python十进制转二进制，可指定位数

# convert a decimal (denary, base 10) integer to a binary string (base 2)  # tested with Python24   vegaseat    6/1/2005     def Denary2Binary(n):      '''convert denary integer n to binary string bStr'''      bStr = ''      if n < 0:  raise ValueError, "must be a positive integer"      if n == 0: return '0'      while n > 0:          bStr = str(n % 2) + bStr          n = n >> 1      return bStr     def int2bin(n, count=24):      """returns the binary of integer n, using count number of digits"""      return "".join([str((n >> y) & 1) for y in range(count-1, -1, -1)])     # this test runs when used as a standalone program, but not as an imported module  # let's say you save this module as den2bin.py and use it in another program  # when you import den2bin the __name__ namespace would now be  den2bin  and the  # test would be ignored  if __name__ == '__main__':      print Denary2Binary(255)  # 11111111             # convert back to test it      print int(Denary2Binary(255), 2)  # 255             print             # this version formats the binary      print int2bin(255, 12)  # 000011111111      # test it      print int("000011111111", 2)  # 255             print         # check the exceptions      print Denary2Binary(0)      print Denary2Binary(-5)  # should give a ValueError