Recipe3.15.Checking a Credit Card Checksum

Recipe 3.15. Checking a Credit Card Checksum

Credit: David Shaw, Miika Keskinen

Problem

You need to check whether a credit card number respects the industry standard Luhn checksum algorithm.

Solution

Luhn mod 10 is the credit card industry's standard for credit card checksums. It's not built into Python, but it's easy to roll our own computation for it:

def cardLuhnChecksumIsValid(card_number):     """ checks to make sure that the card passes a luhn mod-10 checksum """     sum = 0     num_digits = len(card_number)     oddeven = num_digits & 1     for count in range(num_digits):         digit = int(card_number[count])         if not (( count & 1 ) ^ oddeven):             digit = digit * 2         if digit > 9:             digit = digit - 9         sum = sum + digit     return (sum % 10) == 0

Discussion

This recipe was originally written for a now-defunct e-commerce application to be used within Zope.

It can save you time and money to apply this simple validation before trying to process a bad or miskeyed card with your credit card vendor, because you won't waste money trying to authorize a bad card number. The recipe has wider applicability because many government identification numbers also use the Luhn (i.e., modulus 10) algorithm.

A full suite of credit card validation methods is available at http://david.theresistance.net/files/creditValidation.py

If you're into cool one-liners rather than simplicity and clarity, (a) you're reading the wrong book (the Perl Cookbook is a great book that will make you much happier), (b) meanwhile, to keep you smiling while you go purchase a more appropriate oeuvre, try:

checksum = lambda a: (   10 - sum([int(y)*[7,3,1][x%3] for x, y in enumerate(str(a)[::-1])])%10)%10

Recipe3.15.Checking a Credit Card Checksum