luhn-valid

#!/usr/bin/env python
import re
import sys
from getpass import getpass

def luhn_checksum(card_number):
    def digits_of(n):
        return [int(d) for d in str(n)]
    digits = digits_of(card_number)
    odd_digits = digits[-1::-2]
    even_digits = digits[-2::-2]
    checksum = 0
    checksum += sum(odd_digits)
    for d in even_digits:
        checksum += sum(digits_of(d*2))
    return checksum % 10

def is_luhn_valid(card_number):
    return luhn_checksum(card_number) == 0

def redacted(card_number):
    if len(card_number) < 10:
        return card_number
    first6 = card_number[:6]
    last4 = card_number[-4:]
    middle = card_number[6:-4]

    return first6 + '*' * len(middle) + last4

if __name__ == '__main__':
    orig_number = getpass('number: ')
    number = re.sub('\D+', '', orig_number)
    if not number:
        print 'no number provided'
        sys.exit(2)

    print redacted(number),
    if is_luhn_valid(number):
        print 'is luhn valid'
    else:
        print 'is NOT luhn valid'
        sys.exit(1)