Practical Python



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Practical Python MAGNUS LIE HETLAND 066FM 7/15/02 5:16 PM Page i Practical Python Copyright ©2002 by Magnus Lie Hetland All rights reserved. No part of this work may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or by any information storage or retrieval system, without the prior written permission of the copyright owner and the publisher. ISBN (pbk): 1-59059-006-6 Printed and bound in the United States of America 12345678910 Trademarked names may appear in this book. Rather than use a trademark symbol with every occurrence of a trademarked name, we use the names only in an editorial fashion and to the benefit of the trademark owner, with no intention of infringement of the trademark. 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You will need to answer questions pertaining to this book in order to successfully down- load the code. 066FM 7/15/02 5:16 PM Page ii Contents at a Glance About the Author . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .xv About the Technical Reviewers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .xvi Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .xvii Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .xix Chapter 1 Instant Hacking:The Basics . . . . . . . . . . . . . . . . . . . . . . . . . . . .1 Chapter 2 Lists and Tuples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .37 Chapter 3 Working with Strings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .63 Chapter 4 Dictionaries: When Indices Won’t Do . . . . . . . . . . . . . . . . .79 Chapter 5 Conditionals, Loops, and Some Other Statements . . . .96 Chapter 6 Abstraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .129 Chapter 7 More Abstraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .167 Chapter 8 Exceptions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .193 Chapter 9 Magic Methods, Properties, and Iterators . . . . . . . . . .209 Chapter 10 Batteries Included . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .245 Chapter 11 Files and Stuff . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .303 Chapter 12 Graphical User Interfaces . . . . . . . . . . . . . . . . . . . . . . . . . . .325 Chapter 13 Playful Programming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .349 Chapter 14 Project 1: Instant Markup . . . . . . . . . . . . . . . . . . . . . . . . . . .365 Chapter 15 Project 2: Painting a Pretty Picture . . . . . . . . . . . . . .387 Chapter 16 Project 3: XML for All Occasions . . . . . . . . . . . . . . . . . . .401 Chapter 17 Project 4: In the News . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .421 v 066FM 7/15/02 5:16 PM Page v Chapter 18 Project 5: A Virtual Tea Party . . . . . . . . . . . . . . . . . . . . .439 Chapter 19 Project 6: Remote Editing with CGI . . . . . . . . . . . . . . . . .461 Chapter 20 Project 7: Your Own Bulletin Board . . . . . . . . . . . . . . . . .481 Chapter 21 Project 8:File Sharing with XML-RPC . . . . . . . . . . . . . . .501 Chapter 22 Project 9: File Sharing II—Now with GUI! . . . . . . . . . .525 Chapter 23 Project 10: Do-It-Yourself Arcade Game . . . . . . . . . . . .535 Appendix A The Short Version . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .559 Appendix B Python Reference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .571 Appendix C Online Resources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .589 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .593 vi Contents at a Glance 066FM 7/15/02 5:16 PM Page vi CHAPTER 5 Conditionals, Loops, and Some Other Statements BYNOW,I’MSUREyou are getting a bit impatient. All right—all these data types are just dandy, but you can’t really do much with them, can you? Let’s crank up the pace a bit. You’ve already encountered a couple of state- ment types (print statements, import statements, assignments). Let’s first take a look at some more ways to use these before diving into the world of condition- als and loops. Then, you’ll see how list comprehensions work almost like conditionals and loops, even though they are expressions, and finally you take a look at pass, del, and exec. More About print and import As you learn more about Python, you may notice that some aspects of Python that you thought you knew have hidden features just waiting to pleasantly sur- prise you. Let’s take a look at a couple of such nice features in print and import. Printing with Commas You’ve seen how print can be used to print an expression, which is either a string or is automatically converted to one. But you can actually print more than one expression, as long as you separate them with commas: >>> print ‘Age:’, 42 Age: 42 As you can see, a space character is inserted between each argument. 95 066ch05 7/12/02 11:14 AM Page 95 If you add a comma at the end, your next print statement will continue printing on the same line. For instance, the statements print ‘Hello,’, print ‘world!’ print out Hello, world! This behavior can be very useful if you want to combine text and variable values without using the full power of string formatting: >>> name = ‘Gumby’ >>> salutation = ‘Mr.’ >>> greeting = ‘Hello,’ >>> print greeting, salutation, name Hello, Mr. Gumby 96 Chapter 5 NOTE The arguments of print do not form a tuple, as one might expect: >>> 1, 2, 3 (1, 2, 3) >>> print 1, 2, 3 1 2 3 >>> print (1, 2, 3) (1, 2, 3) NOTE If the greeting string had no comma, how would you get the comma in the result? You couldn’t just use print greeting, ‘,’, salutation, name because that would introduce a space before the comma. One solution would be the following: print greeting + ‘,’, salutation, name Here the comma is simply added to the greeting. 066ch05 7/12/02 11:14 AM Page 96 Importing Something as Something Else Usually when you import something from a module you either use import somemodule or from somemodule import somefunction or from somemodule import * The latter should only be used when you are certain that you want to import everything from the given module. But what if you have two modules each contain- ing a function called open, for instance—what do you do then? You could simply import the modules using the first form, and then use the functions as follows: But there is another option: You can add an as clause to the end and supply the name you want to use, either for the entire module: >>> import math as foobar >>> foobar.sqrt(4) 2.0 or for the given function: >>> from math import sqrt as foobar >>> foobar(4) 2.0 For the open functions you might use the following: from module1 import open as open1 from module2 import open as open2 97 Conditionals, Loops, and Some Other Statements 066ch05 7/12/02 11:14 AM Page 97 Assignment Magic The humble assignment statement also has a few tricks up its sleeve. Sequence Unpacking You’ve seen quite a few examples of assignments, both for variables and for parts of data structures (such as positions and slices in a list, or slots in a dictionary), but there is more. You can perform several different assignments simultaneously: >>> x, y, z = 1, 2, 3 >>> print x, y, z 1 2 3 Doesn’t sound useful? Well, you can use it to switch the contents of two variables: >>> x, y = y, x >>> print x, y, z 2 1 3 Actually, what I’m doing here is called “sequence unpacking”—I have a sequence of values, and I unpack it into a sequence of variables. Let me be more explicit: >>> values = 1, 2, 3 >>> values (1, 2, 3) >>> x, y, z = values >>> x 1 This is particularly useful when a function or method returns a tuple; let’s say that you want to retrieve (and remove) a random key-value pair from a dictionary. You can then use the popitem method, which does just that, returning the pair as a tuple. Then you can unpack the returned tuple directly into two variables: >>> scoundrel = {‘name’: ‘Robin’, ‘girlfriend’: ‘Marion’} >>> key, value = scoundrel.popitem() >>> key 98 Chapter 5 066ch05 7/12/02 11:14 AM Page 98 ‘girlfriend’ >>> value ‘Marion’ This allows functions to return more than one value, packed as a tuple, easily accessible through a single assignment. The sequence you unpack must have exactly as many items as the targets you list on the left of the = sign; otherwise Python raises an exception when the assignment is performed. Chained Assignments Chained assignments are used as a shortcut when you want to bind several vari- ables to the same value. This may seem a bit like the simultaneous assignments in the previous section, except that here you are only dealing with one value: x = y = somefunction() is the same as y = somefunction() x = y Note that the statements above may not be the same as x = somefunction() y = somefunction() For more information, see the section about the identity operator (is), later in this chapter. Augmented Assignments Instead of writing x = x + 1 you can just put the expression operator (in this case +) before the assignment operator (=) and write x += 1.This is called an aug- mented assignment, and it works with all the standard operators, such as *, /, %, and so on: >>> x = 2 >>> x += 1 >>> x *= 2 >>> x 6 99 Conditionals, Loops, and Some Other Statements 066ch05 7/12/02 11:14 AM Page 99 It also works with other data types: >>> fnord = ‘foo’ >>> fnord += ‘bar’ >>> fnord ‘foobar’ Augmented assignments can make your code more compact and concise, yet some argue that it can also make it harder to read. 100 Chapter 5 TIP In general, you should not use += with strings, espe- cially if you are building a large string piece by piece in a loop (see the section “Loops” later in this chapter for more infor- mation about loops). Each addition and assignment needs to create a new string, and that takes time, making your pro- gram slower. A much better approach is to append the small strings to a list, and use the string method join to create the big string when your list is finished. Blocks: The Joy of Indentation This isn’t really a type of statement but something you’re going to need when you tackle the next two sections. A block is a group of statements that can be executed if a condition is true (conditional statements), or executed several times (loops), and so on. A block is created by indenting a part of your code; that is, putting spaces in front of it. NOTE You can use tab characters to indent your blocks as well. Python interprets a tab as moving to the next tab stop, with one tab stop every eight spaces, but the standard and preferable style is to use spaces only, no tabs, and specifically four spaces per each level of indentation. Each line in a block must be indented by the same amount. The following is pseudocode (not real Python code) but shows how the indenting works: this is a line this is another line: this is another block continuing the same block 066ch05 7/12/02 11:14 AM Page 100 The following values are considered by the interpreter to mean false: None 0 “” () [] {} That is, the standard value None, numeric zero of all types (including float, long, and so on), all empty sequences (such as empty strings, tuples, and lists), and empty dictionaries. Everything else is interpreted as true. Laura Creighton describes this as discerning between something and nothing, rather than true and false. the last line of this block phew, there we escaped the inner block In many languages a special word or character (for example, “begin” or “{”) is used to start a block, and another (such as “end” or “}”) is used to end it. In Python, a colon (“:”) is used to indicate that a block is about to begin, and then every line in that block is indented (by the same amount). When you go back to the same amount of indentation as some enclosing block, you know that the cur- rent block has ended. Now I’m sure you are curious to know how to use these blocks. So, without further ado, let’s have a look. Conditions and Conditional Statements Until now you’ve only written programs in which each statement is executed, one after the other. It’s time to move beyond that and let your program choose whether or not to execute a block of statements. So That’s What Those Boolean Values Are For Now you are finally going to need those truth values we’ve been bumping into repeatedly. 101 Conditionals, Loops, and Some Other Statements NOTE If you’ve been paying close attention, you noticed the sidebar in Chapter 1, “Sneak Peek: The if Statement,” which describes the if statement. I haven’t really introduced it for- mally until now, and as you’ll see, there is a bit more to it than what I’ve told you so far. 066ch05 7/12/02 11:14 AM Page 101 Conditional Execution and the if Statement Truth values can be combined (which you’ll see in a while), but let’s first see what you can use them for. Try running the following script: name = raw_input(‘What is your name? ‘) if name.endswith(‘Gumby’): print ‘Hello, Mr. Gumby’ This is the if statement, which lets you do conditional execution. That means that if the condition (the expression after if but before the colon) evaluates to true (as defined previously), the following block (in this case, a single print state- ment) is executed. If the condition is false, then the block is not executed (but you guessed that, didn’t you?). Got it? This means that every value in Python can be interpreted as a truth value, which can be a bit confusing at first, but it can also be extremely useful. And even though you have all these truth values to choose from, the “standard” truth values are 0 (for false) and 1 (for true). 102 Chapter 5 NOTE In Python 2.3 a separate Boolean type is introduced, with the values True and False, which are basically equiva- lent to 1 and 0, but will eventually replace them as “standard” truth values. NOTE In the sidebar “Sneak Peek: The if Statement” in Chapter 1, the statement was written on a single line. That is equivalent to using a single-line block, as in the preceding example. else Clauses In the example from the previous section, if you enter a name that ends with “Gumby,” the method name.endswith returns 1, making the if statement enter the block, and the greeting is printed. If you want, you can add an alternative, with the else clause (called a “clause” because it isn’t really a separate statement, just a part of the if statement): 066ch05 7/12/02 11:14 AM Page 102 name = raw_input(‘What is your name? ‘) if name.endswith(‘Gumby’): print ‘Hello, Mr. Gumby’ else: print ‘Hello, stranger’ Here, if the first block isn’t executed (because the condition evaluated to false), you enter the second block instead. This really makes you see how easy it is to read Python code, doesn’t it? Just read the code aloud (from if) and it sound just like a normal (or perhaps not quite normal) sentence. elif Clauses If you want to check for several conditions, you can use elif, which is short for “else if.” It is a combination of an if clause and an else clause—an else clause with a condition: num = input(‘Enter a number: ‘) if num > 0: print ‘The number is positive’ elif num < 0: print ‘The number is negative’ else: print ‘The number is zero’ Nesting Blocks Let’s throw in a few bells and whistles. You can have if statements inside other if statement blocks, as follows: name = raw_input(‘What is your name? ‘) if name.endswith(‘Gumby’): if name.startswith(‘Mr.’): print ‘Hello, Mr. Gumby’ elif name.startswith(‘Mrs.’): print ‘Hello, Mrs. Gumby’ else: print ‘Hello, Gumby’ else: print ‘Hello, stranger’ 103 Conditionals, Loops, and Some Other Statements 066ch05 7/12/02 11:14 AM Page 103 Here, if the name ends with “Gumby,” you check the start of the name as well—in a separate if statement inside the first block. Note the use of elif here. The last alternative (the else clause) has no condition—if no other alternative is chosen, you use the last one. If you want to, you can leave out either of the else clauses. If you leave out the inner else clause, names that don’t start with either “Mr.” or “Mrs.” are ignored (assuming the name was “Gumby”). If you drop the outer else clause, strangers are ignored. More Complex Conditions That’s really all there is to know about if statements. Now let’s return to the con- ditions themselves because they are the really interesting part of conditional execution. Comparison Operators Perhaps the most basic operators used in conditions are the comparison opera- tors. They are used (surprise, surprise) to compare things. The comparison operators are summed up in Table 5-1. Table 5-1. The Python Comparison Operators EXPRESSION DESCRIPTION x == yxequals y x < yxis less than y x > yxis greater than y x >= yxis greater than or equal to y x <= yxis less than or equal to y x != yxis not equal to y x is yxand y are the same object x is not yxand y are different objects x in yxis a member of the container (e.g., sequence) y x not in yx is not a member of the container (e.g., sequence) y Comparisons can be chained in Python, just like assignments—you can put several comparison operators in a chain, like this: 0 < age < 100. 