we continue to execute the optional else-block and then continue to the next statement. The else block is executed when the while loop condition becomes False - this may even be the first time that the condition is checked. If there is an else clause for a while loop, it is always executed unless you break out of the loop with a break statement. The True and False are called Boolean types and you can consider them to be equivalent to the value 1 and 0 respectively. Note for C/C++ Programmers Remember that you can have an else clause for the while loop. The for loop The for..in statement is another looping statement which iterates over a sequence of objects i.e. go through each item in a sequence. We will see more about sequences in detail in later chapters. What you need to know right now is that a sequence is just an ordered collection of items. Example: #!/usr/bin/python # Filename: for.py for i in range(1, 5): print(i) else: print('The for loop is over') Output: $ python for.py 1 2 3 4 The for loop is over How It Works: In this program, we are printing a sequence of numbers. We generate this sequence of numbers using the built-in range function. What we do here is supply it two numbers and range returns a sequence of numbers starting from the first number and up to the second number. For example, range(1,5) Python en:Control Flow 42 gives the sequence [1, 2, 3, 4]. By default, range takes a step count of 1. If we supply a third number to range, then that becomes the step count. For example, range(1,5,2) gives [1,3]. Remember that the range extends up to the second number i.e. it does not include the second number. The for loop then iterates over this range - for i in range(1,5) is equivalent to for i in [1, 2, 3, 4] which is like assigning each number (or object) in the sequence to i, one at a time, and then executing the block of statements for each value of i. In this case, we just print the value in the block of statements. Remember that the else part is optional. When included, it is always executed once after the for loop is over unless a break statement is encountered. Remember that the for..in loop works for any sequence. Here, we have a list of numbers generated by the built-in range function, but in general we can use any kind of sequence of any kind of objects! We will explore this idea in detail in later chapters. Note for C/C++/Java/C# Programmers The Python for loop is radically different from the C/C++ for loop. C# programmers will note that the for loop in Python is similar to the foreach loop in C#. Java programmers will note that the same is similar to for (int i : IntArray) in Java 1.5 . In C/C++, if you want to write for (int i = 0; i < 5; i++), then in Python you write just for i in range(0,5). As you can see, the for loop is simpler, more expressive and less error prone in Python. The break Statement The break statement is used to break out of a loop statement i.e. stop the execution of a looping statement, even if the loop condition has not become False or the sequence of items has been completely iterated over. An important note is that if you break out of a for or while loop, any corresponding loop else block is not executed. Example: #!/usr/bin/python # Filename: break.py while True: s = (input('Enter something : ')) if s == 'quit': break print('Length of the string is', len(s)) print('Done') Output: $ python break.py Enter something : Programming is fun Length of the string is 18 Enter something : When the work is done Length of the string is 21 Python en:Control Flow 43 Enter something : if you wanna make your work also fun: Length of the string is 37 Enter something : use Python! Length of the string is 12 Enter something : quit Done How It Works: In this program, we repeatedly take the user's input and print the length of each input each time. We are providing a special condition to stop the program by checking if the user input is 'quit'. We stop the program by breaking out of the loop and reach the end of the program. The length of the input string can be found out using the built-in len function. Remember that the break statement can be used with the for loop as well. Swaroop's Poetic Python The input I have used here is a mini poem I have written called Swaroop's Poetic Python: Programming is fun When the work is done if you wanna make your work also fun: use Python! The continue Statement The continue statement is used to tell Python to skip the rest of the statements in the current loop block and to continue to the next iteration of the loop. Example: #!/usr/bin/python # Filename: continue.py while True: s = input('Enter something : ') if s == 'quit': break if len(s) < 3: print('Too small') continue print('Input is of sufficient length') # Do other kinds of processing here... Output: $ python test.py Enter something : a Too small Enter something : 12 Too small Python en:Control Flow 44 Enter something : abc Input is of sufficient length Enter something : quit How It Works: In this program, we accept input from the user, but we process them only if they are at least 3 characters long. So, we use the built-in len function to get the length and if the length is less than 3, we skip the rest of the statements in the block by using the continue statement. Otherwise, the rest of the statements in the loop are executed and we can do any kind of processing we want to do here. Note that the continue statement works with the for loop as well. Summary We have seen how to use the three control flow statements - if, while and for along with their associated break and continue statements. These are some of the most often used parts of Python and hence, becoming comfortable with them is essential. Next, we will see how to create and use functions. Previous Next Source: http:/ / www. swaroopch. com/ mediawiki/ index. php? oldid=1664 Contributors: Swaroop, 8 anonymous edits Python en:Functions Introduction Functions are reusable pieces of programs. They allow you to give a name to a block of statements and you can run that block using that name anywhere in your program and any number of times. This is known as calling the function. We have already used many built-in functions such as the len and range. The function concept is probably the most important building block of any non-trivial software (in any programming language), so we will explore various aspects of functions in this chapter. Functions are defined using the def keyword. This is followed by an identifier name for the function followed by a pair of parentheses which may enclose some names of variables and the line ends with a colon. Next follows the block of statements that are part of this function. An example will show that this is actually very simple: Example: #!/usr/bin/python # Filename: function1.py def sayHello(): print('Hello World!') # block belonging to the function # End of function Python en:Functions 45 sayHello() # call the function sayHello() # call the function again Output: $ python function1.py Hello World! Hello World! How It Works: We define a function called sayHello using the syntax as explained above. This function takes no parameters and hence there are no variables declared in the parentheses. Parameters to functions are just input to the function so that we can pass in different values to it and get back corresponding results. Notice that we can call the same function twice which means we do not have to write the same code again. Function Parameters A function can take parameters, which are values you supply to the function so that the function can do something utilising those values. These parameters are just like variables except that the values of these variables are defined when we call the function and are already assigned values when the function runs. Parameters are specified within the pair of parentheses in the function definition, separated by commas. When we call the function, we supply the values in the same way. Note the terminology used - the names given in the function definition are called parameters whereas the values you supply in the function call are called arguments. Example: #!/usr/bin/python # Filename: func_param.py def printMax(a, b): if a > b: print(a, 'is maximum') elif a == b: print(a, 'is equal to', b) else: print(b, 'is maximum') printMax(3, 4) # directly give literal values x = 5 y = 7 printMax(x, y) # give variables as arguments Output: Python en:Functions 46 $ python func_param.py 4 is maximum 7 is maximum How It Works: Here, we define a function called printMax where we take two parameters called a and b. We find out the greater number using a simple if..else statement and then print the bigger number. In the first usage of printMax, we directly supply the numbers i.e. arguments. In the second usage, we call the function using variables. printMax(x, y) causes value of argument x to be assigned to parameter a and the value of argument y assigned to parameter b. The printMax function works the same in both the cases. Local Variables When you declare variables inside a function definition, they are not related in any way to other variables with the same names used outside the function i.e. variable names are local to the function. This is called the scope of the variable. All variables have the scope of the block they are declared in starting from the point of definition of the name. Example: #!/usr/bin/python # Filename: func_local.py x = 50 def func(x): print('x is', x) x = 2 print('Changed local x to', x) func(x) print('x is still', x) Output: $ python func_local.py x is 50 Changed local x to 2 x is still 50 How It Works: In the function, the first time that we use the value of the name x, Python uses the value of the parameter declared in the function. Next, we assign the value 2 to x. The name x is local to our function. So, when we change the value of x in the function, the x defined in the main block remains unaffected. In the last print function call, we display the value of x in the main block and confirm that it is actually unaffected. Python en:Functions 47 Using The global Statement If you want to assign a value to a name defined at the top level of the program (i.e. not inside any kind of scope such as functions or classes), then you have to tell Python that the name is not local, but it is global. We do this using the global statement. It is impossible to assign a value to a variable defined outside a function without the global statement. You can use the values of such variables defined outside the function (assuming there is no variable with the same name within the function). However, this is not encouraged and should be avoided since it becomes unclear to the reader of the program as to where that variable's definition is. Using the global statement makes it amply clear that the variable is defined in an outermost block. Example: #!/usr/bin/python # Filename: func_global.py x = 50 def func(): global x print('x is', x) x = 2 print('Changed global x to', x) func() print('Value of x is', x) Output: $ python func_global.py x is 50 Changed global x to 2 Value of x is 2 How It Works: The global statement is used to declare that x is a global variable - hence, when we assign a value to x inside the function, that change is reflected when we use the value of x in the main block. You can specify more than one global variable using the same global statement. For example, global x, y, z. Python en:Functions 48 Using nonlocal statement We have seen how to access variables in the local and global scope above. There is another kind of scope called "nonlocal" scope which is in-between these two types of scopes. Nonlocal scopes are observed when you define functions inside functions. Since everything in Python is just executable code, you can define functions anywhere. Let's take an example: #!/usr/bin/python # Filename: func_nonlocal.py def func_outer(): x = 2 print('x is', x) def func_inner(): nonlocal x x = 5 func_inner() print('Changed local x to', x) func_outer() Output: $ python func_nonlocal.py x is 2 Changed local x to 5 How It Works: When we are inside func_inner, the 'x' defined in the first line of func_outer is relatively neither in local scope nor in global scope. We declare that we are using this x by nonlocal x and hence we get access to that variable. Try changing the nonlocal x to global x and also by removing the statement itself and observe the difference in behavior in these two cases. Default Argument Values For some functions, you may want to make some of its parameters as optional and use default values if the user does not want to provide values for such parameters. This is done with the help of default argument values. You can specify default argument values for parameters by following the parameter name in the function definition with the assignment operator (=) followed by the default value. Note that the default argument value should be a constant. More precisely, the default argument value should be immutable - this is explained in detail in later chapters. For now, just remember this. Example: Python en:Functions 49 #!/usr/bin/python # Filename: func_default.py def say(message, times = 1): print(message * times) say('Hello') say('World', 5) Output: $ python func_default.py Hello WorldWorldWorldWorldWorld How It Works: The function named say is used to print a string as many times as specified. If we don't supply a value, then by default, the string is printed just once. We achieve this by specifying a default argument value of 1 to the parameter times. In the first usage of say, we supply only the string and it prints the string once. In the second usage of say, we supply both the string and an argument 5 stating that we want to say the string message 5 times. Important Only those parameters which are at the end of the parameter list can be given default argument values i.e. you cannot have a parameter with a default argument value before a parameter without a default argument value in the order of parameters declared in the function parameter list. This is because the values are assigned to the parameters by position. For example, def func(a, b=5) is valid, but def func(a=5, b) is not valid. Keyword Arguments If you have some functions with many parameters and you want to specify only some of them, then you can give values for such parameters by naming them - this is called keyword arguments - we use the name (keyword) instead of the position (which we have been using all along) to specify the arguments to the function. There are two advantages - one, using the function is easier since we do not need to worry about the order of the arguments. Two, we can give values to only those parameters which we want, provided that the other parameters have default argument values. Example: #!/usr/bin/python # Filename: func_key.py def func(a, b=5, c=10): print('a is', a, 'and b is', b, 'and c is', c) func(3, 7) Python en:Functions 50 func(25, c=24) func(c=50, a=100) Output: $ python func_key.py a is 3 and b is 7 and c is 10 a is 25 and b is 5 and c is 24 a is 100 and b is 5 and c is 50 How It Works: The function named func has one parameter without default argument values, followed by two parameters with default argument values. In the first usage, func(3, 7), the parameter a gets the value 3, the parameter b gets the value 7 and c gets the default value of 10. In the second usage func(25, c=24), the variable a gets the value of 25 due to the position of the argument. Then, the parameter c gets the value of 24 due to naming i.e. keyword arguments. The variable b gets the default value of 5. In the third usage func(c=50, a=100), we use keyword arguments completely to specify the values. Notice, that we are specifying value for parameter c before that for a even though a is defined before c in the function definition. VarArgs parameters TODO Should I write about this in a later chapter since we haven't talked about lists and dictionaries yet? Sometimes you might want to define a function that can take any number of parameters, this can be achieved by using the stars: #!/usr/bin/python # Filename: total.py def total(initial=5, *numbers, **keywords): count = initial for number in numbers: count += number for key in keywords: count += keywords[key] return count print(total(10, 1, 2, 3, vegetables=50, fruits=100)) Output: $ python total.py 166 How It Works: Python en:Functions 51 When we declare a starred parameter such as *param, then all the positional arguments from that point till the end are collected as a list called 'param'. Similarly, when we declare a double-starred parameter such as **param, then all the keyword arguments from that point till the end are collected as a dictionary called 'param'. We will explore lists and dictionaries in a later chapter. Keyword- only Parameters If we want to specify certain keyword parameters to be available as keyword-only and not as positional arguments, they can be declared after a starred parameter: #!/usr/bin/python # Filename: keyword_only.py def total(initial=5, *numbers, vegetables): count = initial for number in numbers: count += number count += vegetables return count print(total(10, 1, 2, 3, vegetables=50)) print(total(10, 1, 2, 3)) # Raises error because we have not supplied a default argument value for 'vegetables' Output: $ python keyword_only.py 66 Traceback (most recent call last): File "test.py", line 12, in print(total(10, 1, 2, 3)) TypeError: total() needs keyword-only argument vegetables How It Works: Declaring parameters after a starred parameter results in keyword-only arguments. If these arguments are not supplied a default value, then calls to the function will raise an error if the keyword argument is not supplied, as seen above. If you want to have keyword-only arguments but have no need for a starred parameter, then simply use an empty star without using any name such as def total(initial=5, *, vegetables). Python en:Functions 52 The return Statement The return statement is used to return from a function i.e. break out of the function. We can optionally return a value from the function as well. Example: #!