Luhn validation algorithm for credit cards.

luhn.py

"""
Python implementation of the Luhn algorithm.

Refer to: http://en.wikipedia.org/wiki/Luhn_algorithm

>>> is_luhn_valid(79927398713)
True
>>> is_luhn_valid(79927398714)
False
>>> is_luhn_valid(4532990475689429)
True
>>> is_luhn_valid(4532990475689420)
False
>>> is_luhn_valid(4556278174544876)
True
>>> is_luhn_valid(4556278174544871)
False
>>> is_luhn_valid(5325806143405717)
True
>>> is_luhn_valid(5325806143405712)
False
>>> is_luhn_valid(5459209022644894)
True
>>> is_luhn_valid(5459209022644895)
False

>>> calculate_luhn(7992739871)
3
>>> calculate_luhn(453299047568942)
9
>>> calculate_luhn(455627817454487)
6
>>> calculate_luhn(532580614340571)
7
>>> calculate_luhn(545920902264489)
4
"""

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 calculate_luhn(partial_card_number):
    return 10 - luhn_checksum(int(partial_card_number) * 10)