104 Chapter 5 066ch05 7/12/02 11:14 AM Page 104 Some of these operators deserve some special attention and will be described in the following sections. The Equality Operator If you want to know if two things are equal, you use the equality operator, written as a double equality sign, ==: >>> “foo” == “foo” 1 >>> “foo” == “bar” 0 Double? Why can’t you just use a single equality sign, like they do in mathe- matics? I’m sure you’re clever enough to figure this out for yourself, but let’s try it: >>> “foo” = “foo” SyntaxError: can’t assign to literal The single equality sign is the assignment operator, which is used to change things, which is not what you want to do when you compare things. is: The Identity Operator The is operator is interesting. It seems to work just like ==, but it doesn’t: >>> x = y = [1, 2, 3] >>> z = [1, 2, 3] >>> x == y 1 >>> x == z 1 >>> x is y 1 >>> x is z 0 105 Conditionals, Loops, and Some Other Statements TIP When comparing things, you can also use the built-in function cmp as described in Chapter 2. 066ch05 7/12/02 11:14 AM Page 105 Until the last example, this looks fine, but then you get that strange result, that x is not z even though they are equal. Why? Because is tests for identity, rather than equality. The variables x and y have been bound to the same list, while z is simply bound to another list that happens to contain the same values in the same order. They may be equal, but they aren’t the same object. Does that seem unreasonable? Consider this example: >>> x = [1, 2, 3] >>> y = [2, 4] >>> x is not y 1 >>> del x[2] >>> y[1] = 1 >>> y.reverse() In this example, I start with two different lists, x and y. As you can see, x is not y (just the inverse of x is y), which you already know. I change the lists around a bit, and though they are now equal, they are still two separate lists: >>> x == y 1 >>> x is y 0 Here it is obvious that the two lists are equal but not identical. To summarize: Use == to see if two objects are equal, and use is to see if they are identical (the same object). 106 Chapter 5 CAUTION Avoid the use of is with basic, immutable values such as numbers and strings. The result is unpredictable because of the way Python handles these internally. in: The Membership Operator I have already introduced the in operator (in Chapter 2, in the section “Membership”). It can be used in conditions, just like all the other comparison operators: name = raw_input(‘What is your name? ‘) if ‘s’ in name: 066ch05 7/12/02 11:14 AM Page 106 print ‘Your name contains the letter “s”.’ else: print ‘Your name does not contain the letter “s”.’ Comparing Strings and Sequences Strings are compared according to their order when sorted alphabetically: >>> “alpha” < “beta” 1 If you throw in capital letters, things get a bit messy. (Actually, characters are sorted by their ordinal values. The ordinal value of a letter can be found with the ord function, whose inverse is chr.) To avoid this, use the string methods upper or lower: >>> ‘FnOrD’.lower() == ‘Fnord’.lower() 1 Other sequences are compared in the same manner, except that instead of letters you have other types of elements: >>> [1, 2] < [2, 1] 1 If the sequences contain lists as elements, the same rule applies to these sublists: >>> [2, [1, 4]] < [2, [1, 5]] 1 Boolean Operators Now, you’ve got plenty of things that return truth values. (Given the fact that all values can be interpreted as truth values, all expressions return them.) But you may want to check for more than one condition. For instance, let’s say you want to write a program that reads a number and checks whether it’s between 1 and 10 (inclusive). You can do it like this: number = input(‘Enter a number between 1 and 10: ‘) if number <= 10: 107 Conditionals, Loops, and Some Other Statements 066ch05 7/12/02 11:14 AM Page 107 if number >= 1: print ‘Great!’ else: print ‘Wrong!’ else: print ‘Wrong!’ This will work, but it’s clumsy. The fact that you have to write print ‘Wrong!’ in two places should alert you to this clumsiness. Duplication of effort is not a good thing. So what do you do? It’s so simple: if number <= 10 and number >= 1: print ‘Great!’ else: print ‘Wrong!’ 108 Chapter 5 NOTE In this example, you could (and quite probably should) have made this even simpler by using the following chained comparison: 1 <= number <= 10 The and operator is a so-called Boolean operator (named after George Boole, who did a lot of smart stuff on truth values, also called logical or Boolean values). It takes two truth values, and returns true if both are true, and false otherwise. You have two more of these operators, or and not. With just these three, you can com- bine truth values in any way you like: if ((cash > price) or customer_has_good_credit) and not out_of_stock: give_goods() 066ch05 7/12/02 11:14 AM Page 108 Short-Circuit Logic The Boolean operators have one interesting property: They only evaluate what they need to. For instance, the expression x and y requires both x and y to be true; so if x is false, the expression returns false immediately, without worrying about y. Actually, if x is false, it returns x—otherwise it returns y. (Can you see how this gives the expected meaning?) This behavior is called short-circuit logic: the Boolean operators are often called logical operators, and as you can see, the second value is sometimes “short- circuited.” This works with or, too. In the expression x or y, if x is true, it is returned, otherwise y is returned. (Can you see how this makes sense?) So, how is this useful? Let’s say a user is supposed to enter his or her name, but may opt to enter nothing, in which case you want to use the default value ‘’. You could use an if statement, but you could also state things very succinctly: name = raw_input(‘Please enter your name: ‘) or ‘’ In other words, if the return value from raw_input is true (not an empty string) it is assigned to name (nothing changes); otherwise, the default ‘’ is assigned to name. This sort of short-circuit logic can be used to implement the so-called “ternary operator” (or conditional operator), found in languages such as C and Java. For a thorough explanation, see Alex Martelli’s recipe on the sub- ject in the Python Cookbook ( Python/Recipe/52310). Assertions There is a useful relative of the if statement, which works more or less like this (pseudocode): if not condition: crash program Now, why on earth would you want something like that? Simply because it’s better that your program crashes when an error condition emerges than at a much later time. Basically, you can require that certain things be true. The key- word used in the statement is assert: >>> age = 10 >>> assert 0 < age < 100 >>> age = -1 109 Conditionals, Loops, and Some Other Statements 066ch05 7/12/02 11:14 AM Page 109 >>> assert 0 < age < 100 Traceback (most recent call last): File “”, line 1, in ? AssertionError It can be useful to put the assert statement in your program as a checkpoint, if you know something has to be true for your program to work correctly. A string may be added after the condition, to explain the assertion: >>> age = -1 >>> assert 0 < age < 100, ‘The age must be realistic’ Traceback (most recent call last): File “”, line 1, in ? AssertionError: The age must be realistic Loops Now you know how to do something if a condition is true (or false), but how do you do something several times? For instance, you might want to create a pro- gram that reminds you to pay the rent every month, but with the tools we have looked at until now, you’d have to write the program like this (pseudocode): send mail wait one month send mail wait one month send mail wait one month (...and so on) But what if you wanted it to continue doing this until you stopped it? Basically, you want something like this (again, pseudocode): while we aren’t stopped: send mail wait one month Or, let’s take a simpler example. Let’s say that you want to print out all the numbers from 1 to 100. Again, you could do it the stupid way: print 1 print 2 print 3 110 Chapter 5 066ch05 7/12/02 11:14 AM Page 110 …and so on. But you didn’t start using Python because you wanted to do stupid things, right? while Loops In order to avoid the cumbersome code of the preceding example, it would be useful to be able to do something like this: x = 1 while x <= 100: print x x += 1 Now, how do you do that in Python? You guessed it—you do it just like that. Not that complicated is it? You could also use a loop to ensure that the user enters a name, as follows: name = ‘’ while not name: name = raw_input(‘Please enter your name: ‘) print ‘Hello, %s!’ % name Try running this, and then just pressing the Enter key when asked to enter your name: the question appears again because name is still an empty string, which evaluates to false. 111 Conditionals, Loops, and Some Other Statements TIP What would happen if you entered just a space charac- ter as your name? Try it. It is accepted because a string with one space character is not empty, and therefore not false. This is definitely a flaw in our little program, but easily corrected: just change while not name to while name.isspace(). for Loops The while statement is very flexible. It can be used to repeat a block of code while any condition is true. While this may be very nice in general, sometimes you may want something tailored to your specific needs. One such need is to perform a block of code for each element of a set (or, actually, sequence) of values. You can do this with the for statement: 066ch05 7/12/02 11:14 AM Page 111 words = [‘this’, ‘is’, ‘an’, ‘ex’, ‘parrot’] for word in words: print word Or... range = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9] for number in range: print number Because iterating (another word for “looping”) over a range of numbers is a common thing to do, there is a built-in function to make ranges for you: >>> range(0, 10) [0, 1, 2, 3, 4, 5, 6, 7, 8, 9] Ranges work like slices. They include the first limit (in this case 0), but not the last (in this case 10). Quite often, you want the ranges to start at 0, and this is actually assumed if you only supply one limit (which will then be the last): >>> range(10) [0, 1, 2, 3, 4, 5, 6, 7, 8, 9] 112 Chapter 5 TIP There is also another function called xrange that works just like range in loops, but where range creates the whole sequence at once, xrange creates only one number at a time. This can be useful when iterating over huge sequences more efficiently, but in general you needn’t worry about it. The following program writes out the numbers from 1 to 100: for number in range(1,101): print number Notice that this is much more compact than the while loop I used earlier. 066ch05 7/12/02 11:14 AM Page 112 Iterating over Dictionaries To loop over the keys of a dictionary, you can use a plain for statement, just as you can with sequences: d = {‘x’: 1, ‘y’: 2, ‘z’: 3} for key in d: print key, ‘corresponds to’, d[key] In Python versions before 2.2, you would have used a dictionary method such as keys to retrieve the keys (since direct iteration over dictionaries wasn’t allowed). If only the values were of interest, you could have used d.values instead of d.keys. You may remember that d.items returns key-value pairs as tuples. One great thing about for loops is that you can use sequence unpacking in them: for key, value in d.items(): print key, ‘corresponds to’, value To make your iteration more efficient, you can use the methods iterkeys (equivalent to the plain for loop), itervalues, or iteritems. (These don’t return lists, but iterators. Iterators are explained in Chapter 9, “Magic Methods and Iterators.”) 113 Conditionals, Loops, and Some Other Statements TIP If you can use a for loop rather than a while loop, you should probably do so. NOTE As always, the order of dictionary elements is unde- fined. In other words, when iterating over either the keys or the values of a dictionary, you can be sure that you’ll process all of them, but you can’t know in which order. If the order is important, you can store the keys or values in a separate list and sort it before iterating over it. 066ch05 7/12/02 11:14 AM Page 113 Parallel Iteration Sometimes you want to iterate over two sequences at the same time. Let’s say that you have the following two lists: names = [‘anne’, ‘beth’, ‘george’, ‘damon’] ages = [12, 45, 32, 102] If you want to print out names with corresponding ages, you could do the following: for i in range(len(names)): print names[i], ‘is’, ages[i], ‘years old’ Here I use i as a standard variable name for loop indices (as these things are called). A useful tool for parallel iteration is the built-in function zip, which “zips” together the sequences, returning a list of tuples: >>> zip(names, ages) [(‘anne’, 12), (‘beth’, 45), (‘george’, 32), (‘damon’, 102)] Now I can unpack the tuples in my loop: for name, age in zip(names, ages): print name, ‘is’, age, ‘years old’ The zip function works with as many sequences as you want. It’s important to note what zip does when the sequences are of different lengths: it stops when the shortest sequence is “used up”: >>> zip(range(5), xrange(100000000)) [(0, 0), (1, 1), (2, 2), (3, 3), (4, 4)] I wouldn’t recommend using range instead of xrange in the preceding exam- ple—although only the first five numbers are needed, range calculates all the numbers, and that may take a lot of time. With xrange, this isn’t a problem because it calculates only those numbers needed. 