/usr/bin/python # Filename: func_return.py def maximum(x, y): if x > y: return x else: return y print(maximum(2, 3)) Output: $ python func_return.py 3 How It Works: The maximum function returns the maximum of the parameters, in this case the numbers supplied to the function. It uses a simple if..else statement to find the greater value and then returns that value. Note that a return statement without a value is equivalent to return None. None is a special type in Python that represents nothingness. For example, it is used to indicate that a variable has no value if it has a value of None. Every function implicitly contains a return None statement at the end unless you have written your own return statement. You can see this by running print(someFunction()) where the function someFunction does not use the return statement such as: def someFunction(): pass The pass statement is used in Python to indicate an empty block of statements. Note There is a built-in function called max that already implements the 'find maximum' functionality, so use this built-in function whenever possible. Python en:Functions 53 DocStrings Python has a nifty feature called documentation strings, usually referred to by its shorter name docstrings. DocStrings are an important tool that you should make use of since it helps to document the program better and makes it easier to understand. Amazingly, we can even get the docstring back from, say a function, when the program is actually running! Example: #!/usr/bin/python # Filename: func_doc.py def printMax(x, y): '''Prints the maximum of two numbers. The two values must be integers.''' x = int(x) # convert to integers, if possible y = int(y) if x > y: print(x, 'is maximum') else: print(y, 'is maximum') printMax(3, 5) print(printMax.__doc__) Output: $ python func_doc.py 5 is maximum Prints the maximum of two numbers. The two values must be integers. How It Works: A string on the first logical line of a function is the docstring for that function. Note that DocStrings also apply to modules and classes which we will learn about in the respective chapters. The convention followed for a docstring is a multi-line string where the first line starts with a capital letter and ends with a dot. Then the second line is blank followed by any detailed explanation starting from the third line. You are strongly advised to follow this convention for all your docstrings for all your non-trivial functions. We can access the docstring of the printMax function using the __doc__ (notice the double underscores) attribute (name belonging to) of the function. Just remember that Python treats everything as an object and this includes functions. We'll learn more about objects in the chapter on classes. If you have used help() in Python, then you have already seen the usage of docstrings! What it does is just fetch the __doc__ attribute of that function and displays it in a neat manner for you. You can try it out on the function above - just include help(printMax) in Python en:Functions 54 your program. Remember to press the q key to exit help. Automated tools can retrieve the documentation from your program in this manner. Therefore, I strongly recommend that you use docstrings for any non-trivial function that you write. The pydoc command that comes with your Python distribution works similarly to help() using docstrings. Annotations Functions have another advanced feature called annotations which are a nifty way of attaching additional information for each of the parameters as well as the return value. Since the Python language itself does not interpret these annotations in any way (that functionality is left to third-party libraries to interpret in any way they want), we will skip this feature in our discussion. If you are interested to read about annotations, please see the Python Enhancement Proposal No. 3107 (http:/ / www. python. org/ dev/ peps/ pep-3107/ ). Summary We have seen so many aspects of functions but note that we still haven't covered all aspects of it. However, we have already covered most of what you'll use regarding Python functions on an everyday basis. Next, we will see how to use as well as create Python modules. Previous Next Source: http:/ / www. swaroopch. com/ mediawiki/ index. php? oldid=2379 Contributors: Swaroop, Vages, 7 anonymous edits Python en:Modules 55 Python en:Modules Introduction You have seen how you can reuse code in your program by defining functions once. What if you wanted to reuse a number of functions in other programs that you write? As you might have guessed, the answer is modules. There are various methods of writing modules, but the simplest way is to create a file with a .py extension that contains functions and variables. Another method is to write the modules in the native language in which the Python interpreter itself was written. For example, you can write modules in the C programming language (http:/ / docs. python. org/ extending/ ) and when compiled, they can be used from your Python code when using the standard Python interpreter. A module can be imported by another program to make use of its functionality. This is how we can use the Python standard library as well. First, we will see how to use the standard library modules. Example: #!/usr/bin/python # Filename: using_sys.py import sys print('The command line arguments are:') for i in sys.argv: print(i) print('\n\nThe PYTHONPATH is', sys.path, '\n') Output: $ python using_sys.py we are arguments The command line arguments are: using_sys.py we are arguments The PYTHONPATH is ['', 'C:\\Windows\\system32\\python30.zip', 'C:\\Python30\\DLLs', 'C:\\Python30\\lib', 'C:\\Python30\\lib\\plat-win', 'C:\\Python30', 'C:\\Python30\\lib\\site-packages'] How It Works: First, we import the sys module using the import statement. Basically, this translates to us telling Python that we want to use this module. The sys module contains functionality related to the Python interpreter and its environment i.e. the system. Python en:Modules 56 When Python executes the import sys statement, it looks for the sys module. In this case, it is one of the built-in modules, and hence Python knows where to find it. If it was not a compiled module i.e. a module written in Python, then the Python interpreter will search for it in the directories listed in its sys.path variable. If the module is found, then the statements in the body of that module is run and then the module is made available for you to use. Note that the initialization is done only the first time that we import a module. The argv variable in the sys module is accessed using the dotted notation i.e. sys.argv. It clearly indicates that this name is part of the sys module. Another advantage of this approach is that the name does not clash with any argv variable used in your program. The sys.argv variable is a list of strings (lists are explained in detail in a later chapter. Specifically, the sys.argv contains the list of command line arguments i.e. the arguments passed to your program using the command line. If you are using an IDE to write and run these programs, look for a way to specify command line arguments to the program in the menus. Here, when we execute python using_sys.py we are arguments, we run the module using_sys.py with the python command and the other things that follow are arguments passed to the program. Python stores the command line arguments in the sys.argv variable for us to use. Remember, the name of the script running is always the first argument in the sys.argv list. So, in this case we will have 'using_sys.py' as sys.argv[0], 'we' as sys.argv[1], 'are' as sys.argv[2] and 'arguments' as sys.argv[3]. Notice that Python starts counting from 0 and not 1. The sys.path contains the list of directory names where modules are imported from. Observe that the first string in sys.path is empty - this empty string indicates that the current directory is also part of the sys.path which is same as the PYTHONPATH environment variable. This means that you can directly import modules located in the current directory. Otherwise, you will have to place your module in one of the directories listed in sys.path. Note that the current directory is the directory from which the program is launched. Run import os; print(os.getcwd()) to find out the current directory of your program. Byte- compiled . pyc files Importing a module is a relatively costly affair, so Python does some tricks to make it faster. One way is to create byte-compiled files with the extension .pyc which is an intermediate form that Python transforms the program into (remember the introduction section on how Python works?). This .pyc file is useful when you import the module the next time from a different program - it will be much faster since a portion of the processing required in importing a module is already done. Also, these byte-compiled files are platform-independent. Note These .pyc files are usually created in the same directory as the corresponding .py files. If Python does not have permission to write to files in that directory, then the .pyc files will not be created. Python en:Modules 57 The from . . . import . . . statement If you want to directly import the argv variable into your program (to avoid typing the sys. everytime for it), then you can use the from sys import argv statement. If you want to import all the names used in the sys module, then you can use the from sys import * statement. This works for any module. In general, you should avoid using this statement and use the import statement instead since your program will avoid name clashes and will be more readable. A module's _ _ name_ _ Every module has a name and statements in a module can find out the name of its module. This is handy in the particular situation of figuring out if the module is being run standalone or being imported. As mentioned previously, when a module is imported for the first time, the code in that module is executed. We can use this concept to alter the behavior of the module if the program was used by itself and not when it was imported from another module. This can be achieved using the __name__ attribute of the module. Example: #!/usr/bin/python # Filename: using_name.py if __name__ == '__main__': print('This program is being run by itself') else: print('I am being imported from another module') Output: $ python using_name.py This program is being run by itself $ python >>> import using_name I am being imported from another module >>> How It Works: Every Python module has it's __name__ defined and if this is '__main__', it implies that the module is being run standalone by the user and we can take appropriate actions. Python en:Modules 58 Making Your Own Modules Creating your own modules is easy, you've been doing it all along! This is because every Python program is also a module. You just have to make sure it has a .py extension. The following example should make it clear. Example: #!/usr/bin/python # Filename: mymodule.py def sayhi(): print('Hi, this is mymodule speaking.') __version__ = '0.1' # End of mymodule.py The above was a sample module. As you can see, there is nothing particularly special about compared to our usual Python program. We will next see how to use this module in our other Python programs. Remember that the module should be placed in the same directory as the program that we import it in, or the module should be in one of the directories listed in sys.path. #!/usr/bin/python # Filename: mymodule_demo.py import mymodule mymodule.sayhi() print ('Version', mymodule.__version__) Output: $ python mymodule_demo.py Hi, this is mymodule speaking. Version 0.1 How It Works: Notice that we use the same dotted notation to access members of the module. Python makes good reuse of the same notation to give the distinctive 'Pythonic' feel to it so that we don't have to keep learning new ways to do things. Here is a version utilising the from..import syntax: #!/usr/bin/python # Filename: mymodule_demo2.py from mymodule import sayhi, __version__ sayhi() print('Version', __version__) Python en:Modules 59 The output of mymodule_demo2.py is same as the output of mymodule_demo.py. Notice that if there was already a __version__ name declared in the module that imports mymodule, there would be a clash. This is also likely because it is common practice for each module to declare it's version number using this name. Hence, it is always recommended to prefer the import statement even though it might make your program a little longer. You could also use: from mymodule import * This will import all public names such as sayhi but would not import __version__ because it starts with double underscores. Zen of Python One of Python's guiding principles is that "Explicit is better than Implicit". Run import this to learn more and see this discussion (http:/ / stackoverflow. com/ questions/ 228181/ zen-of-python) which lists examples for each of the principles. The dir function You can use the built-in dir function to list the identifiers that an object defines. For example, for a module, the identifiers include the functions, classes and variables defined in that module. When you supply a module name to the dir() function, it returns the list of the names defined in that module. When no argument is applied to it, it returns the list of names defined in the current module. Example: $ python >>> import sys # get list of attributes, in this case, for the sys module >>> dir(sys) ['__displayhook__', '__doc__', '__excepthook__', '__name__', '__package__', '__s tderr__', '__stdin__', '__stdout__', '_clear_type_cache', '_compact_freelists', '_current_frames', '_getframe', 'api_version', 'argv', 'builtin_module_names', ' byteorder', 'call_tracing', 'callstats', 'copyright', 'displayhook', 'dllhandle' , 'dont_write_bytecode', 'exc_info', 'excepthook', 'exec_prefix', 'executable', 'exit', 'flags', 'float_info', 'getcheckinterval', 'getdefaultencoding', 'getfil esystemencoding', 'getprofile', 'getrecursionlimit', 'getrefcount', 'getsizeof', 'gettrace', 'getwindowsversion', 'hexversion', 'intern', 'maxsize', 'maxunicode ', 'meta_path', 'modules', 'path', 'path_hooks', 'path_importer_cache', Python en:Modules 60 'platfor m', 'prefix', 'ps1', 'ps2', 'setcheckinterval', 'setprofile', 'setrecursionlimit ', 'settrace', 'stderr', 'stdin', 'stdout', 'subversion', 'version', 'version_in fo', 'warnoptions', 'winver'] >>> dir() # get list of attributes for current module ['__builtins__', '__doc__', '__name__', '__package__', 'sys'] >>> a = 5 # create a new variable 'a' >>> dir() ['__builtins__', '__doc__', '__name__', '__package__', 'a', 'sys'] >>> del a # delete/remove a name >>> dir() ['__builtins__', '__doc__', '__name__', '__package__', 'sys'] >>> How It Works: First, we see the usage of dir on the imported sys module. We can see the huge list of attributes that it contains. Next, we use the dir function without passing parameters to it. By default, it returns the list of attributes for the current module. Notice that the list of imported modules is also part of this list. In order to observe the dir in action, we define a new variable a and assign it a value and then check dir and we observe that there is an additional value in the list of the same name. We remove the variable/attribute of the current module using the del statement and the change is reflected again in the output of the dir function. A note on del - this statement is used to delete a variable/name and after the statement has run, in this case del a, you can no longer access the variable a - it is as if it never existed before at all. Note that the dir() function works on any object. For example, run dir(print) to learn about the attributes of the print function, or dir(str) for the attributes of the str class. Python en:Modules 61 Packages By now, you must have started observing the hierarchy of organizing your programs. Variables usually go inside functions. Functions and global variables usually go inside modules. What if you wanted to organize modules? That's where packages come into the picture. Packages are just folders of modules with a special __init__.py file that indicates to Python that this folder is special because it contains Python modules. Let's say you want to create a package called 'world' with subpackages 'asia', 'africa', etc. and these subpackages in turn contain modules like 'india', 'madagascar', etc. This is how you would structure the folders: - / - world/ - __init__.py - asia/ - __init__.py - india/ - __init__.py - foo.py - africa/ - __init__.py - madagascar/ - __init__.py - bar.py Packages are just a convenience to hierarchically organize modules. You will see many instances of this in the standard library. Summary Just like functions are reusable parts of programs, modules are reusable programs. Packages are another hierarchy to organize modules. The standard library that comes with Python is an example of such a set of packages and modules. We have seen how to use these modules and create our own modules. Next, we will learn about some interesting concepts called data structures. Previous Next Source: http:/ / www. swaroopch. com/ mediawiki/ index. php? oldid=2371 Contributors: Swaroop, 7 anonymous edits Python en:Data Structures 62 Python en:Data Structures Introduction Data structures are basically just that - they are structures which can hold some data together. In other words, they are used to store a collection of related data. There are four built-in data structures in Python - list, tuple, dictionary and set. We will see how to use each of them and how they make life easier for us. List A list is a data structure that holds an ordered collection of items i.e. you can store a sequence of items in a list. This is easy to imagine if you can think of a shopping list where you have a list of items to buy, except that you probably have each item on a separate line in your shopping list whereas in Python you put commas in between them. The list of items should be enclosed in square brackets so that Python understands that you are specifying a list. Once you have created a list, you can add, remove or search for items in the list. Since we can add and remove items, we say that a list is a mutable data type i.e. this type can be altered. Quick Introduction To Objects And Classes Although I've been generally delaying the discussion of objects and classes till now, a little explanation is needed right now so that you can understand lists better. We will explore this topic in detail later in its own chapter. A list is an example of usage of objects and classes. When we use a variable i and assign a value to it, say integer 5 to it, you can think of it as creating an object (i.e. instance) i of class (i.e. type) int. In fact, you can read help(int) to understand this better. A class can also have methods i.e. functions defined for use with respect to that class only. You can use these pieces of functionality only when you have an object of that class. For example, Python provides an append method for the list class which allows you to add an item to the end of the list. For example, mylist.append('an item') will add that string to the list mylist. Note the use of dotted notation for accessing methods of the objects. A class can also have fields which are nothing but variables defined for use with respect to that class only. You can use these variables/names only when you have an object of that class. Fields are also accessed by the dotted notation, for example, mylist.field. Example: #!