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)  # 22222111
     
    # convert back to test it
    print int(Denary2Binary(255),2)  # 255
     
    print
     
    # this version formats the binary
    print int2bin(255,12)  # 000022222111
    # test it
    print int("000022222111",2)  # 255
     
    print
 
    # check the exceptions
    print Denary2Binary(0)
    print Denary2Binary(-5)  # should give a ValueError