114 Chapter 5 066ch05 7/12/02 11:14 AM Page 114 Breaking Out of Loops Usually, a loop simply executes a block until its condition becomes false, or until it has used up all sequence elements—but sometimes you may want to interrupt the loop, to start a new iteration (one “round” of executing the block), or to sim- ply end the loop. break To end (break out of) a loop, you use break. Let’s say you wanted to find the largest square (an integer that is the square of another integer) below 100. Then you start at 100 and iterate downwards to 0. When you’ve found a square, there’s no need to continue, so you simply break out of the loop: from math import sqrt for n in range(99, 0, -1): root = sqrt(n) if root == int(root): print n break If you run this program, it will print out 81, and stop. Notice that I’ve added a third argument to range—that’s the step, the difference between every pair of adjacent numbers in the sequence. It can be used to iterate downwards as I did here, with a negative step value, and it can be used to skip numbers: >>> range(0, 10, 2) [0, 2, 4, 6, 8] continue The continue statement is used less often than break. It causes the current itera- tion to end, and to “jump” to the beginning of the next. It basically means “skip the rest of the loop body, but don’t end the loop.” This can be useful if you have a large and complicated loop body and several possible reasons for skipping it— in that case you can use continue as follows: for x in seq: if condition1: continue if condition2: continue if condition3: continue 115 Conditionals, Loops, and Some Other Statements 066ch05 7/12/02 11:14 AM Page 115 do_something() do_something_else() do_another_thing() etc() In many cases, however, simply using an if statement is just as good: for x in seq: if not (condition1 or condition2 or condition3): do_something() do_something_else() do_another_thing() etc() Even though continue can be a useful tool, it is not essential. The break state- ment, however, is something you should get used to because it is used quite often in concert with while 1, as explained in the next section. The while 1/break Idiom The while and for loops in Python are quite flexible, but every once in a while you may encounter a problem that makes you wish you had more functionality. For instance, let’s say you want to do something while a user enters words at a prompt, and you want to end the loop when no word is provided. One way of doing that would be word = ‘dummy’ while word: word = raw_input(‘Please enter a word: ‘) # do something with the word: print ‘The word was ‘ + word Here is an example session: Please enter a word: first The word was first Please enter a word: second The word was second Please enter a word: 116 Chapter 5 066ch05 7/12/02 11:14 AM Page 116 The while 1 part gives you a loop that will never terminate by itself. Instead you put the condition in an if statement inside the loop, which calls break when the condition is fulfilled. Thus you can terminate the loop anywhere inside the loop instead of only at the beginning (as with a normal while loop). The if/break line splits the loop naturally in two parts: The first takes care of setting things up (the part that would be duplicated with a normal while loop), and the other part makes use of the initialization from the first part, provided that the loop con- dition is true. Although you should be wary of using break too often (because it can make your loops harder to read), this specific technique is so common that most Python programmers (including yourself) will probably be able to follow your intentions. This works just like you want it to. (Presumably you’d do something more useful with the word than print it out, though.) However, as you can see, this code is a bit ugly. To enter the loop in the first place, you have to assign a dummy (unused) value to word. Dummy values like this are usually a sign that you aren’t doing things quite right. Let’s try to get rid of it: word = raw_input(‘Please enter a word: ‘) while word: # do something with the word: print ‘The word was ‘ + word word = raw_input(‘Please enter a word: ‘) Here the dummy is gone, but I have repeated code (which is also a bad thing): I have to use the same assignment and call to raw_input in two places. How can I avoid that? I can use the while 1/break idiom: while 1: word = raw_input(‘Please enter a word: ‘) if not word: break # do something with the word: print ‘The word was ‘ + word 117 Conditionals, Loops, and Some Other Statements NOTE An idiom is a common way of doing things that people who know the language are assumed to know. 066ch05 7/12/02 11:14 AM Page 117 else Clauses in Loops When you use break statements in loops, it is often because you have “found” something, or because something has “happened.” It’s easy to do something when you break out (like print n), but sometimes you may want to do something if you didn’t break out. But how do you find out? You could use a Boolean vari- able, set it to 0 before the loop, and set it to 1 when you break out. Then you can use an if statement afterwards to check whether you did break out or not: broke_out = 0 for x in seq: do_something(x) if condition(x): broke_out = 1 break do_something_else(x) if not broke_out: print “I didn’t break out!” A simpler way is to add an else clause to your loop—it is only executed if you didn’t call break. Let’s reuse the example from the preceding section on break: from math import sqrt for n in range(99, 81, -1): root = sqrt(n) if root == int(root): print n break else: print “Didn’t find it!” Notice that I changed the lower (exclusive) limit to 81 to test the else clause. If you run the program, it prints out “Didn’t find it!” because (as you saw in the section on break) the largest square below 100 is 81. You can use continue, break, and else clauses both with for loops and while loops. List Comprehension—Slightly Loopy List comprehension is a way of making lists from other lists (similar to set com- prehension, if you know that term from mathematics). It works in a way similar to for loops, and is actually quite simple: 118 Chapter 5 066ch05 7/12/02 11:14 AM Page 118 >>> [x*x for x in range(10)] [0, 1, 4, 9, 16, 25, 36, 49, 64, 81] The list is composed of x*x for each x in range(10). Pretty straightforward? What if you only want to print out those squares that are divisible by 3? Then you can use the modulo operator—y % 3 returns zero when y is divisible by 3. (Note that x*x is divisible by 3 only if x is divisible by 3.) You put this into your list com- prehension by adding an if part to it: >>> [x*x for x in range(10) if x % 3 == 0] [0, 9, 36, 81] You can also add more for parts: >>> [(x, y) for x in range(3) for y in range(3)] [(0, 0), (0, 1), (0, 2), (1, 0), (1, 1), (1, 2), (2, 0), (2, 1), (2, 2)] This can be combined with an if clause, just like before: >>> girls = [‘alice’, ‘bernice’, ‘clarice’] >>> boys = [‘chris’, ‘arnold’, ‘bob’] >>> [b+’+’+g for b in boys for g in girls if b[0] == g[0]] [‘chris+clarice’, ‘arnold+alice’, ‘bob+bernice’] This gives the pairs of boys and girls who have the same initial letter in their first name. A Better Solution The boy/girl pairing example isn’t particularly efficient because it checks every possible pairing. There are many ways of solving this problem in Python. The following was suggested by Alex Martelli: girls = [‘alice’, ‘bernice’, ‘clarice’] boys = [‘chris’, ‘arnold’, ‘bob’] letterGirls = {} for girl in girls: letterGirls.setdefault(girl[0], []).append(girl) print [b+’+’+g for b in boys for g in letterGirls[b[0]]] This program constructs a dictionary called letterGirl where each entry has a single letter as its key and a list of girls’ names as its value. (The setdefault dictionary method is described in the previous chapter.) After this dictionary 119 Conditionals, Loops, and Some Other Statements 066ch05 7/12/02 11:14 AM Page 119 has been constructed, the list comprehension loops over all the boys and looks up all the girls whose name begins with the same letter as the current boy. This way the list comprehension doesn’t have to try out every possible combination of boy and girl and check whether the first letters match. And Three for the Road To end the chapter, let’s take a quick look at three more statements: pass, del, and exec. Nothing Happened! Sometimes you need to do nothing. This may not be very often, but when it hap- pens, it’s good to know that you have the pass statement: >>> pass >>> Not much going on here. Now, why on earth would you want a statement that does nothing? It can be useful as a placeholder while you are writing code. For instance, you may have written an if statement and you want to try it, but you lack the code for one of your blocks. Consider the following: if name == ‘Ralph Auldus Melish’: print ‘Welcome!’ elif name == ‘Enid’: # Not finished yet... elif name == ‘Bill Gates’: print ‘Access Denied’ This code won’t run because an empty block is illegal in Python. To fix this, simply add a pass statement to the middle block: if name == ‘Ralph Auldus Melish’: print ‘Welcome!’ elif name == ‘Enid’: # Not finished yet... pass 120 Chapter 5 066ch05 7/12/02 11:14 AM Page 120 Deleting with del In general, Python deletes objects that you don’t use anymore: >>> scoundrel = {‘age’: 42, ‘first name’: ‘Robin’, ‘last name’: ‘of Locksley’} >>> robin = scoundrel >>> scoundrel {‘age’: 42, ‘first name’: ‘Robin’, ‘last name’: ‘of Locksley’} >>> robin {‘age’: 42, ‘first name’: ‘Robin’, ‘last name’: ‘of Locksley’} >>> scoundrel = None >>> robin {‘age’: 42, ‘first name’: ‘Robin’, ‘last name’: ‘of Locksley’} >>> robin = None At first, robin and scoundrel both contain (or “point to”) the same dictionary. So when I assign None to scoundrel, the dictionary is still available through robin. But when I assign None to robin as well, the dictionary suddenly floats around in the memory of the computer with no name attached to it. There is no way I can retrieve it or use it, so the Python interpreter (in its infinite wisdom) simply deletes it. (This is called “garbage collection.”) Note that I could have used any value other than None as well. The dictionary would be just as gone. Another way of doing this is to use the del statement (which we used to delete sequence and dictionary elements in Chapters 2 and 4, remember?). This not only removes a reference to an object, it also removes the name itself: >>> x = 1 >>> del x >>> x Traceback (most recent call last): elif name == ‘Bill Gates’: print ‘Access Denied’ 121 Conditionals, Loops, and Some Other Statements NOTE An alternative to the combination of a comment and a pass statement is to simply insert a string. This is especially useful for unfinished functions (see Chapter 6) and classes (see Chapter 7) because they will then act as “docstrings” (explained in Chapter 6). 066ch05 7/12/02 11:14 AM Page 121 File “”, line 1, in ? x NameError: name ‘x’ is not defined This may seem easy, but it can actually be a bit tricky to understand at times. For instance, in the following example, x and y refer to the same list: >>> x = [“Hello”, “world”] >>> y = x >>> y[1] = “Python” >>> x [‘Hello’, ‘Python’] You might assume that by deleting x, you would also delete y, but that is not the case: >>> del x >>> y [‘Hello’, ‘Python’] Why is this? x and y referred to the same list, but deleting x didn’t affect y at all. The reason for this is that you only delete the name, not the list itself (the value). In fact, there is no way to delete values in Python (and you don’t really need to because the Python interpreter does it by itself whenever you don’t use the value anymore). Executing and Evaluating Strings with exec and eval Sometimes you may want to create Python code “on the fly” and execute it as a statement or evaluate it as an expression. This may border on dark magic at times—consider yourself warned. 122 Chapter 5 CAUTION In this section, you learn to execute Python code stored in a string. This is a potential security hole of great dimensions. If you execute a string where parts of the con- tents have been supplied by a user, you have little or no control over what code you are executing. This is especially dangerous in network applications, such as CGI scripts, which you will learn about in Chapter 19. 066ch05 7/12/02 11:14 AM Page 122 exec The statement for executing a string is exec: >>> exec “print ‘Hello, world!’” Hello, world! However, using this simple form of the exec statement is rarely a good thing; in most cases you want to supply it with a namespace, a place where it can put its vari- ables. You want to do this so that the code doesn’t corrupt your namespace (that is, change your variables). For instance, let’s say that the code uses the name sqrt: >>> from math import sqrt >>> exec “sqrt = 1” >>> sqrt(4) Traceback (most recent call last): File “”, line 1, in ? sqrt(4) TypeError: object is not callable: 1 Well, why would you do something like that in the first place, you ask? The exec statement is mainly useful when you build the code string on the fly. And if the string is built from parts that you get from other places, and possibly from the user, you can rarely be certain of exactly what it will contain. So to be safe, you give it a dictionary, which will work as a namespace for it. 123 Conditionals, Loops, and Some Other Statements NOTE The concept of namespaces, or scopes, is a very impor- tant one.You will look at it in depth in the next chapter, but for now you can think of a namespace as a place where you keep your variables, much like an invisible dictionary. So when you execute an assignment like x = 1, you store the key x with the value 1 in the current namespace, which will often be the global namespace (which we have been using, for the most part, up until now), but doesn’t have to be. 066ch05 7/12/02 11:14 AM Page 123 You do this by adding in scope, where scope is some dictionary that will function as the namespace for your code string: >>> from math import sqrt >>> scope = {} >>> exec ‘sqrt = 1’ in scope >>> sqrt(4) 2.0 >>> scope[‘sqrt’] 1 Now you have full control over the variables that are changed by the code because they are all kept inside scope. If you try to print out scope, you see that it contains a lot of stuff because the dictionary called __builtins__ is automatically added and contains all built-in functions and values: >>> len(scope) 2 >>> scope.keys() [‘sqrt’, ‘__builtins__’] eval A built-in function that is similar to exec is eval (for “evaluate”). Just as exec exe- cutes a series of Python statements, eval evaluates a Python expression (written in a string) and returns the value. (exec doesn’t return anything because it is a state- ment itself.) For instance, you can use the following to make a Python calculator: >>> eval(raw_input(“Enter an arithmetic expression: “)) Enter an arithmetic expression: 6 + 18 * 2 42 124 Chapter 5 NOTE The expression eval(raw_input(...)) is, in fact, equiva- lent to input(...). 