/usr/bin/python # Filename: using_list.py # This is my shopping list shoplist = ['apple', 'mango', 'carrot', 'banana'] print('I have', len(shoplist), 'items to purchase.') print('These items are:', end=' ') Python en:Data Structures 63 for item in shoplist: print(item, end=' ') print('\nI also have to buy rice.') shoplist.append('rice') print('My shopping list is now', shoplist) print('I will sort my list now') shoplist.sort() print('Sorted shopping list is', shoplist) print('The first item I will buy is', shoplist[0]) olditem = shoplist[0] del shoplist[0] print('I bought the', olditem) print('My shopping list is now', shoplist) Output: $ python using_list.py I have 4 items to purchase. These items are: apple mango carrot banana I also have to buy rice. My shopping list is now ['apple', 'mango', 'carrot', 'banana', 'rice'] I will sort my list now Sorted shopping list is ['apple', 'banana', 'carrot', 'mango', 'rice'] The first item I will buy is apple I bought the apple My shopping list is now ['banana', 'carrot', 'mango', 'rice'] How It Works: The variable shoplist is a shopping list for someone who is going to the market. In shoplist, we only store strings of the names of the items to buy but you can add any kind of object to a list including numbers and even other lists. We have also used the for..in loop to iterate through the items of the list. By now, you must have realised that a list is also a sequence. The speciality of sequences will be discussed in a later section. Notice the use of the end keyword argument to the print function to indicate that we want to end the output with a space instead of the usual line break. Next, we add an item to the list using the append method of the list object, as already discussed before. Then, we check that the item has been indeed added to the list by printing the contents of the list by simply passing the list to the print statement which prints it neatly. Then, we sort the list by using the sort method of the list. It is important to understand that this method affects the list itself and does not return a modified list - this is different from the way strings work. This is what we mean by saying that lists are mutable and that Python en:Data Structures 64 strings are immutable. Next, when we finish buying an item in the market, we want to remove it from the list. We achieve this by using the del statement. Here, we mention which item of the list we want to remove and the del statement removes it from the list for us. We specify that we want to remove the first item from the list and hence we use del shoplist[0] (remember that Python starts counting from 0). If you want to know all the methods defined by the list object, see help(list) for details. Tuple Tuples are used to hold together multiple objects. Think of them as similar to lists, but without the extensive functionality that the list class gives you. One major feature of tuples is that they are immutable like strings i.e. you cannot modify tuples. Tuples are defined by specifying items separated by commas within an optional pair of parentheses. Tuples are usually used in cases where a statement or a user-defined function can safely assume that the collection of values i.e. the tuple of values used will not change. Example: #!/usr/bin/python # Filename: using_tuple.py zoo = ('python', 'elephant', 'penguin') # remember the parentheses are optional print('Number of animals in the zoo is', len(zoo)) new_zoo = ('monkey', 'camel', zoo) print('Number of cages in the new zoo is', len(new_zoo)) print('All animals in new zoo are', new_zoo) print('Animals brought from old zoo are', new_zoo[2]) print('Last animal brought from old zoo is', new_zoo[2][2]) print('Number of animals in the new zoo is', len(new_zoo)-1+len(new_zoo[2])) Output: $ python using_tuple.py Number of animals in the zoo is 3 Number of cages in the new zoo is 3 All animals in new zoo are ('monkey', 'camel', ('python', 'elephant', 'penguin')) Animals brought from old zoo are ('python', 'elephant', 'penguin') Last animal brought from old zoo is penguin Number of animals in the new zoo is 5 How It Works: The variable zoo refers to a tuple of items. We see that the len function can be used to get the length of the tuple. This also indicates that a tuple is a sequence as well. Python en:Data Structures 65 We are now shifting these animals to a new zoo since the old zoo is being closed. Therefore, the new_zoo tuple contains some animals which are already there along with the animals brought over from the old zoo. Back to reality, note that a tuple within a tuple does not lose its identity. We can access the items in the tuple by specifying the item's position within a pair of square brackets just like we did for lists. This is called the indexing operator. We access the third item in new_zoo by specifying new_zoo[2] and we access the third item within the third item in the new_zoo tuple by specifying new_zoo[2][2]. This is pretty simple once you've understood the idiom. Parentheses Although the parentheses is optional, I prefer always having them to make it obvious that it is a tuple, especially because it avoids ambiguity. For example, print(1,2,3) and print( (1,2,3) ) mean two different things - the former prints three numbers whereas the latter prints a tuple (which contains three numbers). Tuple with 0 or 1 items An empty tuple is constructed by an empty pair of parentheses such as myempty = (). However, a tuple with a single item is not so simple. You have to specify it using a comma following the first (and only) item so that Python can differentiate between a tuple and a pair of parentheses surrounding the object in an expression i.e. you have to specify singleton = (2 , ) if you mean you want a tuple containing the item 2. Note for Perl programmers A list within a list does not lose its identity i.e. lists are not flattened as in Perl. The same applies to a tuple within a tuple, or a tuple within a list, or a list within a tuple, etc. As far as Python is concerned, they are just objects stored using another object, that's all. Dictionary A dictionary is like an address-book where you can find the address or contact details of a person by knowing only his/her name i.e. we associate keys (name) with values (details). Note that the key must be unique just like you cannot find out the correct information if you have two persons with the exact same name. Note that you can use only immutable objects (like strings) for the keys of a dictionary but you can use either immutable or mutable objects for the values of the dictionary. This basically translates to say that you should use only simple objects for keys. Pairs of keys and values are specified in a dictionary by using the notation d = {key1 : value1, key2 : value2 }. Notice that the key-value pairs are separated by a colon and the pairs are separated themselves by commas and all this is enclosed in a pair of curly braces. Remember that key-value pairs in a dictionary are not ordered in any manner. If you want a particular order, then you will have to sort them yourself before using it. The dictionaries that you will be using are instances/objects of the dict class. Example: #!/usr/bin/python # Filename: using_dict.py Python en:Data Structures 66 # 'ab' is short for 'a'ddress'b'ook ab = { 'Swaroop' : 'swaroop@swaroopch.com', 'Larry' : 'larry@wall.org', 'Matsumoto' : 'matz@ruby-lang.org', 'Spammer' : 'spammer@hotmail.com' } print("Swaroop's address is", ab['Swaroop']) # Deleting a key-value pair del ab['Spammer'] print('\nThere are {0} contacts in the address-book\n'.format(len(ab))) for name, address in ab.items(): print('Contact {0} at {1}'.format(name, address)) # Adding a key-value pair ab['Guido'] = 'guido@python.org' if 'Guido' in ab: # OR ab.has_key('Guido') print("\nGuido's address is", ab['Guido']) Output: $ python using_dict.py Swaroop's address is swaroop@swaroopch.com There are 3 contacts in the address-book Contact Swaroop at swaroop@swaroopch.com Contact Matsumoto at matz@ruby-lang.org Contact Larry at larry@wall.org Guido's address is guido@python.org How It Works: We create the dictionary ab using the notation already discussed. We then access key-value pairs by specifying the key using the indexing operator as discussed in the context of lists and tuples. Observe the simple syntax. We can delete key-value pairs using our old friend - the del statement. We simply specify the dictionary and the indexing operator for the key to be removed and pass it to the del statement. There is no need to know the value corresponding to the key for this operation. Next, we access each key-value pair of the dictionary using the items method of the dictionary which returns a list of tuples where each tuple contains a pair of items - the key followed by the value. We retrieve this pair and assign it to the variables name and address correspondingly for each pair using the for..in loop and then print these values in the Python en:Data Structures 67 for-block. We can add new key-value pairs by simply using the indexing operator to access a key and assign that value, as we have done for Guido in the above case. We can check if a key-value pair exists using the in operator or even the has_key method of the dict class. You can see the documentation for the complete list of methods of the dict class using help(dict). Keyword Arguments and Dictionaries On a different note, if you have used keyword arguments in your functions, you have already used dictionaries! Just think about it - the key-value pair is specified by you in the parameter list of the function definition and when you access variables within your function, it is just a key access of a dictionary (which is called the symbol table in compiler design terminology). Sequences Lists, tuples and strings are examples of sequences, but what are sequences and what is so special about them? The major features is that they have membership tests (i.e. the in and not in expressions) and indexing operations. The indexing operation which allows us to fetch a particular item in the sequence directly. The three types of sequences mentioned above - lists, tuples and strings, also have a slicing operation which allows us to retrieve a slice of the sequence i.e. a part of the sequence. Example: #!/usr/bin/python # Filename: seq.py shoplist = ['apple', 'mango', 'carrot', 'banana'] name = 'swaroop' # Indexing or 'Subscription' operation print('Item 0 is', shoplist[0]) print('Item 1 is', shoplist[1]) print('Item 2 is', shoplist[2]) print('Item 3 is', shoplist[3]) print('Item -1 is', shoplist[-1]) print('Item -2 is', shoplist[-2]) print('Character 0 is', name[0]) # Slicing on a list print('Item 1 to 3 is', shoplist[1:3]) print('Item 2 to end is', shoplist[2:]) print('Item 1 to -1 is', shoplist[1:-1]) print('Item start to end is', shoplist[:]) # Slicing on a string Python en:Data Structures 68 print('characters 1 to 3 is', name[1:3]) print('characters 2 to end is', name[2:]) print('characters 1 to -1 is', name[1:-1]) print('characters start to end is', name[:]) Output: $ python seq.py Item 0 is apple Item 1 is mango Item 2 is carrot Item 3 is banana Item -1 is banana Item -2 is carrot Character 0 is s Item 1 to 3 is ['mango', 'carrot'] Item 2 to end is ['carrot', 'banana'] Item 1 to -1 is ['mango', 'carrot'] Item start to end is ['apple', 'mango', 'carrot', 'banana'] characters 1 to 3 is wa characters 2 to end is aroop characters 1 to -1 is waroo characters start to end is swaroop How It Works: First, we see how to use indexes to get individual items of a sequence. This is also referred to as the subscription operation. Whenever you specify a number to a sequence within square brackets as shown above, Python will fetch you the item corresponding to that position in the sequence. Remember that Python starts counting numbers from 0. Hence, shoplist[0] fetches the first item and shoplist[3] fetches the fourth item in the shoplist sequence. The index can also be a negative number, in which case, the position is calculated from the end of the sequence. Therefore, shoplist[-1] refers to the last item in the sequence and shoplist[-2] fetches the second last item in the sequence. The slicing operation is used by specifying the name of the sequence followed by an optional pair of numbers separated by a colon within square brackets. Note that this is very similar to the indexing operation you have been using till now. Remember the numbers are optional but the colon isn't. The first number (before the colon) in the slicing operation refers to the position from where the slice starts and the second number (after the colon) indicates where the slice will stop at. If the first number is not specified, Python will start at the beginning of the sequence. If the second number is left out, Python will stop at the end of the sequence. Note that the slice returned starts at the start position and will end just before the end position i.e. the start position is included but the end position is excluded from the sequence slice. Thus, shoplist[1:3] returns a slice of the sequence starting at position 1, includes position 2 but stops at position 3 and therefore a slice of two items is returned. Similarly, shoplist[:] returns a copy of the whole sequence. Python en:Data Structures 69 You can also do slicing with negative positions. Negative numbers are used for positions from the end of the sequence. For example, shoplist[:-1] will return a slice of the sequence which excludes the last item of the sequence but contains everything else. You can also provide a third argument for the slice, which is the step for the slicing (by default, the step size is 1): >>> shoplist = ['apple', 'mango', 'carrot', 'banana'] >>> shoplist[::1] ['apple', 'mango', 'carrot', 'banana'] >>> shoplist[::2] ['apple', 'carrot'] >>> shoplist[::3] ['apple', 'banana'] >>> shoplist[::-1] ['banana', 'carrot', 'mango', 'apple'] Notice that when the step is 2, we get the items with position 0, 2, ... When the step size is 3, we get the items with position 0, 3, etc. Try various combinations of such slice specifications using the Python interpreter interactively i.e. the prompt so that you can see the results immediately. The great thing about sequences is that you can access tuples, lists and strings all in the same way! Set Sets are unordered collections of simple objects. These are used when the existence of an object in a collection is more important than the order or how many times it occurs. Using sets, you can test for membership, whether it is a subset of another set, find the intersection between two sets, and so on. >>> bri = set(['brazil', 'russia', 'india']) >>> 'india' in bri True >>> 'usa' in bri False >>> bric = bri.copy() >>> bric.add('china') >>> bric.issuperset(bri) True >>> bri.remove('russia') >>> bri & bric # OR bri.intersection(bric) {'brazil', 'india'} How It Works: The example is pretty much self-explanatory because it involves basic set theory mathematics taught in school. Python en:Data Structures 70 References When you create an object and assign it to a variable, the variable only refers to the object and does not represent the object itself! That is, the variable name points to that part of your computer's memory where the object is stored. This is called as binding of the name to the object. Generally, you don't need to be worried about this, but there is a subtle effect due to references which you need to be aware of: Example: #!