>>> def commaInsert(s, groupby=3):
    def grouper(itr, groupby):
        i = iter(itr)
        args = [i] * (len(itr) % groupby)
        if args:
            yield ''.join(map(next, args))
        args = [i] * groupby
        while True:
            yield ''.join(map(next, args))
    return ','.join(grouper(s, groupby))

>>> commaInsert('1994')
'1,994'
>>> commaInsert('Im born in 1994, and you?')
'I,m b,orn, in, 19,94,, an,d y,ou?'