066ch05 7/12/02 11:14 AM Page 124 You can supply a namespace with eval, just as with exec, although expressions rarely rebind variables in the way statements usually do. 125 Conditionals, Loops, and Some Other Statements CAUTION Even though expressions don’t rebind variables as a rule, they certainly can (for instance by calling functions that rebind global variables). Therefore, using eval with an untrusted piece of code is no safer than using exec.For a more secure alternative, see the standard library modules rexec and Bastion, mentioned in Chapter 10. Priming the Scope When supplying a namespace for exec or eval, you can also put some values in before actually using the namespace: >>> scope = {} >>> scope[‘x’] = 2 >>> scope[‘y’] = 3 >>> eval(‘x * y’, scope) 6 In the same way, a scope from one exec or eval call can be used again in another one: >>> scope = {} >>> exec ‘x = 2’ in scope >>> eval(‘x*x’, scope) 4 Actually, exec and eval are not used all that often, but they can be nice tools to keep in your back pocket (figuratively, of course). A Quick Summary In this chapter you’ve seen several kinds of statements: Printing. You can use the print statement to print several values by sepa- rating them with commas. If you end the statement with a comma, later print statements will continue printing on the same line. 066ch05 7/12/02 11:14 AM Page 125 Importing. Sometimes you don’t like the name of a function you want to import—perhaps you’ve already used the name for something else. You can use the statement, to locally rename a function. Assignments. You’ve seen that through the wonder of sequence unpacking and chained assignments, you can assign values to several variables at once, and that with augmented assignments you can change a variable in place. Blocks. Blocks are used as a means of grouping statements through inden- tation. They are used in conditionals and loops, and as you see later in the book, in function and class definitions, among other things. Conditionals. A conditional statement either executes a block or not, depending on a condition (Boolean expression). Several conditionals can be strung together with if/elif/else. Assertions. An assertion simply asserts that something (a Boolean expression) is true, optionally with a string explaining why it has to be so. If the expression happens to be false, the assertion brings your program to a halt (or actually raises an exception—more on that in Chapter 8). It’s bet- ter to find an error early than to let it sneak around your program until you don’t know where it originated. Loops. You either can execute a block for each element in a sequence (such as a range of numbers) or continue executing it while a condition is true. To skip the rest of the block and continue with the next iteration, use the continue statement; to break out of the loop, use the break statement. Optionally, you may add an else clause at the end of the loop, which will be executed if you didn’t execute any break statements inside the loop. List comprehension. These aren’t really statements—they are expressions that look a lot like loops, which is why I grouped them with the looping statements. Through list comprehension you can build new lists from old ones, applying functions to the elements, filtering out those you don’t want, and so on. The technique is quite powerful, but in many cases using plain loops and conditionals (which will always get the job done) may be more readable. pass, del, exec, and eval. The pass statement does nothing, which can be useful as a placeholder, for instance. The del statement is used to delete variables or parts of a datastructure, but cannot be used to delete values. The exec statement is used to execute a string as if it were a Python program. The built-in function eval evaluates an expression written in a string and returns the result. 126 Chapter 5 066ch05 7/12/02 11:14 AM Page 126 New Functions in This Chapter FUNCTION DESCRIPTION chr(n) Returns a one-character string with ordinal n (0 ≤ n ≤ 256) eval(source[, globals[, locals]]) Evaluates a string as an expression and returns the value ord(c) Returns the integer ordinal value of a one- character string range([start,] stop[, step]) Creates a list of integers xrange([start,] stop[, step]) Creates an xrange object, used for iteration zip(seq1, seq2,...) Creates a new sequence suitable for parallel iteration What Now? Now you’ve cleared the basics. You can implement any algorithm you can dream up; you can read in parameters and print out the results. In the next couple of chapters, you learn about something that will help you write larger programs without losing the big picture. That something is called abstraction. 127 Conditionals, Loops, and Some Other Statements 066ch05 7/12/02 11:14 AM Page 127 066ch05 7/12/02 11:14 AM Page 128 Numbers and Symbols = (assignment operator, using in vari- ables, 20 * (asterisk) using as the width or precision (or both), 67 using when collecting parameters, 144–147 += (augmented assignment), making code more compact and con- cise with, 99–100 \ (backslash) character using in Python code, 246 using to escape special characters, 280 using to escape the quotes in a string, 27 \ \; (backticks), converting a Python value to a string with, 29 “ “ (blank space), for putting a blank in front of positive numbers, 68 ^ (caret), using for pattern matching, 282 : (colon) separating the start and stop index of results with, 561 use of in Python statements, 17 using to indicate a block is to begin, 101 , (commas), printing with, 95–96 %s (conversion specifiers) as modulus operator, 11 in strings, 64 $ (dollar sign), using to mark the end of a string for pattern matching, 282 “.” (dot) conversion specifier for specifying number of decimal places in strings, 65 as wildcard in regular expressions, 279, 280 ** (double asterisk) operator gathering keyword arguments with, 146–147 using as exponentiation operator to calculate powers, 12 __ (double underscores), making meth- ods and attributes private with, 175–176 == (equality operator), using in Python, 17 / (forward slash) character, using as division operator, 10–11, 18 using in file paths in Python code, 246 # (hash sign), beginning comments with, 25, 132, 560 - (minus sign), conversion specifier, 65 as subtraction operator, 11 % (percent), using as string formatting operator, 63–64 %%, using to specify a percent sign, 64 | (pipe) character linking several commands together with, 307 using for character set alternatives, 281 + (plus sign) conversion specifier, 65 using for concatenating lists, 565 ‘+’ value, for mode argument of the open function, 304 #! (pound bang or shebang) character sequence adding to your CGI script, 463 using, 23 >>> (prompt). See Python prompt (>>>) ? (question mark) character using after a subpattern in character sets, 281–282 using to make repetition operator non-greedy, 290 0 (zero), conversion specifier, 65 A ‘a’ value, for mode argument of the open function, 304 abs function, using, 18 593 Index 066Index 7/15/02 5:30 PM Page 593 abspath function, getting the absolute path of the data directory with, 475 abstraction, 129–166 defined, 165 and structure, 130–131 accessor methods, Rectangle class code example, 223 Acrobat Reader (Adobe), Web site address for downloading, 387 ActivePython, Web site address, 7, 589 adding, as a sequence operation, 38 addition (plus sign) operator, concate- nating sequences with, 43 address book, using Python dictionaries for, 79–80 algorithm, 9, 34 alignment, conversion specifier, 68 __all__ variable, public interface for a module defined by, 256–257 allow_reuse_address attribute, setting for SimpleXMLRPCServer, 515 Anygui GUI toolkit downloading and installing, 329 getting started with, 329–330 Web site address for documentation, 336 what it is, 328 function, linking compo- nents with event handlers with, 347 append method, appending an object to end of list with, 51 Application class, 347 Application.add method, adding a win- dow to an application with, 347 method, starting the main event loop with, 347 apply function, function of, 164–165 arcade game project basic design process, 535 creating, 535–556 first implementation, 540–545 preparations for creating, 540 second implementation, 545–556 specific goals for creating, 536 useful tools for creating, 536–539 arithmetic sequence, code example of implementation of, 218 ArithmeticSequence class, using, 219–220 as clause, using to import something from a module, 97 assert keyword, used in Python programs as a checkpoint, 109–110 assertions, using, 109–110, 126 assignment, 14 assignment statements function of in Python, 581 tricks, 98–100 assignments, assigning values to several variables at once with, 126 asterisk (*), using as the width or preci- sion (or both), 67 async_chat class, overriding methods in, 445–446 asynchat module, using to create a chat server, 439 asyncore module subclassing the dispatcher class from, 443 using to create a chat server, 439 AttributeError class, built-in exception, 196 attributes making private, 175–176 setting to customize component in Anygui, 332–333 attributes, functions, and methods, 178–179 augmented assignments, using, 99–100 augmented assignment statements, function of in Python, 582 autoexec.bat file, editing PYTHONPATH in, 252–253 automated tests, using to catch bugs in programs, 356 B ‘b’ value, for mode argument of the open function, 304 backslash (\) character using in Python code, 246 using to escape the quotes in a string, 27 backticks (\\), converting objects to string representation with, 29, 565 backtracking, using to solve problems, 235–236 base case of recursive function, 155 using to solve the Eight Queens prob- lem, 237–239 using with recursion, 231 __bases__ attribute accessing for classes, 185 how it works, 186–187 basic (literal) values, 571 Bastion module, function of, 299 594 Index 066Index 7/15/02 5:30 PM Page 594 Beck, Ken, extreme programming (XP) created by, 349 binary files, adding arguments to code for processing, 304 binary mode, reasons for using, 305 binary search definition, 158 implementing, 159–160 bind method, calling to bind a socket to a specific port, 443–444 bisect module, that implements binary search, 160 blank space (“ “), for putting a blank in front of positive numbers, 68 blitting, defined, 537 blocks creating and using, 100–101 grouping statements through inden- tation with, 126 Boolean operators checking for more than one con- dition with, 107–108 Boolean values as truth values, 45 using in conditional statements, 101–102 bottom keyword argument, in Placer lay- out manager, 336 bound methods example of, 214–215 using like functions, 178–179 brackets ([]), using to create a character set, 280 break statement using to break out of a loop, 115 buffering, open function optional parameter for, 305 built-in functions, 18 list of some important in Python, 573–576 bulletin board project building your own, 481–499 edit.cgi script for, 493–495 first implementation, 484–489 main.cgi script for, 489–491 save.cgi script for, 495–496 second implementation, 489–496 specific goals for creating, 482 structuring the system, 489–490 trying it out after creating, 497–498 useful tools for creating, 482–483 view.cgi script for, 492–493 Button class, 347 buttons adding to a window, 331 setting the text of, 332–333 C callable function, defined, 131 Canvas class, methods for drawing, 390 caret (^), using to mark the beginning of a string for pattern matching, 282 catching an exception object, 200–201 ceil function, 19 CGI (Common Gateway Interface), remote editing of Web pages with, 461–480 CGI editor editing page of, 479 opening page of, 478 running, 478–479 cgi module using, 467–468 Web site address for description of, 462 CGI programs, putting in a directory for accessibility via the Web, 463 CGI script adding a pound bang (#!) line to, 463 for creating a simple HTML form, 469–470 first implementation, 465–473 invoking without forms, 469 preparations for making accessible and runnable, 462–465 process for getting the threaded look for replies, 487 second implementation, 473–479 setting the file permissions in, 463–464 specific goals for creating for remote editing, 462 splitting the functionality into several scripts, 473–478 CGI security risks, 465 cgitb module example of a CGI traceback from, 467 using for development of CGI scripts, 462 chained assignments, using, 99 character sets alternatives and subpatterns, 281 creating and inverting, 280 optional and repeated subpatterns, 281–282 using special characters in, 280–281 chat server adding support for identity and com- mand interpretation, 449–450 code for one that accepts con- nections, 443–444 595 Index 066Index 7/15/02 5:30 PM Page 595 596 Index chat server (continued) commands available in, 458 final version of, 454–458 first implementation, 443–449 main additions to new, 453 pointers for a multithreaded, 441–442 preparations for creating, 442–443 second implementation, 449–458 specific goals for creating, 440 useful tools for creating, 440–442 chmod command, for changing UNIX file permissions, 464 chr function, 127 clamp rectangle method, in arcade game project, 545 class, defined, 177 __class__ attribute, finding which class an object belongs to with, 186 class definitions, function of in Python, 587 class keyword, defining classes with, 564 class namespace, using, 182–184 class scope, defining a variable in, 183 class statement indicating a superclass in, 184 method definitions written inside of, 178–179 classes accessing __bases__ attribute for, 185 code example for calling, 564–565 defined, 190 making your own, 178–179 classes and types, 177–189 clear method, removing all items from the dictionary with, 85 client, needed to test your chat server, 442 client interface, for the XML-RPC file sharing program, 515–516, code example, 520–522 close database connection method, 485 closure, defined, 153 cmath (complex math) and complex numbers, 20–21 cmath.sqrt function, 35 cmd module, function of, 299 cmp function, 61 code example adding a method to the TestHandler class, 406 adding a test to a recursive generator, 232 adding buttons to your text editor, 331 adding emphasis handler to markup program, 372 adding line numbers to a Python script, 267 adding PolyLines to your drawing, 394 adding some default arguments to recursive case, 240 adding two buttons with customized text, 333 adding your module directory to PYTHONPATH, 252 addSource and addDestination meth- ods, 427 of a basic chat server with some cleanup, 444–445 calling a class, 564–565 of a CGI script that invokes a trace- back (faulty.cgi), 466 of a CGI script that retrieves a single value from a FieldStorage, 468 of a chat server program with ChatSession class, 446–447 of a chat server that accepts con- nections, 443–444 for checking for usable SAX parser, 403 of a configuration file using ConfigParser module, 354 for connecting to a peer, 513 of Content-type header code, 466 for converting an iterator to a list, 229 of a create SQL command, 483 for creating a destination, 428 for creating a fortune cookie pro- gram, 272 for creating an electronic deck of cards, 272–274 for creating an electronic die- throwing machine, 272 of defaultStart and defaultEnd methods, 414–415 of Dispatcher mixin class dispatch method, 413 for distribute method, 428 of edit.cgi script, 475 for escaping all characters in a string, 286 for fetching the body of an article, 425 of a file containing some nonsensical text, 308 of the files and directories created for the XML project, 418 of the final editor program, 340–341 of final version of chat server, 454–458 066Index 7/15/02 5:30 PM Page 596 for finding the search path in the sys module, 251 for finding yesterday’s time, 424 of the finished GUI client (, 531–534 of flatten generator rewritten as a plain function, 234 for generating a random date in a given range, 271–272 of generator, 230–231 of generic superclass for chat rooms, 451 for getting a list of sunspot data rows, 396 of a greeting script with an HTML form (simple3.cgi), 470 of a GUI implemented with Jython and Swing, 345 of a GUI implemented with Tkinter, 344 of a GUI implemented with wxPython, 344 of a GUI program with only a win- dow, 330 of handlers (, 381–383 for implementation of the NewsItem class, 427 for implementing the program, 370 for importing MySQLdb, 483 for importing psycopg, 483 of index.html file, 474 of iterating over a file without assign- ing file object to a variable, 317 of iterating over characters with read, 315 iterating over lines with