/usr/bin/python # Filename: reference.py print('Simple Assignment') shoplist = ['apple', 'mango', 'carrot', 'banana'] mylist = shoplist # mylist is just another name pointing to the same object! del shoplist[0] # I purchased the first item, so I remove it from the list print('shoplist is', shoplist) print('mylist is', mylist) # notice that both shoplist and mylist both print the same list without # the 'apple' confirming that they point to the same object print('Copy by making a full slice') mylist = shoplist[:] # make a copy by doing a full slice del mylist[0] # remove first item print('shoplist is', shoplist) print('mylist is', mylist) # notice that now the two lists are different Output: $ python reference.py Simple Assignment shoplist is ['mango', 'carrot', 'banana'] mylist is ['mango', 'carrot', 'banana'] Copy by making a full slice shoplist is ['mango', 'carrot', 'banana'] mylist is ['carrot', 'banana'] How It Works: Most of the explanation is available in the comments. Remember that if you want to make a copy of a list or such kinds of sequences or complex objects (not simple objects such as integers), then you have to use the slicing operation to make a copy. If you just assign the variable name to another name, both of them will refer Python en:Data Structures 71 to the same object and this could be trouble if you are not careful. Note for Perl programmers Remember that an assignment statement for lists does not create a copy. You have to use slicing operation to make a copy of the sequence. More About Strings We have already discussed strings in detail earlier. What more can there be to know? Well, did you know that strings are also objects and have methods which do everything from checking part of a string to stripping spaces! The strings that you use in program are all objects of the class str. Some useful methods of this class are demonstrated in the next example. For a complete list of such methods, see help(str). Example: #!/usr/bin/python # Filename: str_methods.py name = 'Swaroop' # This is a string object if name.startswith('Swa'): print('Yes, the string starts with "Swa"') if 'a' in name: print('Yes, it contains the string "a"') if name.find('war') != -1: print('Yes, it contains the string "war"') delimiter = '_*_' mylist = ['Brazil', 'Russia', 'India', 'China'] print(delimiter.join(mylist)) Output: $ python str_methods.py Yes, the string starts with "Swa" Yes, it contains the string "a" Yes, it contains the string "war" Brazil_*_Russia_*_India_*_China How It Works: Here, we see a lot of the string methods in action. The startswith method is used to find out whether the string starts with the given string. The in operator is used to check if a given string is a part of the string. The find method is used to do find the position of the given string in the string or returns -1 if it is not successful to find the substring. The str class also has a neat method to join the items of a sequence with the string acting as a delimiter between each item of the sequence and returns a bigger string generated from this. Python en:Data Structures 72 Summary We have explored the various built-in data structures of Python in detail. These data structures will be essential for writing programs of reasonable size. Now that we have a lot of the basics of Python in place, we will next see how to design and write a real-world Python program. Previous Next Source: http:/ / www. swaroopch. com/ mediawiki/ index. php? oldid=1582 Contributors: Swaroop, 5 anonymous edits Python en:Problem Solving We have explored various parts of the Python language and now we will take a look at how all these parts fit together, by designing and writing a program which does something useful. The idea is to learn how to write a Python script on your own. The Problem The problem is "I want a program which creates a backup of all my important files". Although, this is a simple problem, there is not enough information for us to get started with the solution. A little more analysis is required. For example, how do we specify which files are to be backed up? How are they stored? Where are they stored? After analyzing the problem properly, we design our program. We make a list of things about how our program should work. In this case, I have created the following list on how I want it to work. If you do the design, you may not come up with the same kind of analysis since every person has their own way of doing things, so that is perfectly okay. 1.  The files and directories to be backed up are specified in a list. 2.  The backup must be stored in a main backup directory. 3.  The files are backed up into a zip file. 4.  The name of the zip archive is the current date and time. 5.  We use the standard zip command available by default in any standard Linux/Unix distribution. Windows users can install (http:/ / gnuwin32. sourceforge. net/ downlinks/ zip. php) from the GnuWin32 project page (http:/ / gnuwin32. sourceforge. net/ packages/ zip. htm) and add C:\Program Files\GnuWin32\bin to your system PATH environment variable, similar to what we did for recognizing the python command itself. Note that you can use any archiving command you want as long as it has a command line interface so that we can pass arguments to it from our script. Python en:Problem Solving 73 The Solution As the design of our program is now reasonably stable, we can write the code which is an implementation of our solution. #!/usr/bin/python # Filename: backup_ver1.py import os import time # 1. The files and directories to be backed up are specified in a list. source = ['"C:\\My Documents"', 'C:\\Code'] # Notice we had to use double quotes inside the string for names with spaces in it. # 2. The backup must be stored in a main backup directory target_dir = 'E:\\Backup' # Remember to change this to what you will be using # 3. The files are backed up into a zip file. # 4. The name of the zip archive is the current date and time target = target_dir + os.sep + time.strftime('%Y%m%d%H%M%S') + '.zip' # 5. We use the zip command to put the files in a zip archive zip_command = "zip -qr {0} {1}".format(target, ' '.join(source)) # Run the backup if os.system(zip_command) == 0: print('Successful backup to', target) else: print('Backup FAILED') Output: $ python backup_ver1.py Successful backup to E:\Backup\20080702185040.zip Now, we are in the testing phase where we test that our program works properly. If it doesn't behave as expected, then we have to debug our program i.e. remove the bugs (errors) from the program. If the above program does not work for you, put a print(zip_command) just before the os.system call and run the program. Now copy/paste the printed zip_command to the shell prompt and see if it runs properly on its own. If this command fails, check the zip command manual on what could be wrong. If this command succeeds, then check the Python program if it exactly matches the program written above. How It Works: You will notice how we have converted our design into code in a step-by-step manner. Python en:Problem Solving 74 We make use of the os and time modules by first importing them. Then, we specify the files and directories to be backed up in the source list. The target directory is where store all the backup files and this is specified in the target_dir variable. The name of the zip archive that we are going to create is the current date and time which we find out using the time.strftime() function. It will also have the .zip extension and will be stored in the target_dir directory. Notice the use of os.sep variable - this gives the directory separator according to your operating system i.e. it will be '/' in Linux, Unix, it will be '\\' in Windows and ':' in Mac OS. Using os.sep instead of these characters directly will make our program portable and work across these systems. The time.strftime() function takes a specification such as the one we have used in the above program. The %Y specification will be replaced by the year without the century. The %m specification will be replaced by the month as a decimal number between 01 and 12 and so on. The complete list of such specifications can be found in the Python Reference Manual (http:/ / docs. python. org/ dev/ 3. 0/ library/ time. html#time. strftime). We create the name of the target zip file using the addition operator which concatenates the strings i.e. it joins the two strings together and returns a new one. Then, we create a string zip_command which contains the command that we are going to execute. You can check if this command works by running it on the shell (Linux terminal or DOS prompt). The zip command that we are using has some options and parameters passed. The -q option is used to indicate that the zip command should work quietly. The -r option specifies that the zip command should work recursively for directories i.e. it should include all the subdirectories and files. The two options are combined and specified in a shortcut as -qr. The options are followed by the name of the zip archive to create followed by the list of files and directories to backup. We convert the source list into a string using the join method of strings which we have already seen how to use. Then, we finally run the command using the os.system function which runs the command as if it was run from the system i.e. in the shell - it returns 0 if the command was successfully, else it returns an error number. Depending on the outcome of the command, we print the appropriate message that the backup has failed or succeeded. That's it, we have created a script to take a backup of our important files! Note to Windows Users Instead of double backslash escape sequences, you can also use raw strings. For example, use 'C:\\Documents' or r'C:\Documents'. However, do not use 'C:\Documents' since you end up using an unknown escape sequence \D. Now that we have a working backup script, we can use it whenever we want to take a backup of the files. Linux/Unix users are advised to use the executable method as discussed earlier so that they can run the backup script anytime anywhere. This is called the operation phase or the deployment phase of the software. The above program works properly, but (usually) first programs do not work exactly as you expect. For example, there might be problems if you have not designed the program properly or if you have made a mistake in typing the code, etc. Appropriately, you will have to go back to the design phase or you will have to debug your program. Python en:Problem Solving 75 Second Version The first version of our script works. However, we can make some refinements to it so that it can work better on a daily basis. This is called the maintenance phase of the software. One of the refinements I felt was useful is a better file-naming mechanism - using the time as the name of the file within a directory with the current date as a directory within the main backup directory. First advantage is that your backups are stored in a hierarchical manner and therefore it is much easier to manage. Second advantage is that the length of the filenames are much shorter. Third advantage is that separate directories will help you to easily check if you have taken a backup for each day since the directory would be created only if you have taken a backup for that day. #!/usr/bin/python # Filename: backup_ver2.py import os import time # 1. The files and directories to be backed up are specified in a list. source = ['"C:\\My Documents"', 'C:\\Code'] # Notice we had to use double quotes inside the string for names with spaces in it. # 2. The backup must be stored in a main backup directory target_dir = 'E:\\Backup' # Remember to change this to what you will be using # 3. The files are backed up into a zip file. # 4. The current day is the name of the subdirectory in the main directory today = target_dir + os.sep + time.strftime('%Y%m%d') # The current time is the name of the zip archive now = time.strftime('%H%M%S') # Create the subdirectory if it isn't already there if not os.path.exists(today): os.mkdir(today) # make directory print('Successfully created directory', today) # The name of the zip file target = today + os.sep + now + '.zip' # 5. We use the zip command to put the files in a zip archive zip_command = "zip -qr {0} {1}".format(target, ' '.join(source)) # Run the backup if os.system(zip_command) == 0: print('Successful backup to', target) Python en:Problem Solving 76 else: print('Backup FAILED') Output: $ python backup_ver2.py Successfully created directory E:\Backup\20080702 Successful backup to E:\Backup\20080702\202311.zip $ python backup_ver2.py Successful backup to E:\Backup\20080702\202325.zip How It Works: Most of the program remains the same. The changes is that we check if there is a directory with the current day as name inside the main backup directory using the os.path.exists function. If it doesn't exist, we create it using the os.mkdir function. Third Version The second version works fine when I do many backups, but when there are lots of backups, I am finding it hard to differentiate what the backups were for! For example, I might have made some major changes to a program or presentation, then I want to associate what those changes are with the name of the zip archive. This can be easily achieved by attaching a user-supplied comment to the name of the zip archive. Note The following program does not work, so do not be alarmed, please follow along because there's a lesson in here. #!/usr/bin/python # Filename: backup_ver3.py import os import time # 1. The files and directories to be backed up are specified in a list. source = ['"C:\\My Documents"', 'C:\\Code'] # Notice we had to use double quotes inside the string for names with spaces in it. # 2. The backup must be stored in a main backup directory target_dir = 'E:\\Backup' # Remember to change this to what you will be using # 3. The files are backed up into a zip file. # 4. The current day is the name of the subdirectory in the main directory today = target_dir + os.sep + time.strftime('%Y%m%d') # The current time is the name of the zip archive now = time.strftime('%H%M%S') Python en:Problem Solving 77 # Take a comment from the user to create the name of the zip file comment = input('Enter a comment --> ') if len(comment) == 0: # check if a comment was entered target = today + os.sep + now + '.zip' else: target = today + os.sep + now + '_' + comment.replace(' ', '_') + '.zip' # Create the subdirectory if it isn't already there if not os.path.exists(today): os.mkdir(today) # make directory print('Successfully created directory', today) # 5. We use the zip command to put the files in a zip archive zip_command = "zip -qr {0} {1}".format(target, ' '.join(source)) # Run the backup if os.system(zip_command) == 0: print('Successful backup to', target) else: print('Backup FAILED') Output: $ python backup_ver3.py File "backup_ver3.py", line 25 target = today + os.sep + now + '_' + ^ SyntaxError: invalid syntax How This (does not) Work: This program does not work! Python says there is a syntax error which means that the script does not satisfy the structure that Python expects to see. When we observe the error given by Python, it also tells us the place where it detected the error as well. So we start debugging our program from that line. On careful observation, we see that the single logical line has been split into two physical lines but we have not specified that these two physical lines belong together. Basically, Python has found the addition operator (+) without any operand in that logical line and hence it doesn't know how to continue. Remember that we can specify that the logical line continues in the next physical line by the use of a backslash at the end of the physical line. So, we make this correction to our program. This correction of the program when we find errors is called bug fixing. Python en:Problem Solving 78 Fourth Version #!/usr/bin/python # Filename: backup_ver4.py import os import time # 1. The files and directories to be backed up are specified in a list. source = ['"C:\\My Documents"', 'C:\\Code'] # Notice we had to use double quotes inside the string for names with spaces in it. # 2. The backup must be stored in a main backup directory target_dir = 'E:\\Backup' # Remember to change this to what you will be using # 3. The files are backed up into a zip file. # 4. The current day is the name of the subdirectory in the main directory today = target_dir + os.sep + time.strftime('%Y%m%d') # The current time is the name of the zip archive now = time.strftime('%H%M%S') # Take a comment from the user to create the name of the zip file comment = input('Enter a comment --> ') if len(comment) == 0: # check if a comment was entered target = today + os.sep + now + '.zip' else: target = today + os.sep + now + '_' + \ comment.replace(' ', '_') + '.zip' # Create the subdirectory if it isn't already there if not os.path.exists(today): os.mkdir(today) # make directory print('Successfully created directory', today) # 5. We use the zip command to put the files in a zip archive zip_command = "zip -qr {0} {1}".format(target, ' '.join(source)) # Run the backup if os.system(zip_command) == 0: print('Successful backup to', target) else: print('Backup FAILED') Output: Python en:Problem Solving 79 $ python backup_ver4.py Enter a comment --> added new examples Successful backup to E:\Backup\20080702\202836_added_new_examples.zip $ python backup_ver4.py Enter a comment --> Successful backup to E:\Backup\20080702\202839.zip How It Works: This program now works! Let us go through the actual enhancements that we had made in version 3. We take in the user's comments using the input function and then check if the user actually entered something by finding out the length of the input using the len function. If the user has just pressed enter without entering anything (maybe it was just a routine backup or no special changes were made), then we proceed as we have done before. However, if a comment was supplied, then this is attached to the name of the zip archive just before the .zip extension. Notice that we are replacing spaces in the comment with underscores - this is because managing filenames without spaces are much easier. More Refinements The fourth version is a satisfactorily working script for most users, but there is always room for improvement. For example, you can include a verbosity level for the program where you can specify a -v option to make your program become more talkative. Another possible enhancement would be to allow extra files and directories to be passed to the script at the command line. We can get these names from the sys.argv list and we can add them to our source list using the extend method provided by the list class. The most important refinement would be to not use the os.system way of creating archives and instead using the zipfile or tarfile built-in module to create these archives. They are part of the standard library and available already for you to use without external dependencies on the zip program to be available on your computer. However, I have been using the os.system way of creating a backup in the above examples purely for pedagogical purposes, so that the example is simple enough to be understood by everybody but real enough to be useful. Can you try writing the fifth version that uses the zipfile (http:/ / docs. python. org/ dev/ 3. 