Python: Put comma(s) to numbers

This script will put commas between each group of three digits number. For example, 1234567 will become 1,234,567.

import re

putComma = lambda x: (",".join(re.findall("d{1,3}",
str(x)[::-1])))[::-1]

print putComma(1234567) # 1,234,567
print putComma(12345678) # 12,345,678
print putComma(123456789) # 123,456,789
print putComma(1212) # 1,212