Generate grey code

grey.py

#!/usr/bin/env python
import sys

def toGrey(num):
    return (num >> 1) ^ num

def generate(n):
    for i in xrange(2**n):
        yield toGrey(i)

if __name__ == '__main__':
    n = int(sys.argv[1])
    for i in generate(n):
        print bin(i)[2:].zfill(n)