fileinput, 316 of iterating over lines with readlines, 315 iterating over lines with xreadlines, 316 iterator for a sequence of Fibonacci numbers, 227 of a line numbering program with line numbers added, 267 of linking several commands together with pipes, 307 of a list with an access counter, 221–222 of load function, 340 of loop that finds the subject within NNTP headers, 425 of looping over characters with read method, 314 of the main bulletin board (simple_main.cgi), 488–489 markup program (, 385–386 of a minimal chat server program, 443 for a minimal XML parser to parse website.xml, 406 of a modified text file, 312 of a module with conditional test code, 250 of a new Node implementation (, 517–520 of, 432–436 of Node class pseudocode, 507 of a Node controller interface (, 520–522 for placing all components with basic layout, 334–335 for printing the subject and body of an article, 425 of a program for finding the sender of an e-mail, 292–293 of a program using the Loggerclass, 362 of property function, 224–225 of pseudocode for the headline rule, 375–376 for putting program state infor- mation into a file, 361 for re match objects, 287–288 for read file method, 311–312 for readline file method, 312 for readlines file method, 312 of Rectangle class, 223 of Rectangle example with magic methods, 226 for retrieving all messages in the bulletin board database, 486 for reversing and printing command- line arguments, 262 for the Rule class in the rules mod- ule, 376 of rules (, 383–384 for running a GUI-enabled file shar- ing program, 530 for running the Squish arcade game, 546 for running, 543 of a sample plain text document (test_input.txt), 367–368 of save function, 340 of the saving script (save.cgi), 477–478 597 Index 066Index 7/15/02 5:30 PM Page 597 code example (continued) for sequence and mapping protocol, 218–220 of a set of fictitious e-mail headers, 291 for setting file permissions, 464 showing the file object being closed automatically, 312 of a simple Anygui version of a text editor, 342–343 of a simple application that uses the shelve module, 275–277 of a simple chat server (, 448–449 of a simple configuration file, 353–354 of simple default header and footer methods, 414 for a simple falling weights animation (, 541–544 of a simple logger class, 361 of a simple markup program (, 369–371 of a simple module, 245 of a simple module containing afunction, 248 of a simple module with some prob- lematic test code, 249 of a simple news gathering agent (, 426 for a simple Node implementation, 510–512 of a simple page maker script, 409–410 of a simple ReportLab program, 391 of simple script that counts the words in sys.stdin, 308 of a simple template, 297 of a simple text file, 311 of a simple Web editor (simple_edit.cgi), 472 of a simple Web site represented as an XML file, 404–405 of simplest possible CGI script, 465 of the Squish configuration file, 547 of the Squish game objects file, 547–549 of the Squish main game module, 550–556 of statement for logging, 360 for telephone book, 83–84 of a template, 298 of template definitions, 297 for a template system, 295 of test program for computing area of arectangle, 355 of test program for my_math module, 359 of a text block generator (, 368–369 that uses the Meerkat to find some Python articles, 504–505 useful for testing an SQL database, 486–487 for using a load function as an event handler, 339 using CounterList class, 222 of using for loops to iterate over a file (in Python 2.2), 317 of using recursive case to solve Eight Queens problem, 239–240 of using the default Anygui layout scheme, 337–338 of using the getvalue method of a FieldStorage, 468 of the Web site constructor, 416–418 of writing a read method loop with while 1/break, 314 for the XML project page handlers, 416 code reuse, using modules to facilitate, 248–249 colon (:) use of in Python statements, 17 using to indicate a block is to begin, 101 columns, creating a polyline for each in a data set, 393–394 command-line interpreter, using cmd module to write, 299 command prompt, running Python scripts from, 23 CommandHandler class, code for simple implementation of, 450 commas (,), printing with, 95–96 comments defined, 25 documenting your functions with, 132–133 commit database connection method, 485 Common Gateway Interface (CGI). See CGI (Common Gateway Interface) comparison function, defining your own for advanced sorting, 57 comparison operators, 104–109 compile function, in re module, 283 complex numbers, cmath and, 20–21 components adding to a window, 332 code example for placing with basic layout, 334–335 598 Index 066Index 7/15/02 5:30 PM Page 598 setting attributes to lay out in Anygui application, 334 compound statements, function of in Python, 586–587 computer game. See arcade game project concatenating strings, 27–28 concatenation vs. extend method, 52 conditional execution, and the if state- ment, 102 conditional statements and conditions, 101–110 conditionals, loops, and other state- ments, 95–127 conditions, and conditional statements, 101–110 ConfigParser module, Web site address for information about, 353 file in arcade game project, 545 code example, 547 configuration extracting symbolic constants from your code, 351–352 in your programs, 350 configuration files, creating, 353–354 conflict function, using to find con- flicts in Eight Queens problem, 237 connect database method, connecting to your database with, 485 constants, writing and placing in global variables, 352 constructors calling with parameters, 211 creating, 210–211 overriding, 211–214 continue statement function of in Python, 584 using in loops, 115–116 conversion flags, 65 conversion specifiers anatomy of basic, 65 in string formatting, 64 conversion types, in string formatting, 66 copy function vs. the deepcopy function, 258 using the help function on, 257–258 copy method, copying a dictionary with, 86–87 count method, counting element occur- rences in a list with, 52 CounterList class, code example of using, 222 cracking vs. hacking, 1 create SQL command code example using MySQL, 484 code example using PostgreSQL, 483 create_socket method, calling to create a socket, 443–444 Ctrl-D, exiting interactive interpreter with, 4 Ctrl+Break, using as a keyboard inter- rupt in DOS, 444 Ctrl+C, using as a keyboard interrupt in UNIX, 444 Ctrl+F5, using to run programs, 22 cursor database connection method, using, 485 custom exception classes, creating by subclassing Exception, 208 D Dalke, Andrew, Web site address for “Sorting Mini-HOWTO”, 58 data structures, defined, 37–61 database code example for retrieving all mes- sages in, 486 core functionality needed for bulletin board project, 485–486 creating, 483–484 fields created by the SQL statements, 484 protecting with a password, 485 setting permissions in, 484 database application interaction between program and user, 278 interesting features of program that uses shelve module, 277 database modules, Web site address for a list of, 482 date tuples, fields of, 268 deck of cards, code example for creating an electronic, 272–274 deepcopy function benefits of using instead of shallow copy, 87 vs. the copy function, 258 from copy module, 87 def keyword, using to define functions, 562 def (or function definition) statement, example, 131–132 __del__ method, 211 del statement, 126 deleting elements from lists with, 49 function of in Python, 582 using, 121–122 __delattr__ method, function of, 225 599 Index 066Index 7/15/02 5:30 PM Page 599 deleting elements from lists, 48–49, 50 __delitem__ method, function of in sequences and mapping pro- tocol, 217 destructor, __del__ method as, 211 dict function, 93 __dict__ attribute, checking all values stored in an object with, 189 dict function, using, 81 dictfetchone database cursor method, using, 486 dictionaries, 79–93 basic operations of, 82–83 function of in Python, 562 iterating over keys of with for loops, 113 string formatting with, 84–85 syntax for, 81 dictionary methods, 85–92, 92 example of, 91–92 list of, 577–578 die-throwing machine, code example for creating an electronic, 272 difflib library, function of, 299 digital telephone/address book, using Python dictionaries for, 79–80 dir function listing the contents of a module with, 195–196, 256 returning an alphabetized list of attri- bute names, 301 direction keyword argument, in Placer layout manager, 336 discussion forum building your own, 481–499 first implementation, 484–489 main.cgi script for, 489–491 second implementation, 489–496 specific goals for creating, 482 structuring the system, 489–490 useful tools for creating, 482–483 dispatcher class, subclassing from asyncore, 443 division operator, additional available in Python 2.2, 11 division program, printing more instruc- tive error message in, 203–204 DocBook XML, Web site address for information about, 402 docstring adding to a function, 132–133 defined, 258 doctest module support for testing in standard libraries, 356–358 Web site address for information about, 358 documentation, for modules, 258–260 dollar sign ($), using to mark the end of a string for pattern matching, 282 DOM (Document Object Model), Web site address for information about, 405 double-clicking, using, 24–25 double quotes (“), use of in Python strings, 26–27 downloading and installing Python on a Windows machine, 1–3 Python on Linux and UNIX machines, 3–6 E edit.cgi script for the bulletin board project, 490, 493–495 as part of a CGI script, 473–476 password handling in, 476 Eight Queens problem, 236 using a tuple to represent a possible solution, 236–237 using recursive case to solve the problem, 239–240 using the base case to solve the prob- lem, 237–239 electronic die-throwing machine, code example for creating, 272 elements, using slice assignments to insert into lists, 50 elif (else-if) clauses, using to check for several conditions, 103 else clauses function of, 208 in loops, 118 using to stop execution when some- thing goes wrong, 202–204 using with if statements, 102–103 empty lists, creating with None value, 43 encapsulation defined, 190 hiding unnecessary details from the world with, 172–176 encoding, possible problems you may encounter, 419 end match object method, function of, 287 environment variable, including your module directory in, 252–253 equality operator (==) 600 Index 066Index 7/15/02 5:30 PM Page 600 checking if two things are equal with, 105 using, 17 error message (traceback), returned if exception object is not han- dled, 193 escape function, in re module, 283 escaping quotes, single-quoted strings and, 26–34 eval function evaluating a Python expression with, 124–125 function of, 126 event handling, 338–339 events, default for some common Anygui components, 338 example of dictionary method, 91–92 of indexing, 40–41 of sequence membership, 46 of sequence (string) multiplication, 44 of slicing, 41–42 of string formatting, 68–69 of supplying and receiving parame- ters, 149–150 except clause adding more than one to the try/except statement, 199–200 catching exceptions with, 208 omitting exception class from, 201–202 Exception built-in exception class, 196 exception classes, building your own custom, 196–197 exception object, catching with the except clause, 200–201 exception objects, 193, 207 exception traceback, seeing when file does not exist, 303–304 exceptions, 193–208 catching, 197–199 catching all in a piece of code, 201–202 catching two with one block, 200 creating and raising your own, 194–197 and functions, 205 list of most important built in, 196 the Zen of, 206–207 exceptions and functions, function of, 208 exclusive indices, 41–42 exec statement, 126 executing a string with, 123–124 function of in Python, 585 execute database cursor method, using, 485 exponentiation (or power) operator, using, 12 expression statements, function of in Python, 581 expressions, 34, 571–580 vs. statements in Python, 15 extend method appending several values at once with, 52–53 vs. concatenation, 52 extending and embedding the Python interpreter, Web site address for, 590 extreme programming (XP), created by Ken Beck, 349 F factorial function, recursive imple- mentation, 155–156 falling weights animation, code exam- ple, 541–544 faulty.cgi script, code example, 466 fetchone database cursor method, using, 486 Fibonacci numbers, 129 Fibs object, making and using in a for loop, 228 fields (or key-value pairs), using the FieldStorage class to retrieve, 467–468 FieldStorage class, using to retrieve fields from the cgi module, 467–468 FIFO (First-In-First-Out), 55 file function, 322 file handling, adding to your simple3.cgi script, 471–473 file iterators, new in Python 2.2, 317–318 file-like objects, defined, 320 file methods, basic, 306–313 file name, specifying for files down- loaded with urlretrieve, 321 file permissions setting in your CGI script, 463–464 UNIX command for changing, 464 __file__ property, examining to find Python modules source code, 259–260 file sharing with GUI, 525–534 with XML-RPC, 501–523 601 Index 066Index 7/15/02 5:30 PM Page 601 file sharing program definition of peer-to-peer, 501 preparations for creating with XML- RPC, 505 file type, calling as a function, 303 fileinput module included in standard library, 300 iterating over all the lines in a series of text files with, 265–268 lazy line iteration with, 316 list of some important functions in, 265 using to find the sender of an e-mail, 292–293 fileinput.close function, closing the entire chain of files with, 266 fileinput.filelineno function, return- ing number of current line within current file with, 266 fileinput.filename function, returning name of current file with, 266 fileinput.input function, using, 266 fileinput.isfirstline function, using, 266 fileinput.isstdin function, using, 266 fileinput.lineno function, returning number of the current line with, 266 fileinput.nextfile function, closing current file and skipping to next with, 266, 293 file.readline method, reading lines with, 309 files, 303–323 closing, 310 converting in lists of strings, 317–318 iterating over contents, 313–318 opening, 303–305 Filter class, filtering sequences with, 184–185 filter function, function of, 161–162 filterwarnings function, filtering warn- ings so they don’t print with, 194 finally clause function of, 208 using to do housekeeping after a pos- sible exception, 204–205 find method, finding a substring within alarger string with, 70–71 findall function, in re module, 283 flags, using before width and precision numbers, 67–68 flags parameter, Web site address for information about, 286 flip function, in pygame.display mod- ule, 538 float function, 19 floating-point numbers (floats), use of, 10 floor function, 18 flush method, using to update files while you are working on them, 311 FoodExpert class, creating and initializ- ing its taste buds, 180–182 for loops looping over the keys of a dictionary with, 113 using, 111–113 using to iterate over file lines in Python 2.2, 317–318 vs. while loops, 112–113 fortune cookie program, code example for creating, 272 forward slash (/) character, using in Python code, 246 found_terminator method, of async_chat objects, 447 from ... import ... statements, using, 20 function calls, 18 function definitions defined, 165 function of in Python, 587 functional programming defined, 165 useful functions for, 160–165 functions, 34 adding a docstring to, 132–133 creating your own, 131–134 documenting, 132–133 and exceptions, 205 vs. generators, 230 making one to initialize a data