def _add_commas(number, comma_sep=","):
    """Returns a number separated with commas"""
    triplets_num = len(number) // 3
    remainder = len(number) % 3
    triplets = [number[:remainder]] if remainder else []
    triplets += [
        number[remainder + i * 3 : remainder + 3 + i * 3] for i in range(triplets_num)
    ]
    return comma_sep.join(triplets)