I'm bored for posting this, but here it is:
def add_commas(str):
str_list = list(str)
str_len = len(str)
for i in range(3, str_len, 3):
str_list.insert(str_len - i, ',')
return ''.join(str_list)-- http://mail.python.org/mailman/listinfo/python-list