struc- ture, 138–139 that don’t return anything, 133–134 using, 17–18 using pow in place of exponentiation operator, 17–18 writing one for getting names, 139 writing one that stores names in your structure, 139–140 __future__ module, using, 21 G garbage collection, 121 generators, 229–233 components of, 233 defined, 233, 243 vs. functions, 230 602 Index 066Index 7/15/02 5:30 PM Page 602 making, 230–231 simulating with normal functions, 234 geometry attribute, setting for compo- nent layout in Anygui application, 334 get method, using to access dictionary items, 87–88 __getattr__ method, function of, 225 getattr function, using to access an attribute, 188 __getitem__ method, function of in sequences and mapping pro- tocol, 217 get_surface function, in pygame.display module, 538 getvalue method, using to fetch values in a CGI script, 468 global scope, function of, 151 global statement, function of in Python, 585 global variables encapsulating in objects, 173–174 rebinding, 152 GNU bash, linking several commands together with pipes in, 307 Gnutella, Web site address for, 501 graphical user interfaces (GUIs), making for your Python programs, 325–347 graphics, creating, 387–399 greedy and non-greedy patterns, 290 greeting script, with an HTML form (simple3.cgi), 470 grok, defined, 255 Group class, function of, 539 group numbers and functions, using in substitutions, 288–289 group match object method, function of, 287 groups, in regular expressions, defined, 286 GUI application code example of one with only a win- dow, 330 how elements of the interface can be used, 326–327 requirements for, 325 writing a basic for text editing, 325–347 GUI client code example for, 527–529 code for getting a listing from a Node, 530 code for running, 530 files needed to run, 529 first implementation, 526–530 preparations for creating, 526 second implementation, 530–534 specific goals for in file sharing sys- tem, 525 using to make file sharing system easier to use, 525 GUI toolkits available for Python, 325, 327–328 selecting one to use, 346 Web site addresses for some well- known, 591 H hacking the basics, 1–36 vs. cracking, 1, code example, 381–383 hasattr function, 191 hash sign (#), using, 25 has_key method, using to check a dic- tionary for a given key, 88 height attribute, setting in Anygui appli- cation, 334 help function getting help with, 257–258 using in the interactive interpreter, 133 interactive help or help about a spe- cific object, 301 hexadecimals and octals, 13–14 hmove keyword argument, in Placer lay- out manager, 336 Holden, Steve, Python Web Programming by, 440 hstretch keyword argument, in Placer layout manager, 336 HTML, marking up a file with for display in a Web browser, 365–366 HTML forms, 469 HyperDriveError class, building, 197 I ICQ chat service, 439 identity operator, is operator as, 105–106 if __name__ == “__main__” statement, using, 249–250 if statement conditional execution and, 102 function of, 17 function of in Python, 586 if statement blocks, nesting if state- ments inside of, 103–104 603 Index 066Index 7/15/02 5:30 PM Page 603 imaginary number, square root of a neg- ative number as, 20 immutable parameter, 141 import statement importing functions from external modules with, 245 using to import modules into Python, 19 import-only-once behavior, of modules, 247 import ... as ... statement, locally renaming a function with, 126 importing, something from a module, 97 in operator checking for a value in a sequence with, 45–46 as the membership operator, 106–107 inclusive indices, 41–42 index, as assigned number in a sequence, 37 index method, finding the index of first occurrence of a value with, 53 IndexError class, built-in exception, 196 indexing example, 40–41 function of in Python, 561 as a sequence operation, 38–41 indexing notation, using, 48 indices, as limits for your slice, 41–42 infinite recursion, defined, 155 inflate rectangle method, in arcade game project, 545 information-gathering agent adding methods for distributing news item, 427–428 first implementation, 423–426 methods for adding sources and des- tinations to, 427 second implementation, 426–436 specific goals for creating, 422 useful tools for writing, 422 ingredients, in algorithms, 9 inheritance defined, 190 investigating, 185–186 using when creating a new class, 176 init function, in pygame module, 537 __init__ method, creating constructors with, 210 file (module), adding to a package, 254–255 initialization, of empty lists, 43 inplace parameter, care needed when using, 267 input function, 16 vs. raw_input function, 30 using to get user input, 560 insert method, inserting an object into a list with, 53–54 inspect module, function of, 189 instances, of a class, 177 Instant Python Web tutorial, Web site address, 559 instructions, in algorithms, 9 int function, 19 integers, using large, 12–13 IDLE running, 2 saving and executing programs in, 22 Web site address for information and documentation, 2 __iter__ method, using, 227–228 interactive interpreter, 7–9 code for telling where to look for a module, 246 exploring for built-in exception classes, 195–196 investigating modules in, 256 keyboard shorcut for exiting, 4 problem with writing programs in, 21 using as a calculator, 10–14 using the help function in, 133 interfaces checking to see if required methods are present in, 188 and introspection, 187–189, 191 invisible dictionary, defined, 151 IOError class, built-in exception, 196 IRC chat service, 439 is operator, testing for identity with, 105–106 isinstance function for checking if an object is an instance of a class, 185–186 using, 169 issubclass function, checking to see if a class is a subclass of another with, 185 item assignments, changing lists with, 48 items, for dictionaries, 81 items method, returning all items of a dictionary with, 88 iter function, extracting an iterator from an object with, 228, 243 iterator protocol, 227–228 iterators, 227–229 code for converting to a list, 229 defined, 243 making sequences from, 229 special (or magic), 209 604 Index 066Index 7/15/02 5:30 PM Page 604 iteritems method, returning an iterator instead of a list with, 88–89 iterkeys method, returning an iterator over the keys with, 89 itervalues method, returning an itera- tor of the values in the dictionary with, 91 J Java Swing GUI toolkit, available for Python, 327 join method, joining the elements of a sequence with, 71–72 ju-jitsu of programming, 349–350 Jython, Web site address for, 7, 345, 589 Jython and Swing, 345 K KaZaA, Web site address for, 501 key-value pairs (or fields), using the FieldStorage class to retrieve, 467–468 key-value pairs, constructing dictionar- ies from, 81 keyboard interrupts, in UNIX and DOS, 444 KeyError class, built-in exception, 196 keys method, returning a list of keys in the dictionary with, 89 keyword arguments, see keyword parameters for default Anygui layout scheme, 336 keyword parameters and defaults, 142–144 Kofler, Michael, MySQL by (Apress, 2001), 483 L lambda expressions, 160–165 using with the filter function, 161–162 using with the map function, 161 using with the reduce function, 162–164 LATEX, 366 layout advanced for Anygui application, 336–338 code example of using the default scheme, 337–338 problems with in Anygui application, 334–335 layout managers, for arranging compo- nents when window is resized, 336–338 left keyword argument, in Placer lay- out manager, 336 len function, getting length of a sequence with, 46–47 __len__ method, function of in sequences and mapping pro- tocol, 217 LIFO (Last-In-First-Out), 54 Line-by-Line Chimp Tutorial, Web site address for, 537 lines, reading and writing, 309–310 link function, linking components to event handlers with, 338–339 Linux machines, downloading and installing Python on, 3–6 list comprehension using, 126 using to filter dir function results, 256 using with loops, 118–120 list, dict, and str types, subclassing, 220–222 list function, 61 using to convert an iterator to a list, 229 list methods, 50–58 list of in Python, 577 listen method, calling to listen for incoming connections, 443–444 lists deleting elements from, 48–49 expression for converting back to a string, 47 vs. tuples, 37 and tuples, 37–61 ways you can change, 47–58 the Python workhorse, 47–58 local scope, storing parameters in, 136 local variables vs. global variables, 151 logger class, code examples of a pro- gram using, 361–362 logging, importance of for discovering problems and bugs, 360–362 long function, 19 long integers (longs), 12–13 long strings, using triple quotes with, 31 longs (long integers), 12–13 loops breaking out of, 115–117 else clauses in, 118 using, 110–118, 126 lower method, returning a lowercase version of a string with, 72–73 605 Index 066Index 7/15/02 5:30 PM Page 605 M Macintosh, installing Python on, 7 Macintosh Library modules, Web site address for, 590 MacPython, Web site address, 589 magic methods, 242 for advanced use, 222–223 implementing properties with old- style classes with, 225–226 main.cgi script, for the bulletin board project, 489–491 maketrans function, for making a trans- lation table, 74–75 map function, function of, 161 mappings, 92 vs. sequences, 38 markup program ( basic steps for creating, 369 code example of, 369–371, code example, 385–386 Martelli, Alex, Python Cookbook, 109 match objects and groups, returned by re functions, 286–299 match function, in re module, 283 math module, importing into Python, 19 math.ceil function, 35 math.floor function, 35 math.sqrt function, 35 max function, 61 returning largest element of a sequence with, 46–47 using with the reduce function, 163 MAX_HISTORY_LENGTH, setting, 509 maximum field width, for converted strings, 65 md5 module, function of, 299 Meerkat news service, code for using to find Python articles, 504–505 membership checking for as a sequence operation, 38 function of, 60 message editor, code example for, 494–495 message viewer, code example for, 492–493 methods, 60. See also dictionary meth- ods; list methods; string methods calling, 51 making private, 175–176 polymorphism in, 170 special (or magic), 209 throwing around, 180–182 MIME (Multipurpose Internet Mail Extensions) types, Web site address for, 466 min function, 61 returning smallest element of a sequence with, 46–47 minimum field width, for converted strings, 65 minus sign (-), conversion specifier, 65 mixin class, implementing to handle administrative details, 412–414 mode argument, possible values for, 304 module.function, writing to use functions from a module, 19 modules, 35 code for making both importable and runnable, 568 creating your own, 245–247 exploring, 255–260, 300 finding on the Web, 255 function of, 300 importing into Python, 19 making yours available, 250–254 naming in Python, 254 other interesting standard, 299 saving, 245–246 in standard Python installation, 245–260 structuring by grouping into pack- ages, 254–255 telling the interpreter where to look for them, 252–253 using __file__ property to examine source code for, 259–260 using __name__ variable in, 249–250 using to define things, 248–250 using to facilitate code reuse, 248–249 Web site addresses for third party used in projects, 591 what they contain, 256–257 modulus (remainder or %) operator, using, 11–12 multiple inheritance, using, 186–187 multiplication example, 44 of sequences, 43–44 multiplying, as a sequence operation, 38 multithreaded chat server, pointers for, 441–442 music, Web site address for information about using XML to represent, 402 my_math module code example of part of, 359 executing the test program for, 359 606 Index 066Index 7/15/02 5:30 PM Page 606 MySQL (Michael Kofler, Apress 2001), 483 MySQL database, Web site address for, 482 MySQLdb module, Web site address for, 482 N name-storing program collecting parameters in, 144–147 writing one that stores names in your structure, 139–141 NameError class, built-in exception, 196 namespace, 123 nested scopes, support for in Python 2.2, 153–154 nesting blocks, 103–104 network programming, standard library modules for, 318–319 new-style classes, introduced in Python 2.2, 242 NEWNEWS command, importance of NNTP server support for, 422, code example, 426 code example, 432–436 code for running, 431 newsgroups and mailing lists, Web site addresses for, 592 newsreader. See NNTP client (news- reader) NNTP (Network News Transfer Protocol), defined, 421 NNTP client (newsreader), 421 NNTP server code for fetching the body of an arti- cle, 425 code for instantiating and calling the newnews method, 424 importance of support for NEWNEWS command, 422 testing, 423 nntplib library introduction to, 421–437 using to write an information- gathering agent, 421–437 non-greedy patterns, 290 None value using for empty lists, 43 using to leave a list position empty, 48 numbering, the lines of a Python script, 266–268 numbers and expressions, 10–14 Numerical Python, Web site address for information about, 536 O object-oriented design, 189–191 object-oriented model, sketch of, 189–190 object-oriented programming, benefits of, 167–168 objects accessing attributes of from the out- side, 174–175 changes in how they work in Python 2.2, 209–210 defined, 190 encapsulating global variables in, 173–174 the magic of, 167–176 most important benefits of, 167–168 file in arcade game project, 545 code example, 547–549 octal numbers, how they are written, 14 official Python distribution, Web site address, 589 official Python tutorial, Web site address for, 590 online resources directory of, 589–592 for finding third party modules, 388 open function, opening files with, 303–305 operators, list of in Python, 572–573 ord function, 127 os module getting access to operating services with, 262–264 included in standard library, 300 some important functions and vari- ables in, 263 os.environ mapping, environment vari- ables contained in, 263 os.linesep variable, line separator string used in text files, 263 os.makedirs function, making necessary directories in a given path with, 415 os.path.isdir function, checking if a path is a directory with, 415 os.path.join function, joining several paths with the correct separa- tor with, 415 os.pathsep variable, using when group- ing several paths, 263 os.sep variable, separator used in path- names, 263 os.startfile function, using to start aWeb browser, 264 607 Index 066Index 7/15/02 5:30 PM Page 607 os.system function example of starting a Web browser with, 264 using to run external programs, 263 overriding 242–243 P package directory, adding modules to, 254–255 package layout, example of simple, 254 packages function of, 300 grouping modules into, 254–255, code example, 409–410 parallel execution, handling, 441–442 parallel iteration, for iterating over two sequences at the same time, 114 parameter gathering, reversing the pro- cess, 147–149 parameters changing inside a function, 135–137 collecting, 144–150 combining positional and keyword, 143 defined, 165 distributing, 147–149 example of supplying and receiving, 149–150 keyword, 142–144 the magic of, 134–150 modifying by wrapping your value in a list, 141 reasons for wanting to modify, 137–141 where the values come from, 134–135 parse function, using to parse an XML file, 405–408 pass statement, 126 function of in Python, 582 using, 120–121 password, protecting your database with, 485 password handling, in edit.cgi script, 476 path configuration file, filename extension for, 253 pattern matching, the beginning and end of a string, 282 (pattern)* operator, for repeating a