0/ library/ zipfile. html) module instead of the os.system call? Python en:Problem Solving 80 The Software Development Process We have now gone through the various phases in the process of writing a software. These phases can be summarised as follows: 1.  What (Analysis) 2.  How (Design) 3.  Do It (Implementation) 4.  Test (Testing and Debugging) 5.  Use (Operation or Deployment) 6.  Maintain (Refinement) A recommended way of writing programs is the procedure we have followed in creating the backup script: Do the analysis and design. Start implementing with a simple version. Test and debug it. Use it to ensure that it works as expected. Now, add any features that you want and continue to repeat the Do It-Test-Use cycle as many times as required. Remember, Software is grown, not built. Summary We have seen how to create our own Python programs/scripts and the various stages involved in writing such programs. You may find it useful to create your own program just like we did in this chapter so that you become comfortable with Python as well as problem-solving. Next, we will discuss object-oriented programming. Previous Next Source: http:/ / www. swaroopch. com/ mediawiki/ index. php? oldid=1395 Contributors: Swaroop, 3 anonymous edits Python en:Object Oriented Programming 81 Python en:Object Oriented Programming Introduction In all the programs we wrote till now, we have designed our program around functions i.e. blocks of statements which manipulate data. This is called the procedure-oriented way of programming. There is another way of organizing your program which is to combine data and functionality and wrap it inside something called an object. This is called the object oriented programming paradigm. Most of the time you can use procedural programming, but when writing large programs or have a problem that is better suited to this method, you can use object oriented programming techniques. Classes and objects are the two main aspects of object oriented programming. A class creates a new type where objects are instances of the class. An analogy is that you can have variables of type int which translates to saying that variables that store integers are variables which are instances (objects) of the int class. Note for Static Language Programmers Note that even integers are treated as objects (of the int class). This is unlike C++ and Java (before version 1.5) where integers are primitive native types. See help(int) for more details on the class. C# and Java 1.5 programmers will find this similar to the boxing and unboxing concept. Objects can store data using ordinary variables that belong to the object. Variables that belong to an object or class are referred to as fields. Objects can also have functionality by using functions that belong to a class. Such functions are called methods of the class. This terminology is important because it helps us to differentiate between functions and variables which are independent and those which belong to a class or object. Collectively, the fields and methods can be referred to as the attributes of that class. Fields are of two types - they can belong to each instance/object of the class or they can belong to the class itself. They are called instance variables and class variables respectively. A class is created using the class keyword. The fields and methods of the class are listed in an indented block. The self Class methods have only one specific difference from ordinary functions - they must have an extra first name that has to be added to the beginning of the parameter list, but you do not give a value for this parameter when you call the method, Python will provide it. This particular variable refers to the object itself, and by convention, it is given the name self. Although, you can give any name for this parameter, it is strongly recommended that you use the name self - any other name is definitely frowned upon. There are many advantages to using a standard name - any reader of your program will immediately recognize it and even specialized IDEs (Integrated Development Environments) can help you if you use self. Python en:Object Oriented Programming 82 Note for C++/Java/C# Programmers The self in Python is equivalent to the this pointer in C++ and the this reference in Java and C#. You must be wondering how Python gives the value for self and why you don't need to give a value for it. An example will make this clear. Say you have a class called MyClass and an instance of this class called myobject. When you call a method of this object as myobject.method(arg1, arg2), this is automatically converted by Python into MyClass.method(myobject, arg1, arg2) - this is all the special self is about. This also means that if you have a method which takes no arguments, then you still have to have one argument - the self. Classes The simplest class possible is shown in the following example. #!/usr/bin/python # Filename: simplestclass.py class Person: pass # An empty block p = Person() print(p) Output: $ python simplestclass.py <__main__.Person object at 0x019F85F0> How It Works: We create a new class using the class statement and the name of the class. This is followed by an indented block of statements which form the body of the class. In this case, we have an empty block which is indicated using the pass statement. Next, we create an object/instance of this class using the name of the class followed by a pair of parentheses. (We will learn more about instantiation in the next section). For our verification, we confirm the type of the variable by simply printing it. It tells us that we have an instance of the Person class in the __main__ module. Notice that the address of the computer memory where your object is stored is also printed. The address will have a different value on your computer since Python can store the object wherever it finds space. Python en:Object Oriented Programming 83 Object Methods We have already discussed that classes/objects can have methods just like functions except that we have an extra self variable. We will now see an example. #!/usr/bin/python # Filename: method.py class Person: def sayHi(self): print('Hello, how are you?') p = Person() p.sayHi() # This short example can also be written as Person().sayHi() Output: $ python method.py Hello, how are you? How It Works: Here we see the self in action. Notice that the sayHi method takes no parameters but still has the self in the function definition. The _ _ init_ _ method There are many method names which have special significance in Python classes. We will see the significance of the __init__ method now. The __init__ method is run as soon as an object of a class is instantiated. The method is useful to do any initialization you want to do with your object. Notice the double underscores both at the beginning and at the end of the name. Example: #!/usr/bin/python # Filename: class_init.py class Person: def __init__(self, name): self.name = name def sayHi(self): print('Hello, my name is', self.name) p = Person('Swaroop') p.sayHi() # This short example can also be written as Person('Swaroop').sayHi() Output: Python en:Object Oriented Programming 84 $ python class_init.py Hello, my name is Swaroop How It Works: Here, we define the __init__ method as taking a parameter name (along with the usual self). Here, we just create a new field also called name. Notice these are two different variables even though they are both called 'name'. The dotted notation allows us to differentiate between them. Most importantly, notice that we do not explicitly call the __init__ method but pass the arguments in the parentheses following the class name when creating a new instance of the class. This is the special significance of this method. Now, we are able to use the self.name field in our methods which is demonstrated in the sayHi method. Class And Object Variables We have already discussed the functionality part of classes and objects (i.e. methods), now let us learn about the data part. The data part, i.e. fields, are nothing but ordinary variables that are bound to the namespaces of the classes and objects. This means that these names are valid within the context of these classes and objects only. That's why they are called name spaces. There are two types of fields - class variables and object variables which are classified depending on whether the class or the object owns the variables respectively. Class variables are shared - they can be accessed by all instances of that class. There is only one copy of the class variable and when any one object makes a change to a class variable, that change will be seen by all the other instances. Object variables are owned by each individual object/instance of the class. In this case, each object has its own copy of the field i.e. they are not shared and are not related in any way to the field by the same name in a different instance. An example will make this easy to understand: #!/usr/bin/python # Filename: objvar.py class Robot: '''Represents a robot, with a name.''' # A class variable, counting the number of robots population = 0 def __init__(self, name): '''Initializes the data.''' self.name = name print('(Initializing {0})'.format(self.name)) # When this person is created, the robot # adds to the population Robot.population += 1 Python en:Object Oriented Programming 85 def __del__(self): '''I am dying.''' print('{0} is being destroyed!'.format(self.name)) Robot.population -= 1 if Robot.population == 0: print('{0} was the last one.'.format(self.name)) else: print('There are still {0:d} robots working.'.format(Robot.population)) def sayHi(self): '''Greeting by the robot. Yeah, they can do that.''' print('Greetings, my masters call me {0}.'.format(self.name)) def howMany(): '''Prints the current population.''' print('We have {0:d} robots.'.format(Robot.population)) howMany = staticmethod(howMany) droid1 = Robot('R2-D2') droid1.sayHi() Robot.howMany() droid2 = Robot('C-3PO') droid2.sayHi() Robot.howMany() print("\nRobots can do some work here.\n") print("Robots have finished their work. So let's destroy them.") del droid1 del droid2 Robot.howMany() Output: (Initializing R2-D2) Greetings, my masters call me R2-D2. We have 1 robots. (Initializing C-3PO) Greetings, my masters call me C-3PO. We have 2 robots. Python en:Object Oriented Programming 86 Robots can do some work here. Robots have finished their work. So let's destroy them. R2-D2 is being destroyed! There are still 1 robots working. C-3PO is being destroyed! C-3PO was the last one. We have 0 robots. How It Works: This is a long example but helps demonstrate the nature of class and object variables. Here, population belongs to the Robot class and hence is a class variable. The name variable belongs to the object (it is assigned using self) and hence is an object variable. Thus, we refer to the population class variable as Robot.population and not as self.population. We refer to the object variable name using self.name notation in the methods of that object. Remember this simple difference between class and object variables. Also note that an object variable with the same name as a class variable will hide the class variable! The howMany is actually a method that belongs to the class and not to the object. This means we can define it as either a classmethod or a staticmethod depending on whether we need to know which class we are part of. Since we don't need such information, we will go for staticmethod. We could have also achieved the same using decorators (http:/ / www. ibm. com/ developerworks/ linux/ library/ l-cpdecor. html): @staticmethod def howMany(): '''Prints the current population.''' print('We have {0:d} robots.'.format(Robot.population)) Decorators can be imagined to be a shortcut to calling an explicit statement, as we have seen in this example. Observe that the __init__ method is used to initialize the Robot instance with a name. In this method, we increase the population count by 1 since we have one more robot being added. Also observe that the values of self.name is specific to each object which indicates the nature of object variables. Remember, that you must refer to the variables and methods of the same object using the self only. This is called an attribute reference. In this program, we also see the use of docstrings for classes as well as methods. We can access the class docstring at runtime using Robot.__doc__ and the method docstring as Robot.sayHi.__doc__ Just like the __init__ method, there is another special method __del__ which is called when an object is going to die i.e. it is no longer being used and is being returned to the computer system for reusing that piece of memory. In this method, we simply decrease the Robot.population count by 1. Python en:Object Oriented Programming 87 The __del__ method is run when the object is no longer in use and there is no guarantee when that method will be run. If you want to explicitly see it in action, we have to use the del statement which is what we have done here. Note for C++/Java/C# Programmers All class members (including the data members) are public and all the methods are virtual in Python. One exception: If you use data members with names using the double underscore prefix such as __privatevar, Python uses name-mangling to effectively make it a private variable. Thus, the convention followed is that any variable that is to be used only within the class or object should begin with an underscore and all other names are public and can be used by other classes/objects. Remember that this is only a convention and is not enforced by Python (except for the double underscore prefix). Inheritance One of the major benefits of object oriented programming is reuse of code and one of the ways this is achieved is through the inheritance mechanism. Inheritance can be best imagined as implementing a type and subtype relationship between classes. Suppose you want to write a program which has to keep track of the teachers and students in a college. They have some common characteristics such as name, age and address. They also have specific characteristics such as salary, courses and leaves for teachers and, marks and fees for students. You can create two independent classes for each type and process them but adding a new common characteristic would mean adding to both of these independent classes. This quickly becomes unwieldy. A better way would be to create a common class called SchoolMember and then have the teacher and student classes inherit from this class i.e. they will become sub-types of this type (class) and then we can add specific characteristics to these sub-types. There are many advantages to this approach. If we add/change any functionality in SchoolMember, this is automatically reflected in the subtypes as well. For example, you can add a new ID card field for both teachers and students by simply adding it to the SchoolMember class. However, changes in the subtypes do not affect other subtypes. Another advantage is that if you can refer to a teacher or student object as a SchoolMember object which could be useful in some situations such as counting of the number of school members. This is called polymorphism where a sub-type can be substituted in any situation where a parent type is expected i.e. the object can be treated as an instance of the parent class. Also observe that we reuse the code of the parent class and we do not need to repeat it in the different classes as we would have had to in case we had used independent classes. The SchoolMember class in this situation is known as the base class or the superclass. The Teacher and Student classes are called the derived classes or subclasses. We will now see this example as a program. #!/usr/bin/python # Filename: inherit.py Python en:Object Oriented Programming 88 class SchoolMember: '''Represents any school member.''' def __init__(self, name, age): self.name = name self.age = age print('(Initialized SchoolMember: {0})'.format(self.name)) def tell(self): '''Tell my details.''' print('Name:"{0}" Age:"{1}"'.format(self.name, self.age), end=" ") class Teacher(SchoolMember): '''Represents a teacher.''' def __init__(self, name, age, salary): SchoolMember.__init__(self, name, age) self.salary = salary print('(Initialized Teacher: {0})'.format(self.name)) def tell(self): SchoolMember.tell(self) print('Salary: "{0:d}"'.format(self.salary)) class Student(SchoolMember): '''Represents a student.''' def __init__(self, name, age, marks): SchoolMember.