Answer to "How to print number with commas as thousands separators?"

format_money.py

# http://stackoverflow.com/a/39301723/2649222
def format_money(f, delimiter=',', frac_digits=2):
    '''
    >>> format_money(1.7777)
    '1.78'
    >>> format_money(-1.7777)
    '-1.78'
    >>> format_money(12.7777)
    '12.78'
    >>> format_money(-12.7777)
    '-12.78'
    >>> format_money(123.7777)
    '123.78'
    >>> format_money(-123.7777)
    '-123.78'
    >>> format_money(1234.7777)
    '1,234.78'
    >>> format_money(-1234.7777)
    '-1,234.78'
    >>> format_money(12345.7777)
    '12,345.78'
    >>> format_money(-12345.7777)
    '-12,345.78'
    >>> format_money(123456.7777)
    '123,456.78'
    >>> format_money(-123456.7777)
    '-123,456.78'
    >>> format_money(1234567.7777)
    '1,234,567.78'
    >>> format_money(-1234567.7777)
    '-1,234,567.78'
    >>> format_money(12345678.7777)
    '12,345,678.78'
    >>> format_money(-12345678.7777)
    '-12,345,678.78'
    >>> format_money(123456789.7777)
    '123,456,789.78'
    >>> format_money(-123456789.7777)
    '-123,456,789.78'
    '''

    negative_fix = int(f < 0)

    s = '%.*f' % (frac_digits, f)
    if len(s) < 5 + frac_digits + negative_fix:
        return s

    l = list(s)
    l_fix = l[negative_fix:]
    p = len(l_fix) - frac_digits - 5
    l_fix[p::-3] = [i + delimiter for i in l_fix[p::-3]]

    return ''.join(l[:negative_fix] + l_fix)