pat- tern zero or more times, 282 (pattern)+ operator, for repeating a pat- tern one or more times, 282 (pattern){m, n} operator, for repeating a pattern from m to n times, 282 patterns, greedy and non-greedy, 290 PDF (Portable Document Format), 387 basic module for generating, 390 pdfgen module, for creating PDF files, 390 peek_sum, peek_max, using with the reduce function, 164 peer-to-peer file sharing program creating peers in, 513 defined, 501 requirements program must satisfy, 502–503 useful tools for creating, 503–505 peers code for introducing the first to the second, 514 connecting to, 513 creating in a peer-to-peer file sharing program, 513 percent (%), using as string formatting operator, 63–64 period (“dot”) character, as regexp wild- card, 279 Piddle project page, Web site address for, 399 pipe (|) character linking several commands together with, 307 using for character set alternatives, 281 pipeline, command example, 307 Placer layout manager, layout argu- ments for, 336 playful programming, 349–364 plus sign (+) conversion specifier, 65 using for concatenating lists, 565 PolyLines code example for adding to your drawing, 394 creating, 392–394 polymorphism creating an example with, 172–173 defined, 168, 190 the many forms of, 171–172 using to create an online payment system, 168–172 pop method, modifying a list and return- ing a value with, 54–55 popitem method removing and processing dictionary items with, 89 port numbers choosing for your chat server, 442 position attribute, setting for compo- nent layout in Anygui application, 334 608 Index 066Index 7/15/02 5:30 PM Page 608 positional parameters, 142 PostgreSQL database, Web site address for, 482 pound bang (#!) line, adding to your CGI script, 463 pow function, using in place of exponen- tiation operator, 17–18 power function, recursive implemen- tation, 156–157 pprint function, using instead of the print statement, 251 precision and width, for conversion specifiers, 67 prettyprint, adding a helper function to, 241 principles of abstraction, polymorphism and encapsulation as, 172 print statement function of in Python, 583 using to print several values, 95–96, 125 printing, with commas, 95–96 programming aspects of practice of flexibility in, 350 planning for change, 356 programs, saving and executing, 21–25 projects basic structure of all, 363 building your own bulletin board, 481–499 creating a chat server, 439–460 file sharing with XML-RPC, 501–524 file sharing II, with GUI, 525–534 information-gathering agent, 421–437 instant markup, 365–386 in the news, 421–437 painting a pretty picture (graphics), 387–399 remote editing with CGI, 461–480 XML for all occasions, 401 prompt (>>>). See Python prompt (>>>) properties, 223–227 defined, 224 special (or magic), 209 property function, using, 224–225 protocol, use of, 216 prototyping your programs, 350–351 pseudo-code, defined, 157 psycopg front-end, for PostgreSQL, 482 .pth filename extension, for a path con- figuration file, 253 PuTTY terminal emulator, Web site address for information about, 442 .py filename extension importance of using for created files, 21 using .pyw instead of in Windows, 254 .pyc filename extension, defined, 246 Pydoc Online, Web site address for, 590 Pygame contents of module, 537 Web site address for documentation, 535 Web site address for downloading, 536 pygame.display module, contents of, 537–538 pygame.event module, function of, 539 pygame.font module, function of, 538 pygame.image module, function of, 539 pygame.locals module, contents of, 537 pygame.mouse module, function of, 539 pygame.sprite module, contents of, 538 PyGTK GUI toolkit, available for Python, 327 PyQt GUI toolkit, available for Python, 327 PyRXP XML parser, Web site address for, 405 Python assignment in, 14 the basics, 559 checking for on Linux and UNIX sys- tems, 3 compiling from sources, 5–6 dictionaries, 79–93 downloading and installing, 1–7 downloading and installing on aWindows machine, 1–3 hexadecimals and octals in, 13–14 how large integers behave in older versions, 13 installing on a Macintosh, 7 list of dictionary methods in, 577–578 list of list methods in, 577 list of operators in, 572–573 list of some important built-in functions in, 573–576 list of string methods in, 578–580 listing of basic (literal) values in, 571 long integers (longs) in, 12–13 a minimal introduction to, 559–569 numbers and expressions, 10–14 objects and stuff, 564–568 online resources for distributions, 589 other distributions, 7 programming in, 9 prompt (>>>), 8 609 Index 066Index 7/15/02 5:30 PM Page 609 Python (continued) reference, 571–580 saving and executing programs in, 21–25 statements in, 14–15 summary of statements in, 581–587 taking control of your computer with, 1–36 useful tools and modules, 591 useful tools for XML support, 403 using equality operator (==) in, 17 using exponentiation (or power) operator in, 12 using large integers in, 12–13 using remainder (modulus or %) operator in, 11–12 using the input function in, 16 using the interactive interpreter as a calculator, 10–14 variables in, 14 version implemented in Java, 7 Web site address for information about setting up in Mac OS, 253 Web site address for information about using DOM in, 405 Python 2.2 additional division operator available in, 11 file iterators as new feature in, 317–318 the help function in, 257–258 new-style classes introduced in, 242 subclassing built-in types in, 221 types and classes in, 178 using super function in, 215 Python Cookbook, Web site address, 109 Python Database Topic Guide, Web site address for information about, 482 Python date tuples, fields of, 268 Python DB-API, using to interface with the server, 482 Python dictionaries, some arbitrary uses of, 79–80 Python documentation, Web site address for, 590–591 Python Enhancement Proposal number 245 Web site address for, 188 Python Enhancement Proposals, Web site address for, 591 Python interpreter, Web site address for extending and embedding, 590 Python Language Reference, Web site address for, 590 Python Library Reference for description of string module, 69 finding built-in classes available in, 195 finding module information in, 259 for information about using DOM in Python, 405 slice objects described in, 220 Web site address, 301, 590 Python programs. See scripts Python prompt (>>>), 8 Python reference, 571–587 Python Reference Manual, Web site address, 220, 223 Python scripts example for numbering the lines of, 266–268 running from a command prompt, 23 Python shell, IDLE interactive, 3 Python Web Programming (Steve Holden), for information about asyncore and asynchat, 440 Python Web site address, for download- ing Python documentation, 259 PYTHONPATH, code for adding your mod- ule directory to, 252 PythonWare, Web site address, 7, 589 PythonWin GUI toolkit, available for Python, 327 .pyw filename extension, using instead of .py, 254 PyXML package, Web site address for downloading, 403 Q queens generator, solutions given by, 240 question mark (?) character, using after a subpattern in character sets, 281–282 quote function, urllib module utility, 321 quote_plus function, urllib module utility, 321 R r prefix, use of in raw strings, 33–34 ‘r’ value, for mode argument of the open function, 304 Raggett, Dave, Getting Started with HTML by, 365 raise statement function of, 208, 584 610 Index 066Index 7/15/02 5:30 PM Page 610 using with an argument to raise an exception, 195–196 using without arguments, 198–199 random access, file-object methods used for, 308–309 random module included in standard library, 300 some important functions in, 270 random numbers, functions in random module that return, 270–271 random.choice function, 191, 271 returning a random element from the sequence seq with, 270 random.random function, 270 random.randrange function, using, 271 random.shuffle function, shuffling the elements of a sequence ran- domly with, 271 random.uniform function, getting a random real number with, 270 using, 271 range function, 127 for iterating over a range of numbers, 112 raw strings, using, 32–33 raw_input function, 35 vs. input function, 30 re functions, using VERBOSE flag in, 289 re match objects, some important methods of, 287 re module contents of, 283–286 included in standard library, 301 some important functions in, 283 support for regular expressions con- tained in, 279–299 read method iterating over characters with, 315 looping over file characters with, 314 reading and writing files, 306–307 readline method, using in a while loop, 315 readlines method, iterating over lines with, 315 re.compile function, transforming a reg- exp to a pattern object with, 283 Rectangle class, code example for using, 223 recursion, defined, 154–155, 165 recursive case of recursive function, 155 using to solve the Eight Queens prob- lem, 239–240 using with recursion, 231 recursive functions parts of, 155 two classics, 155–157 recursive generator adding a test to beginning of, 232 creating, 231–233 Red Hat Linux Package Manager (RPM), steps for installing Python RPM packages, 4–5 reduce function, 166 function of, 162–164 re.escape function, escaping all charac- ters in a string with, 285–286 refactoring, defined, 421 re.findall function, returning a list of all occurrences of a pattern with, 285 regexp (regular expression) defined, 279 period (“dot”) character as wildcard in, 279 Web site address for complete listing of operators, 282 register_function method of SimpleXMLRPCServer, 506 register_instance method of SimpleXMLRPCServer, 505–506 regular expressions, using VERBOSE flag to make more readable, 289 reload function function of, 301 using to reload modules, 247 remainder (modulus or %) operator, using, 11–12 re.match function, matching a regexp at beginning of a string with, 284 remote editing, of documents with CGI via the Web, 461–480 remote files accessing with urllib module, 318–321 opening with urlopen from the url- lib module, 319–320 retrieving, 320–321 remove method, removing the first occurrence of an value in a list with, 55 repetition operators, making non- greedy, 290 replace method, using, 73 reportlab module, code for importing, 389 ReportLab package drawing with, 390–392 611 Index 066Index 7/15/02 5:30 PM Page 611 ReportLab package (continued) learning the basics of for a graphics project, 388 PolyLine class in, 392–394 Web site address for downloading, 388 package, creating objects with, 390 module, using the LinePlot class from, 397–398 repr function, 35 using, 28–29 polymorphic use, 171–172 requirement specifications, importance of writing precise, 354–355 function, searching a string with, 283–284 re.split function, splitting a string by occurrences of a pattern with, 284–285 re.sub function, 283 substituting occurrences of a pattern with a replacement with, 285 using group numbers in the substi- tution string, 288 return statement function of in Python, 583 using, 132 using to end a function, 134 using to return a value from a function, 562–563 reverse method, reversing elements in a list with, 55–56 rexec module, function of, 299 right keyword argument, in Placer lay- out manager, 336 round function, 35 using, 18 RPC (Remote Procedure Call), 501 RPM packages, locating on for your sys- tem, 5 rules module, code example for Rule class in, 376, code example, 383–384 S save.cgi script for the bulletin board project, 490, 495–496 as part of a CGI script, 473, 476–478 saving script code example for, 495–496 code example of, 477–478 SAX (Simple API for XML), processing XML files with, 401, 405 scopes defined, 165 nested, 153–154 scoping, 150–154 scripts making executable before running, 24 making them behave like normal programs, 23–25 running from a command prompt, 23 search path, code example for finding in the sys module, 251 seek method, for random access of files, 308–309 self parameter, 178–179 sequence and mapping protocol basic, 217–220 code example for, 218–220 sequence types, built-in, 37 sequence unpacking, for performing several assignments simulta- neously, 98 sequences, 60 as basic data structures in Python, 37 common operations, 38–47 comparing, 107 concatenating with the addition operator, 43 making from iterators, 229 vs. mappings, 38 multiplying, 43–44 sequences and mapping protocol, 243 extra requirements imposed upon methods, 217 serve_forever method, calling, 506, 510, code example, 517–520 set_caption function, in pygame.display module, 538 setattr function, setting the attributes of an object with, 188 __setattr__ method, function of, 225 setdefault dictionary method, 90 __setitem__ method, function of in sequences and mapping pro- tocol, 217 set_mode function, in pygame.display module, 538 set_terminator method, setting the line terminator with, 447 SHA (Secure Hash Algorithm), 476 sha module code example of how you can use, 477 function of, 299 using in password handling, 476–478 shadowing, the problem of, 152 612 Index 066Index 7/15/02 5:30 PM Page 612 shallow copy, 86 Shelf object, using to store things, 274–275 shelve module code example of a simple database application that uses, 275–277 included in standard library, 301 using for a simple storage solution, 274–275 function, potential trap when using, 274–275 short-circuit logic, 107–108 signs, alignment, and zero-padding, 67–68 Simple API for XML (SAX). See SAX (Simple API for XML) simple generators. See generators simple statements, function of in Python, 581 simple2.cgi, code example, 468 simple3.cgi, code example, 470, code example, 448–449 simple_edit.cgi, code example, 472, code example for, 527–529 simple_main.cgi, code example, 488–489 program adding components to in second implementation, 371–386 adding handlers (renderers) to, 371–372 code example for implementing, 370 making rules a separate object for, 375–376 code example for, 510–512 example of how it is used, 513–514 single quote (‘), use of in Python strings, 26–27 single-quoted strings and escaping quotes, 26–34 site-packages directory, storing modules in, 251–252 six degrees of separation hypothesis, Web site address for, 509 size attribute, setting for component layout in Anygui application, 334 Slashdot discussion forum, Web site address, 481 slice assignments, using, 49–50 slices assigning, 49–50 deleting from lists, 50 slicing function of in Python, 561 as a sequence operation, 38 shortcut for, 41–42 using to access ranges of elements, 41–42 socket module, using functions in to create a chat server, 440–441 sockets, defined, 439 SolarWolf game (Pete Shinners), Web site address for, 557 sort list method sorting lists in place with, 56–57 sorting, defining your own comparison function for advanced, 57 Sorting Mini-HOWTO, Web site address for, 58 sorting order, for upper- and lowercase letters, 293 source code, reading to better under- stand Python, 259–260 space character, conversion specifier, 65 Space Environment Center, Web site address for, 389 space keyword argument, in Placer lay- out manager, 336 SPAMFilter, important points in defi- nition of, 185 span match object method, function of, 287 special characters escaping, 280 rules for using in character sets, 280–281 Sping graphics and plotting package, Web site address for, 399 split method, splitting a string into a sequence with, 73 split function, in re module, 283 Sprite class, function of, 538 SQL databases creating, 483–484 fields created by the SQL statements, 483–484 setting permissions in, 484 two freely available, 482 SQL programming, Web site address for practicing, 483 sqrt