__init__(self, name, age) self.marks = marks print('(Initialized Student: {0})'.format(self.name)) def tell(self): SchoolMember.tell(self) print('Marks: "{0:d}"'.format(self.marks)) t = Teacher('Mrs. Shrividya', 40, 30000) s = Student('Swaroop', 25, 75) print() # prints a blank line members = [t, s] for member in members: member.tell() # works for both Teachers and Students Output: $ python inherit.py (Initialized SchoolMember: Mrs. Shrividya) Python en:Object Oriented Programming 89 (Initialized Teacher: Mrs. Shrividya) (Initialized SchoolMember: Swaroop) (Initialized Student: Swaroop) Name:"Mrs. Shrividya" Age:"40" Salary: "30000" Name:"Swaroop" Age:"25" Marks: "75" How It Works: To use inheritance, we specify the base class names in a tuple following the class name in the class definition. Next, we observe that the __init__ method of the base class is explicitly called using the self variable so that we can initialize the base class part of the object. This is very important to remember - Python does not automatically call the constructor of the base class, you have to explicitly call it yourself. We also observe that we can call methods of the base class by prefixing the class name to the method call and then pass in the self variable along with any arguments. Notice that we can treat instances of Teacher or Student as just instances of the SchoolMember when we use the tell method of the SchoolMember class. Also, observe that the tell method of the subtype is called and not the tell method of the SchoolMember class. One way to understand this is that Python always starts looking for methods in the actual type, which in this case it does. If it could not find the method, it starts looking at the methods belonging to its base classes one by one in the order they are specified in the tuple in the class definition. A note on terminology - if more than one class is listed in the inheritance tuple, then it is called multiple inheritance. Summary We have now explored the various aspects of classes and objects as well as the various terminologies associated with it. We have also seen the benefits and pitfalls of object-oriented programming. Python is highly object-oriented and understanding these concepts carefully will help you a lot in the long run. Next, we will learn how to deal with input/output and how to access files in Python. Previous Next Source: http:/ / www. swaroopch. com/ mediawiki/ index. php? oldid=2359 Contributors: Horstjens, Swaroop, 6 anonymous edits Python en:Input Output 90 Python en:Input Output Introduction There will be situations where your program has to interact with the user. For example, you would want to take input from the user and then print some results back. We can achieve this using the input() and print() functions respectively. For output, we can also use the various methods of the str (string) class. For example, you can use the rjust method to get a string which is right justified to a specified width. See help(str) for more details. Another common type of input/output is dealing with files. The ability to create, read and write files is essential to many programs and we will explore this aspect in this chapter. Input from user #!/usr/bin/python # user_input.py def reverse(text): return text[::-1] def is_palindrome(text): return text == reverse(text) something = input('Enter text: ') if (is_palindrome(something)): print("Yes, it is a palindrome") else: print("No, it is not a palindrome") Output: $ python user_input.py Enter text: sir No, it is not a palindrome $ python user_input.py Enter text: madam Yes, it is a palindrome $ python user_input.py Enter text: racecar Yes, it is a palindrome How It Works: We use the slicing feature to reverse the text. We've already seen how we can make slices from sequences using the seq[a:b] code starting from position a to position b. We can also provide a third argument that determines the step by which the slicing is done. The Python en:Input Output 91 default step is 1 because of which it returns a continuous part of the text. Giving a negative step, i.e., -1 will return the text in reverse. The input() function takes a string as argument and displays it to the user. Then it waits for the user to type something and press the return key. Once the user has entered, the input() function will then return that text. We take that text and reverse it. If the original text and reversed text are equal, then the text is a palindrome (http:/ / en. wiktionary. org/ wiki/ palindrome). Homework exercise: Checking whether a text is a palindrome should also ignore punctuation, spaces and case. For example, "Rise to vote, sir." is also a palindrome but our current program doesn't say it is. Can you improve the above program to recognize this palindrome? Files You can open and use files for reading or writing by creating an object of the file class and using its read, readline or write methods appropriately to read from or write to the file. The ability to read or write to the file depends on the mode you have specified for the file opening. Then finally, when you are finished with the file, you call the close method to tell Python that we are done using the file. Example: #!/usr/bin/python # Filename: using_file.py poem = '''\ Programming is fun When the work is done if you wanna make your work also fun: use Python! ''' f = open('poem.txt', 'w') # open for 'w'riting f.write(poem) # write text to file f.close() # close the file f = open('poem.txt') # if no mode is specified, 'r'ead mode is assumed by default while True: line = f.readline() if len(line) == 0: # Zero length indicates EOF break print(line, end='') f.close() # close the file Output: $ python using_file.py Programming is fun Python en:Input Output 92 When the work is done if you wanna make your work also fun: use Python! How It Works: First, open a file by using the built-in open function and specifying the name of the file and the mode in which we want to open the file. The mode can be a read mode ('r'), write mode ('w') or append mode ('a'). We can also by dealing with a text file ('t') or a binary file ('b'). There are actually many more modes available and help(open) will give you more details about them. By default, open() considers the file to be a 't'ext file and opens it in 'r'ead mode. In our example, we first open the file in write text mode and use the write method of the file object to write to the file and then we finally close the file. Next, we open the same file again for reading. We don't need to specify a mode because 'read text file' is the default mode. We read in each line of the file using the readline method in a loop. This method returns a complete line including the newline character at the end of the line. When an empty string is returned, it means that we have reached the end of the file and we 'break' out of the loop. By deafult, the print() function prints the text as well as an automatic newline to the screen. We are suppressing the newline by specifying end='' because the line that is read from the file already ends with a newline character. Then, we finally close the file. Now, check the contents of the poem.txt file to confirm that the program has indeed written and read from that file. Pickle Python provides a standard module called pickle using which you can store any Python object in a file and then get it back later. This is called storing the object persistently. Example: #!/usr/bin/python # Filename: pickling.py import pickle # the name of the file where we will store the object shoplistfile = 'shoplist.data' # the list of things to buy shoplist = ['apple', 'mango', 'carrot'] # Write to the file f = open(shoplistfile, 'wb') pickle.dump(shoplist, f) # dump the object to a file f.close() del shoplist # destroy the shoplist variable # Read back from the storage Python en:Input Output 93 f = open(shoplistfile, 'rb') storedlist = pickle.load(f) # load the object from the file print(storedlist) Output: $ python pickling.py ['apple', 'mango', 'carrot'] How It Works: To store an object in a file, we have to first open the file in 'w'rite 'b'inary mode and then call the dump function of the pickle module. This process is called pickling. Next, we retrieve the object using the load function of the pickle module which returns the object. This process is called unpickling. Summary We have discussed various types of input/output and also file handling and using the pickle module. Next, we will explore the concept of exceptions. Previous Next Source: http:/ / www. swaroopch. com/ mediawiki/ index. php? oldid=1583 Contributors: Horstjens, Swaroop, 1 anonymous edits Python en:Exceptions Introduction Exceptions occur when certain exceptional situations occur in your program. For example, what if you are going to read a file and the file does not exist? Or what if you accidentally deleted it when the program was running? Such situations are handled using exceptions. Similarly, what if your program had some invalid statements? This is handled by Python which raises its hands and tells you there is an error. Errors Consider a simple print function call. What if we misspelt print as Print? Note the capitalization. In this case, Python raises a syntax error. >>> Print('Hello World') Traceback (most recent call last): File "", line 1, in Print('Hello World') NameError: name 'Print' is not defined >>> print('Hello World') Hello World Python en:Exceptions 94 Observe that a NameError is raised and also the location where the error was detected is printed. This is what an error handler for this error does. Exceptions We will try to read input from the user. Press ctrl-d and see what happens. >>> s = input('Enter something --> ') Enter something --> Traceback (most recent call last): File "", line 1, in s = input('Enter something --> ') EOFError: EOF when reading a line Python raises an error called EOFError which basically means it found an end of file symbol (which is represented by ctrl-d) when it did not expect to see it. Handling Exceptions We can handle exceptions using the try..except statement. We basically put our usual statements within the try-block and put all our error handlers in the except-block. #!/usr/bin/python # Filename: try_except.py try: text = input('Enter something --> ') except EOFError: print('Why did you do an EOF on me?') except KeyboardInterrupt: print('You cancelled the operation.') else: print('You entered {0}'.format(text)) Output: $ python try_except.py Enter something --> # Press ctrl-d Why did you do an EOF on me? $ python try_except.py Enter something --> # Press ctrl-c You cancelled the operation. $ python try_except.py Enter something --> no exceptions You entered no exceptions How It Works: We put all the statements that might raise exceptions/errors inside the try block and then put handlers for the appropriate errors/exceptions in the except clause/block. The except Python en:Exceptions 95 clause can handle a single specified error or exception, or a parenthesized list of errors/exceptions. If no names of errors or exceptions are supplied, it will handle all errors and exceptions. Note that there has to be at least one except clause associated with every try clause. Otherwise, what's the point of having a try block? If any error or exception is not handled, then the default Python handler is called which just stops the execution of the program and prints an error message. We have already seen this in action above. You can also have an else clause associated with a try..except block. The else clause is executed if no exception occurs. In the next example, we will also see how to get the exception object so that we can retrieve additional information. Raising Exceptions You can raise exceptions using the raise statement by providing the name of the error/exception and the exception object that is to be thrown. The error or exception that you can arise should be class which directly or indirectly must be a derived class of the Exception class. #!/usr/bin/python # Filename: raising.py class ShortInputException(Exception): '''A user-defined exception class.''' def __init__(self, length, atleast): Exception.__init__(self) self.length = length self.atleast = atleast try: text = input('Enter something --> ') if len(text) < 3: raise ShortInputException(len(text), 3) # Other work can continue as usual here except EOFError: print('Why did you do an EOF on me?') except ShortInputException as ex: print('ShortInputException: The input was {0} long, expected at least {1}'\ .format(ex.length, ex.atleast)) else: print('No exception was raised.') Output: $ python raising.py Enter something --> a Python en:Exceptions 96 ShortInputException: The input was 1 long, expected at least 3 $ python raising.py Enter something --> abc No exception was raised. How It Works: Here, we are creating our own exception type. This new exception type is called ShortInputException. It has two fields - length which is the length of the given input, and atleast which is the minimum length that the program was expecting. In the except clause, we mention the class of error which will be stored as the variable name to hold the corresponding error/exception object. This is analogous to parameters and arguments in a function call. Within this particular except clause, we use the length and atleast fields of the exception object to print an appropriate message to the user. Try . . Finally Suppose you are reading a file in your program. How do you ensure that the file object is closed properly whether or not an exception was raised? This can be done using the finally block. Note that you can use an except clause along with a finally block for the same corresponding try block. You will have to embed one within another if you want to use both. #!/usr/bin/python # Filename: finally.py import time try: f = open('poem.txt') while True: # our usual file-reading idiom line = f.readline() if len(line) == 0: break print(line, end='') time.sleep(2) # To make sure it runs for a while except KeyboardInterrupt: print('!! You cancelled the reading from the file.') finally: f.close() print('(Cleaning up: Closed the file)') Output: $ python finally.py Programming is fun When the work is done if you wanna make your work also fun: !! You cancelled the reading from the file. Python en:Exceptions 97 (Cleaning up: Closed the file) How It Works: We do the usual file-reading stuff, but we have arbitrarily introduced sleeping for 2 seconds after printing each line using the time.sleep function so that the program runs slowly (Python is very fast by nature). When the program is still running, press ctrl-c to interrupt/cancel the program. Observe that the KeyboardInterrupt exception is thrown and the program quits. However, before the program exits, the finally clause is executed and the file object is always closed. The with statement Acquiring a resource in the try block and subsequently releasing the resource in the finally block is a common pattern. Hence, there is also a with statement that enables this to be done in a clean manner: #!/usr/bin/python # Filename: using_with.py with open("poem.txt") as f: for line in f: print(line, end='') How It Works: The output should be same as the previous example. The difference here is that we are using the open function with the with statement - we leave the closing of the file to be done automatically by with open. What happens behind the scenes is that there is a protocol used by the with statement. It fetches the object returned by the open statement, let's call it "thefile" in this case. It always calls the thefile.__enter__ function before starting the block of code under it and always calls thefile.__exit__ after finishing the block of code. So the code that we would have written in a finally block is should be taken care of automatically by the __exit__ method. This is what helps us to avoid having to use explicit try..finally statements repeatedly. More discussion on this topic is beyond scope of this book, so please refer PEP 343 (http:/ / www. python. org/ dev/ peps/ pep-0343/ ) for comprehensive explanation. Summary We have discussed the usage of the try..except and try..finally statements. We have seen how to create our own exception types and how to raise exceptions as well. Next, we will explore the Python Standard Library. Previous Next Source: http:/ / www. swaroopch. com/ mediawiki/ index. php? oldid=1470 Contributors: Swaroop, 2 anonymous edits Python en:Standard Library 98 Python en:Standard Library Introduction The Python Standard Library contains a huge number of useful modules and is part of every standard Python installation. It is important to become familiar with the Python Standard Library since many problems can be solved quickly if you are familiar with the range of things that these libraries can do. We will explore some of the commonly used modules in this library. You can find complete details for all of the modules in the Python Standard Library in the 'Library Reference' section (http:/ / docs. python. org/ dev/ 3. 0/ library/ ) of the documentation that comes with your Python installation. Let us explore a few useful modules. Note If you find the topics in this chapter too advanced, you may skip this chapter. However, I highly recommend coming back to this chapter when you are more comfortable with programming using Python. sys module The sys module contains system-specific functionality. We have already seen that the sys.argv list contains the command-line arguments. Suppose we want to check the version of the Python command being used so that, say, we want to ensure that we are using at least version 3. The sys module gives us such functionality. >>> import sys >>> sys.version_info (3, 0, 0, 'beta', 2) >>> sys.version_info[0] >= 3 True How It Works: The sys module has a version_info tuple that gives us the version information. The first entry is the major version. We can check this to, for example, ensure the program runs only under Python 3.0: #!/usr/bin/python # Filename: versioncheck.py import sys, warnings if sys.version_info[0] < 3: warnings.warn("Need Python 3.0 for this program to run", RuntimeWarning) else: print('Proceed as normal') Output: Python en:Standard Library 99 $ python2.5 versioncheck.py versioncheck.py:6: RuntimeWarning: Need Python 3.0 for this program to run RuntimeWarning) $ python3 versioncheck.py Proceed as normal How It Works: We use another module from the standard library called warnings that is used to display warnings to the end-user. If the Python version number is not at least 3, we display a corresponding warning. logging module What if you wanted to have some debugging messages or important messages to be stored somewhere so that you can check whether your program has been running as you would expect it? How do you "store somewhere" these messages? This can be achieved using the logging module. #!/usr/bin/python # Filename: use_logging.py import os, platform, logging if platform.platform().startswith('Windows'): logging_file = os.path.join(os.getenv('HOMEDRIVE'), os.getenv('HOMEPATH'), 'test.log') else: logging_file = os.path.join(os.getenv('HOME'), 'test.log') logging.basicConfig( level=logging.DEBUG, format='%(asctime)s : %(levelname)s : %(message)s', filename = logging_file, filemode = 'w', ) logging.debug("Start of the program") logging.info("Doing something") logging.warning("Dying now") Output: $python use_logging.py Logging to C:\Users\swaroop\test.log If we check the contents of test.log, it will look something like this: 2008-09-03 13:18:16,233 : DEBUG : Start of the program 2008-09-03 13:18:16,233 : INFO : Doing something 2008-09-03 13:18:16,233 : WARNING : Dying now Python en:Standard Library 100 How It Works: We use three modules from the standard library - the os module for interacting with the operating system, the platform module for information about the platform i.e. the operating system and the logging module to log information. First, we check which operating system we are using by checking the string returned by platform.platform() (for more information, see import platform; help(platform)). If it is Windows, we figure out the home drive, the home folder and the filename where we want to store the information. Putting these three parts together, we get the full location of the file. For other platforms, we need to know just the home folder of the user and we get the full location of the file. We use the os.path.join() function to put these three parts of the location together. The reason to use a special function rather than just adding the strings together is because this function will ensure the full location matches the format expected by the operating system. We configure the logging module to write all the messages in a particular format to the file we have specified. Finally, we can put messages that are either meant for debugging, information, warning or even critical messages. Once the program has run, we can check this file and we will know what happened in the program, even though no information was displayed to the user running the program. urllib and json modules How much fun would it be if we could write our own program that will get search results from the web? Let us explore that now. This can be achieved using a few modules. First is the urllib module that we can use to fetch any webpage from the internet. We will make use of Yahoo! Search to get the search results and luckily they can give us the results in a format called JSON which is easy for us to parse because of the built-in json module in the standard library. TODO This program doesn't work yet which seems to be a bug in Python 3.0 beta 2 (http:/ / bugs. python. org/ issue3763). #!/usr/bin/python # Filename: yahoo_search.py import sys if sys.version_info[0] != 3: sys.exit('This program needs Python 3.0') import json import urllib, urllib.parse, urllib.request, urllib.response # Get your own APP ID at http://developer.yahoo.com/wsregapp/ YAHOO_APP_ID = 'jl22psvV34HELWhdfUJbfDQzlJ2B57KFS_qs4I8D0Wz5U5_yCI1Awv8.lBSfPhwr' SEARCH_BASE = 'http://search.yahooapis.com/WebSearchService/V1/webSearch' Python en:Standard Library 101 class YahooSearchError(Exception): pass # Taken from http://developer.yahoo.com/python/python-json.html def search(query, results=20, start=1, **kwargs): kwargs.update({ 'appid': YAHOO_APP_ID, 'query': query, 'results': results, 'start': start, 'output': 'json' }) url = SEARCH_BASE + '?' + urllib.parse.urlencode(kwargs) result = json.load(urllib.request.urlopen(url)) if 'Error' in result: raise YahooSearchError(result['Error']) return result['ResultSet'] query = input('What do you want to search for? ') for result in search(query)['Result']: print("{0} : {1}".format(result['Title'], result['Url'])) Output: TODO How It Works: We can get the search results from a particular website by giving the text we are searching for in a particular format. We have to specify many options which we combine using key1=value1&key2=value2 format which is handled by the urllib.parse.urlencode() function. So for example, open this link in your web browser (http:/ / search. yahooapis. com/ WebSearchService/ V1/ webSearch?query=byte+ of+ python& appid=jl22psvV34HELWhdfUJbfDQzlJ2B57KFS_qs4I8D0Wz5U5_yCI1Awv8. lBSfPhwr& results=20& start=1& output=json) and you will see 20 results, starting from the first result, for the words "byte of python", and we are asking for the output in JSON format. We make a connection to this URL using the urllib.request.urlopen() function and pass that file handle to json.load() which will read the content and simultaneously convert it to a Python object. We then loop through these results and display it to the end-user. Python en:Standard Library 102 Module of the Week Series There is much more to be explored in the standard library such as debugging (http:/ / docs. python. org/ dev/ library/ pdb. html), handling command line options (http:/ / docs. python. org/ dev/ 3. 0/ library/ getopt. html), regular expressions (http:/ / www. diveintopython. org/ regular_expressions/ index. html) and so on. The best way to further explore the standard library is to read Doug Hellmann's excellent Python Module of the Week (http:/ / www. doughellmann. com/ projects/ PyMOTW/ ) series. Summary We have explored some of the functionality of many modules in the Python Standard Library. It is highly recommended to browse through the Python Standard Library documentation (http:/ / docs. python. org/ dev/ 3. 0/ library/ ) to get an idea of all the modules that are available. Next, we will cover various aspects of Python that will make our tour of Python more complete. Previous Next Source: http:/ / www. swaroopch. com/ mediawiki/ index. php? oldid=1156 Contributors: Swaroop, 2 anonymous edits Python en:More Introduction So far we have covered majority of the various aspects of Python that you will use. In this chapter, we will cover some more aspects that will make our knowledge of Python more well-rounded. Passing tuples around Ever wished you could return two different values from a function? You can. All you have to do is use a tuple. >>> def get_error_details(): ... return (2, 'second error details') ... >>> errnum, errstr = get_error_details() >>> errnum 2 >>> errstr 'second error details' Notice that the usage of a, b = interprets the result of the expression as a tuple with two values. If you want to interpret the results as (a, ), then you just need to star it just like you would in function parameters: Python en:More 103 >>> a, *b = [1, 2, 3, 4] >>> a 1 >>> b [2, 3, 4] This also means the fastest way to swap two variables in Python is: >>> a = 5; b = 8 >>> a, b = b, a >>> a, b (8, 5) Special Methods There are certain methods such as the __init__ and __del__ methods which have special significance in classes. Special methods are used to mimic certain behaviors of built-in types. For example, if you want to use the x[key] indexing operation for your class (just like you use it for lists and tuples), then all you have to do is implement the __getitem__() method and your job is done. If you think about it, this is what Python does for the list class itself! Some useful special methods are listed in the following table. If you want to know about all the special methods, see the manual (http:/ / docs. python. org/ dev/ 3. 0/ reference/ datamodel. html#special-method-names). Name Explanation __init__(self, ...) This method is called just before the newly created object is returned for usage. __del__(self) Called just before the object is destroyed __str__(self) Called when we use the print function or when str() is used. __lt__(self, other) Called when the less than operator (<) is used. Similarly, there are special methods for all the operators (+, >, etc.) __getitem__(self, key) Called when x[key] indexing operation is used. __len__(self) Called when the built-in len() function is used for the sequence object. Single Statement Blocks We have seen that each block of statements is set apart from the rest by its own indentation level. Well, there is one caveat. If your block of statements contains only one single statement, then you can specify it on the same line of, say, a conditional statement or looping statement. The following example should make this clear: >>> flag = True >>> if flag: print 'Yes' ... Yes Notice that the single statement is used in-place and not as a separate block. Although, you can use this for making your program smaller, I strongly recommend avoiding this short-cut method, except for error checking, mainly because it will be much easier to add an extra Python en:More 104 statement if you are using proper indentation. Lambda Forms A lambda statement is used to create new function objects and then return them at runtime. #!/usr/bin/python # Filename: lambda.py def make_repeater(n): return lambda s: s * n twice = make_repeater(2) print(twice('word')) print(twice(5)) Output: $ python lambda.py wordword 10 How It Works: Here, we use a function make_repeater to create new function objects at runtime and return it. A lambda statement is used to create the function object. Essentially, the lambda takes a parameter followed by a single expression only which becomes the body of the function and the value of this expression is returned by the new function. Note that even a print statement cannot be used inside a lambda form, only expressions. TODO Can we do a list.sort() by providing a compare function created using lambda? points = [ { 'x' : 2, 'y' : 3 }, { 'x' : 4, 'y' : 1 } ] # points.sort(lambda a, b : cmp(a['x'], b['x'])) List Comprehension List comprehensions are used to derive a new list from an existing list. Suppose you have a list of numbers and you want to get a corresponding list with all the numbers multiplied by 2 only when the number itself is greater than 2. List comprehensions are ideal for such situations. #!/usr/bin/python # Filename: list_comprehension.py listone = [2, 3, 4] listtwo = [2*i for i in listone if i > 2] print(listtwo) Output: Python en:More 105 $ python list_comprehension.py [6, 8] How It Works: Here, we derive a new list by specifying the manipulation to be done (2*i) when some condition is satisfied (if i > 2). Note that the original list remains unmodified. The advantage of using list comprehensions is that it reduces the amount of boilerplate code required when we use loops to process each element of a list and store it in a new list. Receiving Tuples and Lists in Functions There is a special way of receiving parameters to a function as a tuple or a dictionary using the * or ** prefix respectively. This is useful when taking variable number of arguments in the function. >>> def powersum(power, *args): ... '''Return the sum of each argument raised to specified power.''' ... total = 0 ... for i in args: ... total += pow(i, power) ... return total ... >>> powersum(2, 3, 4) 25 >>> powersum(2, 10) 100 Because we have a * prefix on the args variable, all extra arguments passed to the function are stored in args as a tuple. If a ** prefix had been used instead, the extra parameters would be considered to be key/value pairs of a dictionary. exec and eval The exec function is used to execute Python statements which are stored in a string or file, as opposed to written in the program itself. For example, we can generate a string containing Python code at runtime and then execute these statements using the exec statement: >>> exec('print("Hello World")') Hello World Similarly, the eval function is used to evaluate valid Python expressions which are stored in a string. A simple example is shown below. >>> eval('2*3') 6 Python en:More 106 The assert statement The assert statement is used to assert that something is true. For example, if you are very sure that you will have at least one element in a list you are using and want to check this, and raise an error if it is not true, then assert statement is ideal in this situation. When the assert statement fails, an AssertionError is raised. >>> mylist = ['item'] >>> assert len(mylist) >= 1 >>> mylist.pop() 'item' >>> mylist [] >>> assert len(mylist) >= 1 Traceback (most recent call last): File "", line 1, in AssertionError The assert statement should be used judiciously. Most of the time, it is better to catch exceptions, either handle the problem or display an error message to the user and then quit. The repr function The repr function is used to obtain a canonical string representation of the object. The interesting part is that you will have eval(repr(object)) == object most of the time. >>> i = [] >>> i.append('item') >>> repr(i) "['item']" >>> eval(repr(i)) ['item'] >>> eval(repr(i)) == i True Basically, the repr function is used to obtain a printable representation of the object. You can control what your classes return for the repr function by defining the __repr__ method in your class. Summary We have covered some more features of Python in this chapter and yet we haven't covered all the features of Python. However, at this stage, we have covered most of what you are ever going to use in practice. This is sufficient for you to get started with whatever programs you are going to create. Next, we will discuss how to explore Python further. Python en:More 107 Previous Next Source: http:/ / www. swaroopch. com/ mediawiki/ index. php? oldid=1463 Contributors: Swaroop, 1 anonymous edits Python en:What Next If you have read this book thoroughly till now and practiced writing a lot of programs, then you must have become comfortable and familiar with Python. You have probably created some Python programs to try out stuff and to exercise your Python skills as well. If you have not done it already, you should. The question now is 'What Next?'. I would suggest that you tackle this problem: Create your own command-line address-book program using which you can browse, add, modify, delete or search for your contacts such as friends, family and colleagues and their information such as email address and/or phone number. Details must be stored for later retrieval. This is fairly easy if you think about it in terms of all the various stuff that we have come across till now. If you still want directions on how to proceed, then here's a hint. Hint (Don't read) Create a class to represent the person's information. Use a dictionary to store person objects with their name as the key. Use the pickle module to store the objects persistently on your hard disk. Use the dictionary built-in methods to add, delete and modify the persons. Once you are able to do this, you can claim to be a Python programmer. Now, immediately send me a mail (http:/ / www. swaroopch. com/ contact/ ) thanking me for this great book ;-) . This step is optional but recommended. Also, please consider making a donation, contributing improvements or volunteering translations to support the continued development of this book. If you found that program easy, here's another one: Implement the replace command (http:/ / unixhelp. ed. ac. uk/ CGI/ man-cgi?replace). This command will replace one string with another in the list of files provided. The replace command can be as simple or as sophisticated as you wish, from simple string substitution to looking for patterns (regular expressions). After that, here are some ways to continue your journey with Python: Python en:What Next 108 Example Code The best way to learn a programming language is to write a lot of code and read a lot of code: •  The PLEAC project (http:/ / pleac. sourceforge. net/ pleac_python/ index. html) •  Rosetta code repository (http:/ / www. rosettacode. org/ wiki/ Category:Python) •  Python examples at java2s (http:/ / www. java2s. com/ Code/ Python/ CatalogPython. htm) •  Python Cookbook (http:/ / code. activestate. com/ recipes/ langs/ python/ ) is an extremely valuable collection of recipes or tips on how to solve certain kinds of problems using Python. This is a must-read for every Python user. Questions and Answers •  Official Python Dos and Don'ts (http:/ / docs. python. org/ dev/ howto/ doanddont. html) •  Official Python FAQ (http:/ / www. python. org/ doc/ faq/ general/ ) •  Norvig's list of Infrequently Asked Questions (http:/ / norvig. com/ python-iaq. html) •  Python Interview Q & A (http:/ / dev. fyicenter. com/ Interview-Questions/ Python/ index. html) •  StackOverflow questions tagged with python (http:/ / beta. stackoverflow. com/ questions/ tagged/ python) Tips and Tricks •  Python Tips & Tricks (http:/ / www. siafoo. net/ article/ 52) •  Advanced Software Carpentry using Python (http:/ / ivory. idyll. org/ articles/ advanced-swc/ ) •  Charming Python (http:/ / gnosis. cx/ publish/ tech_index_cp. html) is an excellent series of Python-related articles by David Mertz. Books, Papers, Tutorials, Videos The logical next step after this book is to read Mark Pilgrim's awesome Dive Into Python (http:/ / www. diveintopython. org) book which you can read fully online as well. The Dive Into Python book explores topics such as regular expressions, XML processing, web services, unit testing, etc. in detail. Other useful resources are: •  ShowMeDo videos for Python (http:/ / showmedo. com/ videos/ python) •  GoogleTechTalks videos on Python (http:/ / youtube. com/ results?search_query=googletechtalks+ python) •  Awaretek's comprehensive list of Python tutorials (http:/ / www. awaretek. com/ tutorials. html) •  The Effbot's Python Zone (http:/ / effbot. org/ zone/ ) •  Links at the end of every Python-URL! email (http:/ / groups. google. com/ group/ comp. lang. python. announce/ t/ 37de95ef0326293d) •  Python Papers (http:/ / pythonpapers. org) Python en:What Next 109 Discussion If you are stuck with a Python problem, and don't know whom to ask, then the comp.lang.python discussion group (http:/ / groups. google. com/ group/ comp. lang. python/ topics) is the best place to ask your question. Make sure you do your homework and have tried solving the problem yourself first. News If you want to learn what is the latest in the world of Python, then follow the Official Python Planet (http:/ / planet. python. org) and/or the Unofficial Python Planet (http:/ / www. planetpython. org). Installing libraries There are a huge number of open source libraries at the Python Package Index (http:/ / pypi. python. org/ pypi) which you can use in your own programs. To install and use these libraries, you can use Philip J. Eby's excellent EasyInstall tool (http:/ / peak. telecommunity. com/ DevCenter/ EasyInstall#using-easy-install). Graphical Software Suppose you want to create your own graphical programs using Python. This can be done using a GUI (Graphical User Interface) library with their Python bindings. Bindings are what allow you to write programs in Python and use the libraries which are themselves written in C or C++ or other languages. There are lots of choices for GUI using Python: PyQt This is the Python binding for the Qt toolkit which is the foundation upon which the KDE is built. Qt is extremely easy to use and very powerful especially due to the Qt Designer and the amazing Qt documentation. PyQt is free if you want to create open source (GPL'ed) software and you need to buy it if you want to create proprietary closed source software. Starting with Qt 4.5 you can use it to create non-GPL software as well. To get started, read the PyQt tutorial (http:/ / zetcode. com/ tutorials/ pyqt4/ ) or the PyQt book (http:/ / www. qtrac. eu/ pyqtbook. html). PyGTK This is the Python binding for the GTK+ toolkit which is the foundation upon which GNOME is built. GTK+ has many quirks in usage but once you become comfortable, you can create GUI apps fast. The Glade graphical interface designer is indispensable. The documentation is yet to improve. GTK+ works well on Linux but its port to Windows is incomplete. You can create both free as well as proprietary software using GTK+. To get started, read the PyGTK tutorial (http:/ / www. pygtk. org/ tutorial. html). wxPython This is the Python bindings for the wxWidgets toolkit. wxPython has a learning curve associated with it. However, it is very portable and runs on Linux, Windows, Mac and even embedded platforms. There are many IDEs available for wxPython which include GUI designers as well such as SPE (Stani's Python Editor) (http:/ / spe. pycs. net/ ) and Python en:What Next 110 the wxGlade (http:/ / wxglade. sourceforge. net/ ) GUI builder. You can create free as well as proprietary software using wxPython. To get started, read the wxPython tutorial (http:/ / zetcode. com/ wxpython/ ). TkInter This is one of the oldest GUI toolkits in existence. If you have used IDLE, you have seen a TkInter program at work. It doesn't have one of the best look & feel because it has an old-school look to it. TkInter is portable and works on both Linux/Unix as well as Windows. Importantly, TkInter is part of the standard Python distribution. To get started, read the Tkinter tutorial (http:/ / www. pythonware. com/ library/ tkinter/ introduction/ ). For more choices, see the GuiProgramming wiki page at the official python website (http:/ / www. python. org/ cgi-bin/ moinmoin/ GuiProgramming). Summary of GUI Tools Unfortunately, there is no one standard GUI tool for Python. I suggest that you choose one of the above tools depending on your situation. The first factor is whether you are willing to pay to use any of the GUI tools. The second factor is whether you want the program to run only on Windows or on Mac and Linux or all of them. The third factor, if Linux is a chosen platform, is whether you are a KDE or GNOME user on Linux. For a more detailed and comprehensive analysis, see Page 26 of the The Python Papers, Volume 3, Issue 1 (http:/ / archive. pythonpapers. org/ ThePythonPapersVolume3Issue1. pdf). Various Implementations There are usually two parts a programming language - the language and the software. A language is how you write something. The software is what actually runs our programs. We have been using the CPython software to run our programs. It is referred to as CPython because it is written in the C language and is the Classical Python interpreter. There are also other software that can run your Python programs: Jython (http:/ / www. jython. org) A Python implementation that runs on the Java platform. This means you can use Java libraries and classes from within Python language and vice-versa. IronPython (http:/ / www. codeplex. com/ Wiki/ View. aspx?ProjectName=IronPython) A Python implementation that runs on the .NET platform. This means you can use .NET libraries and classes from within Python language and vice-versa. PyPy (http:/ / codespeak. net/ pypy/ dist/ pypy/ doc/ home. html) A Python implementation written in Python! This is a research project to make it fast and easy to improve the interpreter since the interpreter itself is written in a dynamic language (as opposed to static languages such as C, Java or C# in the above three implementations) Stackless Python (http:/ / www. stackless. com) A Python implementation that is specialized for thread-based performance. Python en:What Next 111 There are also others such as CLPython (http:/ / common-lisp. net/ project/ clpython/ ) - a Python implementation written in Common Lisp and IronMonkey (http:/ / wiki. mozilla. org/ Tamarin:IronMonkey) which is a port of IronPython to work on top of a JavaScript interpreter which could mean that you can use Python (instead of JavaScript) to write your web-browser ("Ajax") programs. Each of these implementations have their specialized areas where they are useful. Summary We have now come to the end of this book but, as they say, this is the the beginning of the end!. You are now an avid Python user and you are no doubt ready to solve many problems using Python. You can start automating your computer to do all kinds of previously unimaginable things or write your own games and much much more. So, get started! Previous Next Source: http:/ / www. swaroopch. com/ mediawiki/ index. php? oldid=1845 Contributors: Swaroop, 2 anonymous edits Python en:Appendix FLOSS Free/ Libre and Open Source Software (FLOSS) FLOSS (http:/ / en. wikipedia. org/ wiki/ FLOSS) is based on the concept of a community, which itself is based on the concept of sharing, and particularly the sharing of knowledge. FLOSS are free for usage, modification and redistribution. If you have already read this book, then you are already familiar with FLOSS since you have been using Python all along and Python is an open source software! Here are some examples of FLOSS to give an idea of the kind of things that community sharing and building can create: •  Linux. This is a FLOSS operating system that the whole world is slowly embracing! It was started by Linus Torvalds as a student. Now, it is giving competition to Microsoft Windows. [  Linux Kernel (http:/ / www. kernel. org) ] •  Ubuntu. This is a community-driven distribution, sponsored by Canonical and it is the most popular Linux distribution today. It allows you to install a plethora of FLOSS available and all this in an easy-to-use and easy-to-install manner. Best of all, you can just reboot your computer and run Linux off the CD! This allows you to completely try out the new OS before installing it on your computer. [  Ubuntu Linux (http:/ / www. ubuntu. com) ] •  OpenOffice.org. This is an excellent office suite with a writer, presentation, spreadsheet and drawing components among other things. It can even open and edit MS Word and MS PowerPoint files with ease. It runs on almost all platforms. [  OpenOffice (http:/ / www. openoffice. org) ] •  Mozilla Firefox. This is the next generation web browser which is giving great competition to Internet Explorer. It is blazingly fast and has gained critical acclaim for its sensible and impressive features. The extensions concept allows any kind of plugins to be used. Python en:Appendix FLOSS 112 •  Its companion product Thunderbird is an excellent email client that makes reading email a snap. [  Mozilla Firefox (http:/ / www. mozilla. org/ products/ firefox), Mozilla Thunderbird (http:/ / www. mozilla. org/ products/ thunderbird) ] •  Mono. This is an open source implementation of the Microsoft .NET platform. It allows .NET applications to be created and run on Linux, Windows, FreeBSD, Mac OS and many other platforms as well. [  Mono (http:/ / www. mono-project. com), ECMA (http:/ / www. ecma-international. org), Microsoft .NET (http:/ / www. microsoft. com/ net) ] •  Apache web server. This is the popular open source web server. In fact, it is the most popular web server on the planet! It runs nearly more than half of the websites out there. Yes, that's right - Apache handles more websites than all the competition (including Microsoft IIS) combined. [  Apache (http:/ / httpd. apache. org) ] •  MySQL. This is an extremely popular open source database server. It is most famous for it's blazing speed. It is the M in the famous LAMP stack which runs most of the websites on the internet. [  MySQL (http:/ / www. mysql. com) ] •  VLC Player. This is a video player that can play anything from DivX to MP3 to Ogg to VCDs and DVDs to ... who says open source ain't fun? ;-) [  VLC media player (http:/ / www. videolan. org/ vlc/ ) ] •  GeexBox is a Linux distribution that is designed to play movies as soon as you boot up from the CD! [  GeexBox (http:/ / geexbox. org/ en/ start. html) ] This list is just intended to give you a brief idea - there are many more excellent FLOSS out there, such as the Perl language, PHP language, Drupal content management system for websites, PostgreSQL database server, TORCS racing game, KDevelop IDE, Xine - the movie player, VIM editor, Quanta+ editor, Banshee audio player, GIMP image editing program, ... This list could go on forever. To get the latest buzz in the FLOSS world, check out the following websites: •  linux.com (http:/ / www. linux. com) •  LinuxToday (http:/ / www. linuxtoday. com) •  NewsForge (http:/ / www. newsforge. com) •  DistroWatch (http:/ / www. distrowatch. com) Visit the following websites for more information on FLOSS: •  SourceForge (http:/ / www. sourceforge. net) •  FreshMeat (http:/ / www. freshmeat. net) So, go ahead and explore the vast, free and open world of FLOSS! Previous Next Source: http:/ / www. swaroopch. com/ mediawiki/ index. php? oldid=1580 Contributors: Swaroop, 1 anonymous edits Python en:Appendix About 113 Python en:Appendix About Colophon Almost all of the software that I have used in the creation of this book are free and open source software. Birth of the Book In the first draft of this book, I had used Red Hat 9.0 Linux as the foundation of my setup and in the sixth draft, I used Fedora Core 3 Linux as the basis of my setup. Initially, I was using KWord to write the book (as explained in the History Lesson in the preface). Teenage Years Later, I switched to DocBook XML using Kate but I found it too tedious. So, I switched to OpenOffice which was just excellent with the level of control it provided for formatting as well as the PDF generation, but it produced very sloppy HTML from the document. Finally, I discovered XEmacs and I rewrote the book from scratch in DocBook XML (again) after I decided that this format was the long term solution. In the sixth draft, I decided to use Quanta+ to do all the editing. The standard XSL stylesheets that came with Fedora Core 3 Linux were being used. The standard default fonts are used as well. The standard fonts are used as well. However, I had written a CSS document to give color and style to the HTML pages. I had also written a crude lexical analyzer, in Python of course, which automatically provides syntax highlighting to all the program listings. Now For this seventh draft, I'm using MediaWiki (http:/ / www. mediawiki. org) as the basis of my setup (http:/ / www. swaroopch. com/ notes/ ). Now I edit everything online and the readers can directly read/edit/discuss within the wiki website. I still use Vim for editing thanks to the ViewSourceWith extension for Firefox (https:/ / addons. mozilla. org/ en-US/ firefox/ addon/ 394) that integrates with Vim. Python en:Appendix About 114 About The Author http:/ / www. swaroopch. com/ about/ Previous Next Source: http:/ / www. swaroopch. com/ mediawiki/ index. php? oldid=236 Contributors: Swaroop Python en:Appendix Revision History •  1.90 •  04/09/2008 and still in progress •  Revival after a gap of 3.5 years! •  Updating to Python 3.0 •  Rewrite using MediaWiki (again) •  1.20 •  13/01/2005 •  Complete rewrite using Quanta+ on FC3 with lot of corrections and updates. Many new examples. Rewrote my DocBook setup from scratch. •  1.15 •  28/03/2004 •  Minor revisions •  1.12 •  16/03/2004 •  Additions and corrections. •  1.10 •  09/03/2004 •  More typo corrections, thanks to many enthusiastic and helpful readers. •  1.00 •  08/03/2004 •  After tremendous feedback and suggestions from readers, I have made significant revisions to the content along with typo corrections. •  0.99 •  22/02/2004 •  Added a new chapter on modules. Added details about variable number of arguments in functions. •  0.98 •  16/02/2004 •  Wrote a Python script and CSS stylesheet to improve XHTML output, including a crude-yet-functional lexical analyzer for automatic VIM-like syntax highlighting of the program listings. •  0.97 •  13/02/2004 •  Another completely rewritten draft, in DocBook XML (again). Book has improved a lot - it is more coherent and readable. Python en:Appendix Revision History 115 •  0.93 •  25/01/2004 •  Added IDLE talk and more Windows-specific stuff •  0.92 •  05/01/2004 •  Changes to few examples. •  0.91 •  30/12/2003 •  Corrected typos. Improvised many topics. •  0.90 •  18/12/2003 •  Added 2 more chapters. OpenOffice format with revisions. •  0.60 •  21/11/2003 •  Fully rewritten and expanded. •  0.20 •  20/11/2003 •  Corrected some typos and errors. •  0.15 •  20/11/2003 •  Converted to DocBook XML. •  0.10 •  14/11/2003 •  Initial draft using KWord. → Previous → Back to Table of Contents Source: http:/ / www. swaroopch. com/ mediawiki/ index. php? oldid=240 Contributors: Swaroop Python en:Appendix Changes for Python 3000 116 Python en:Appendix Changes for Python 3000 •  Vim and Emacs editors •  http:/ / henry. precheur. org/ 2008/ 4/ 18/ Indenting%20Python%20with%20VIM. html •  http:/ / www. enigmacurry. com/ 2008/ 05/ 09/ emacs-as-a-powerful-python-ide/ •  String - unicode only •  http:/ / docs. python. org/ dev/ 3. 0/ tutorial/ introduction. html#about-unicode •  Non-ASCII identifiers allowed •  http:/ / www. python. org/ dev/ peps/ pep-3131/ •  print() function •  http:/ / www. python. org/ dev/ peps/ pep-3105/ •  raw_input() becomes input() •  http:/ / www. python. org/ dev/ peps/ pep-3111/ •  Integer Literal Support and Syntax •  http:/ / www. python. org/ dev/ peps/ pep-3127/ •  nonlocal statement •  http:/ / www. python. org/ dev/ peps/ pep-3104/ •  Functions can take * argument (varargs) for lists and keyword-only arguments •  http:/ / www. python. org/ dev/ peps/ pep-3102/ •  Functions can have annotations (make a passing note?) •  http:/ / www. python. org/ dev/ peps/ pep-3107/ •  Better explanation of modules, packages and their organization (including __init__.py, etc.) •  http:/ / ivory. idyll. org/ articles/ advanced-swc/ #packages •  String .format() instead of % operator •  http:/ / www. python. org/ dev/ peps/ pep-3101/ •  http:/ / docs. python. org/ dev/ library/ string. html#formatstrings •  Dict method changes •  http:/ / www. python. org/ dev/ peps/ pep-3106/ •  Built-in set class, in data structures chapter •  Problem Solving •  Use http:/ / gnuwin32. sourceforge. net/ packages/ zip. htm on Windows •  Classes •  http:/ / docs. python. org/ dev/ 3. 0/ reference/ datamodel. html •  Metaclasses •  http:/ / www. python. org/ dev/ peps/ pep-3115/ •  Abstract Base Classes •  http:/ / www. python. org/ dev/ peps/ pep-3119/ •  Not sure if any changes required for New I/O •  http:/ / www. python. org/ dev/ peps/ pep-3116/ •  Exception handling Python en:Appendix Changes for Python 3000 117 •  http:/ / www. python. org/ dev/ peps/ pep-0352/ •  http:/ / www. python. org/ dev/ peps/ pep-3109/ •  http:/ / www. python. org/ dev/ peps/ pep-3110/ •  Standard Library - interesting additions •  Reorganization : http:/ / www. python. org/ dev/ peps/ pep-3108/ •  http:/ / docs. python. org/ dev/ library/ warnings. html •  http:/ / docs. python. org/ dev/ library/ logging. html (important) •  http:/ / docs. python. org/ dev/ library/ urllib. html •  http:/ / docs. python. org/ dev/ library/ json. html •  Debugging •  http:/ / docs. python. org/ dev/ library/ pdb. html •  http:/ / docs. python. org/ dev/ 3. 0/ library/ trace. html •  eval, repr/ascii functions •  getopt/optparse - how to write a standard command-line program using python? •  something like replace? •  http:/ / hpux. connect. org. uk/ hppd/ hpux/ Users/ replace-2. 24/ man. html •  More •  Unpacking can take * argument •  http:/ / www. python. org/ dev/ peps/ pep-3132/ •  with statement •  http:/ / www. python. org/ dev/ peps/ pep-0343/ •  What Next? •  Implement 'replace' •  http:/ / unixhelp. ed. ac. uk/ CGI/ man-cgi?replace •  Mention use of PyPI •  Q&A •  http:/ / docs. python. org/ dev/ howto/ doanddont. html •  http:/ / www. python. org/ doc/ faq/ general/ •  http:/ / norvig. com/ python-iaq. html •  Books & Resources •  http:/ / www. coderholic. com/ free-python-programming-books/ •  http:/ / pythonpapers. org •  http:/ / www. mobilepythonbook. org •  http:/ / effbot. org/ zone/ •  Links at the end of every Python-URL! email •  http:/ / groups. google. com/ group/ comp. lang. python. announce/ t/ 37de95ef0326293d •  Examples •  http:/ / www. rosettacode. org •  http:/ / dev. fyicenter. com/ Interview-Questions/ Python/ index. html •  http:/ / www. java2s. com/ Code/ Python/ CatalogPython. htm •  Tips & Tricks •  http:/ / www. siafoo. net/ article/ 52 Source: http:/ / www. swaroopch. com/ mediawiki/ index. php? oldid=242 Python en:Appendix Changes for Python 3000 118 Contributors: Swaroop License Creative Commons Attribution-Share Alike 3.0 Unported http:/ / creativecommons. org/ licenses/ by-sa/ 3. 0/