function, 19–20 square root, using sqrt function to cal- culate, 20–21 Squish arcade game basic design process, 535 first implementation, 540–545 preparations for creating, 540 second implementation, 545–556 613 Index 066Index 7/15/02 5:30 PM Page 613 Squish arcade game (continued) some screenshots of, 546 specific goals for creating, 536 steps for creating animation of falling16-ton weight, 540–541 useful tools for creating, 536–539 file in arcade game project, 545 code example, 550–556 code for executing to run the game, 546 stack trace, example of, 205 standard libraries, support for testing in, 356–360 standard library, included in standard Python installation, 260–300 standard library reference, Web site address for time module information, 270 star. See * (asterisk); asterisk (*) start match object method, function of, 287 statements, 14–15, 34 vs. expressions in Python, 15 summary of, 581–587 use of colon character in, 17 StopIteration exception, raising, 227 store function steps performed by, 140 testing, 140–141 str function, using, 28–29 streams, three standard, 306 strftime function, formatting time as strings with, 424 string formatting advanced template mechanism, 294 conversion types, 66 with dictionaries, 84–85, 92 example, 68–69 the long version, 64–69 operator, 63–64 the short version, 63–64 a simple conversion, 66 for working with strings, 76 string literals, indexing, 39 string methods, 69–75 list of in Python, 578–580 using to check membership, 45 for working with strings, 76 string module useful values from, 70 Web site address for description of, 69 string representations, 28–29 string.maketrans function, making atranslation table with, 77 strings, 26–34, 35 basic operations, 63 comparing, 107 concatenating, 27–28 statements for executing and evaluat- ing, 122–125 working with, 63–78 strip method, returning a string where whitespace has been stripped, 74 sub method, in Handler class, 374 subclass, defined, 177 subpatterns and alternatives in character sets, 281 making optional, 281–282 operators that allow you to repeat more than once, 282 optional and repeated in character sets, 281–282 sunspot graph program code example for final, 397–398 first prototype for, 394–395 second implementation, 395–398 sunspots data downloading from the Space Environment Center Web site, 389–390 using urllib to fetch files across the Internet, 396 super function using in Python 2.2, 215 why it is better, 216 superclass defined, 177 specifying, 184–185 superclass constructor, calling the unbound, 214–215 superclasses, multiple, 186–187 SyntaxError class, built-in exception, 196 sys module included in standard library, 300 some important functions and vari- ables in, 261 sys.argv variable, contents of, 261 sys.exit function, exiting the current program with, 261 sys.modules mapping, mapping module names to actual modules with, 261 sys.path variable, using, 261 sys.platform variable, 261 sys.stderr variable, 262 sys.stdin variable a simple script that counts the words in, 308 614 Index 066Index 7/15/02 5:30 PM Page 614 using to iterate over lines in standard input, 317 where Python gets its input from, 262 sys.stdout, where Python prints to, 262 T tab characters, using to indent blocks, 100 telephone book code example for, 83–84 using Python dictionaries for, 79–80 tell method, for random access of files, 308–309 telnet, using to connect to any socket server, 442 template system code example, 295 code example of output from, 298 code example of template defi- nitions, 297 making, 294–299 a simple template example, 297 what the program does, 296 terminal emulator software, 442–443 ternary operator, implementing, 109 test document, code example, 367–368 test program, for computing the area of arectangle, 355 TestCase class, writing a test program for my_math module with, 359 testing automated in you Python programs, 350 setting up for your programs, 354–362 testmod function, running from doctest, 357 TEX typesetting program, LAT EX markup system based on, 366 TEX Users Group home page, Web site address, 366 text areas vs. text fields, 327 text editor adding buttons to, 331 adding components to, 332 creating windows and components, 330–332 the finished program, 339–342 sketch of, 326 steps for trying out the finished pro- gram, 341 writing a basic, 325–347 text fields vs. text areas, 327 text files, writing a basic program for editing, 325–347 TextArea class, 347 TextField class, 347 The Covers Project, code example for connecting to its server, 504 third party modules, Web site addresses for those used in the ten proj- ects, 591 threading module, information sources, 441–442 threads, alternative approach to creating a chat server, 439 time module checking the standard library refer- ence for information about, 270 fields of Python date tuples, 268 functions needed from for the information-gathering agent, 423 included in standard library, 300 list of some important functions in, 269 using, 268–270 time stamps, getting for your sunspot graph program, 393–394 time.asctime function, formatting the current time as a string with, 269 time.gmtime function, displaying univer- sal time with, 269 time.localtime function, converting seconds to a date tuple, local time with, 269 time.mktime function, converting time tuple to local time with, 269 time.sleep function, makes interpreter wait for a given number of seconds, 269 time.strptime function, parsing a string into a time tuple with, 269 time.time function, returning the cur- rent time as seconds since epoch with, 269 TinyFugue terminal emulator, Web site address for, 443 Tkinter GUI toolkit available for Python, 327–328 Web site addresses for information about, 343 tools deciding which you need to write your programs, 366–367 useful for the graphic-generating package, 388–389 top keyword argument, in Placer layout manager, 336 615 Index 066Index 7/15/02 5:30 PM Page 615 traceback (error message), returned if exception object is not han- dled, 193 translate method, replacing parts of a string with, 74–75 translation table contents of, 75 making before you can use translate method, 74–75 triple quotes (‘’’ or “””), using with long strings, 31 truth values, 101–102 try/except statement avoiding program termination with, 206–207 checking for object attributes with, 207 using to catch exceptions, 197–199 try/finally statement, avoiding pro- gram termination with, 206–207 tuple function function of, 59 using in Python 2.2 and later, 169 tuples basic operations of, 59 as immutable sequences, 58–59 important reasons for using, 60 and lists, 37–61 vs. lists, 37 type checking, reasons to use, 220 type function, using, 169 TypeError class, built-in exception, 196 types and classes, in Python 2.2, 178 U u prefix, use of in Unicode strings, 34 unbound methods, example of, 214–215 unbound superclass constructor, call- ing, 214–215 Unicode, Web site address for infor- mation about, 33 Unicode strings, using, 33–34 unittest module support for testing in standard libraries, 358–360 Web site address for information about, 359 universal time, displaying with the time.gmtime function, 269 UNIX, code for making an executable script in, 569 UNIX machines downloading and installing Python on, 3–6 making Python scripts behave like normal programs on, 23–25 update function, in pygame.display module, 538 update method, updating one dictionary with items of another with, 90–91 urlencode(query[, doweq]) function, urllib module utility, 321 urllib module accessing remote files with, 318–321 utilities available in, 321 urllib.encode function, 322 urllib.quote_plus function, 322 urllib.quote function, 322 urllib.unquote function, 322 urllib.unquote_plus function, 322 urllib.urlcleanup function, 321 urllib.urlopen opening remote files with, 319–320 urllib.urlretrieve retrieving remote files with, 320–321 Usenet, Web site address for information about, 421 UserDict module, in the Python stan- dard library, 221 UserList module, in the Python stan- dard library, 221 UserString module, in the Python stan- dard library, 221 V -v switch, using to get more input from doctest.testmod function, 357–358 ValueError class, built-in exception, 196 values, getting for your sunspot graph program, 393–394 values method, returning a list of values in the dictionary with, 91 van Rossum, Guido, Python’s Benevolent Dictator For Life (BDFL), 590–591 variables, 14, 34 making global, 152 naming, 14 storing from an enclosing scope, 154 switching the contents of two with sequence unpacking, 98 using to refer to functions in Python, 20 vars function, using to return a name- space or scope, 150–151 Vaults of Parnassus finding a graphics-generating pack- age in, 388 searching for GUI toolkits in, 327 VERBOSE flag, making regular expressions more readable with, 289 616 Index 066Index 7/15/02 5:30 PM Page 616 view.cgi script, for the bulletin board project, 489, 492–493 vmove keyword argument, in Placer lay- out manager, 336 vstretch keyword argument, in Placer layout manager, 336 W ‘w’ value, for mode argument of the open function, 304 W3Schools Web site, for a tutorial on XML, 401 warnings, function of, 194 warnings.filterwarnings function, using to filter out warnings, 194 Web browser, example of starting with the os.system function, 264 Web editor, code example of a simple, 472 Web pages, editing remotely with CGI, 461–480 Web Programming Topic Guide, Web site address for information about, 461 Web security, Web site address for infor- mation about, 465 Web server, putting your programs on, 462–463 Web site address for ActivePython, 7 ActivePython, 589 for Andrew Dalke’s “Sorting Mini- HOWTO”, 58 Anygui GUI toolkit, 328–329 for Anygui GUI toolkit documen- tation, 336 for Cameron Laird’s Web page of GUI toolkits, 327 for a complete listing of regexp oper- ators, 282 for concise description of what XML is, 401 for The Covers Project, 504 for Dave Raggett’s Getting Started with HTML guide, 365 for description of six degrees of sepa- ration hypothesis, 509 for description of string module, 69 for detailed information about asyncore, 440 for downloading Acrobat Reader, 387 for downloading Instant Python Web tutorial, 559 for downloading new version of, 503 for downloading Pygame, 536 for downloading PyXML package, 403 for downloading ReportLab package, 388 for finding NNTP server resources, 422–423 for game development Web sites, 557 for GNU bash information, 307 for Gnutella, 501 for Gordon McMillan’s Socket Programming HOWTO, 439 for IDLE information and documen- tation, 2 for information about DocBook XML, 402 for information about encodings, 419 for information about extreme pro- gramming (XP), 349 for information about graphs and trees, 235 for information about HTML and HTML forms, 469 for information about SAX, 401 for information about setting up Python in Mac OS, 253 for information about the ConfigParser module, 353 for information about the doctest module, 358 for information about the unittest module, 359 for information about Usenet, 421 for information about using DOM in Python, 405 for information about using XML for representing music, 402 for information about using XML to describe any physical object, 402 for information about Web security, 465 for information about XHTML, 402 for information about XML-RPC, 501 for Jython, 7, 589 for KaZaA, 501 for the Line-by-Line Chimp Tutorial, 537 for a list of chat services, 439 for a list of database modules, 482 for a list of official MIME types, 466 for Macintosh Library modules, 590 MacPython, 589 for MySQL database, 482 for MySQLdb module, 482 for newsgroups and mailing lists, 592 for the official Python distribution, 589 for the official Python tutorial, 590 617 Index 066Index 7/15/02 5:30 PM Page 617 Web site address (continued) for PostgreSQL database, 482 for practicing SQL programming, 483 for psycopg front-end for PostgreSQL, 482 for PuTTY terminal emulator, 442 for Pydoc Online, 590 for Pygame documentation, 535 for the Python Database Topic Guide, 482 for Python documentation, 590–591 for the Python Enhancement Proposal number 245, 188 for Python Enhancement Proposals, 591 for the Python Library Reference, 259, 301 for Python Web page, 319 for the Python win32all Windows extensions, 589 for PythonWare, 7, 389 for Slashdot discussion forum, 481 for the SolarWolf game by Pete Shinners, 557 for Space Environment Center, 389 for the standard SocketServer mod- ule documentation, 441 for the standard threading module documentation, 441 TEX Users Group home page, 366 for tutorial about Meerkat XML-RPC API, 505 for Unicode information, 33 for using XML to represent human moods, emotions, and char- acter traits, 402 for Vaults of Parnassus, 388 for the Web Programming Topic Guide, 461 for the World Wide Web Consortium, 365 webbrowser module, using to start a Web browser, 264–265 website.xml code example, 404–405 code for a minimal XML parser to parse, 406 code that uses methods to build a list of the h1 elements for, 407–408 weight.png image file, in arcade game project, 545 code example, 541–544 code example for running, 541–544 explanation of important points, 544–545 while 1/break idiom using, 116–117 using to iterate over file contents, 314 while loops vs. for loops, 112–113 using, 111 using to iterate over file contents, 315 while statement, function of in Python, 586 width and precision, for conversion specifiers, 67 width attribute, setting for component layout in Anygui application, 334 win32all, Web site address for the Python Windows extensions, 589 window adding to your text editor, 330–331 example of one with layout problems, 333 Window class, Window instantiating to create a text editor window, 330–332 Window.add method, adding a compo- nent to a window with, 347 Windows machines, downloading and installing Python on, 1–3 wrap function, defined, 432 write file method, writing lines with, 310 wxPython, using, 344 wyPython GUI toolkit, available for Python, 327 X XHTML, Web site address for infor- mation about, 402 XML using to represent human moods, emotions, and character traits, 402 Web site address for description of what it does, 401 Web site address for sampling of existing applications, 402 XML Cover Pages, Web site address, 402 XML file, using to generate a complete Web site, 402 XML for all occasions project, defining the problem, 401–402 XML format, main concepts for design- ing your Web page, 403–404 XML parsing, looking at how it works, 405 618 Index 066Index 7/15/02 5:30 PM Page 618 XML-RPC defining result codes for, 508 file sharing with, 501–523 Web site address for information about, 501 XML-RPC file sharing program calling the fetch method in, 508 calling the query method in, 508 client interface, 515–516 exceptions handling in, 516 first implementation, 505–514 implementing the hello method, 508 second implementation, 514–522 some flaws in the first implement- ation, 514–515 starting the program, 517 testing first implementation, 506 testing the second implementation, 517–522 validating file names in, 516–517 xmlrpclib module, connecting to XML- RPC servers with, 503–505 xrange function, 127 using, 112 xreadlines object, lazy line iteration with, 316 Y yield statement, 230–231 Z zero (0), conversion specifier, 65 zero padding, of numbers in strings, 67 ZeroDivisionError class, built-in exception, 196 zip function, for zipping sequences together, 114 619 Index 066Index 7/15/02 5:30 PM Page 619 066Index 7/15/02 5:30 PM Page 620



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