Title: Validate credit card numbers in Python

Have you ever wondered how a website knows when you mistype your credit card number? The answer is checksums. A checksum is a digit or digits added to a value to verify that the number was typed correctly.

For example, a checksum might add up all of the digits in a number and then append the ones digit of the result. To calculate the checksum for the number 1729346, you add up 1 + 7 + 2 + 9 + 3 + 4 + 6 = 32. The last digit of the sum is 2 so the checksum is 2 and the encoded number would be 17293462.

Credit Card Checksums

1. Starting at the rightmost digit before the check digit, move left doubling every other digit. If the result is greater than 9, add the digits.
2. Add up the doubled digits and the non-doubled digits.
3. Subtract the total from the smallest multiple of 10 that's greater than or equal to the sum.

Here's an example.

371449635398431 Double these: ^ ^ ^ ^ ^ ^ ^ Doubled: 5 8 9 6 6 7 6 Non-doubled: 3 1 4 6 5 9 4 Add: 5 + 8 + 9 + 6 + 6 + 7 + 6 + 3 + 1 + 4 + 6 + 5 + 9 + 4 = 79 Subtract: 80 - 79 = 1

Python Code

def checksum(number): '''Calculate a credit card checksum.''' clean = number.replace(' ', '') reversed = clean[:-1][::-1] sum = 0 double_it = True for ch in reversed: value = int(ch) if double_it: value *= 2 if value >= 10: value = (value // 10) + (value % 10) sum += value double_it = not double_it sum = 10 - sum % 10 if sum == 10: sum = 0 return sum

The following card_is_valid function uses checksum to see if a credit card number seems valid.

def card_is_valid(number): '''Return True if the credit card number is valid.''' sum = checksum(number) return int(number[-1]) == sum

Card Companies

This example uses the following class to store BIN information.

class bin: def __init__(self, brand, min_bin, max_bin): self.brand = brand self.min_bin = min_bin self.max_bin = max_bin def matches(self, number): '''Return True if this number matches the bin.''' bin_length = len(self.min_bin) return self.min_bin <= number[:bin_length] <= self.max_bin

The class's constructor simply saves the card name and BIN range. The matches method return True if a given number lies between the minimum and maximum BIN values. The calculation is a little odd because the numbers are treated as strings and not all credit card numbers have the same length.

The following find_brand function builds a small BIN database and then uses it to return a card's brand.

def find_brand(number): # Example bins from https://chargebacks911.com/bank-identification-numbers/ # There's a more complete list at https://bintable.com/card-schemes, although # it's hard to use and the site has many spam advertizing/download links. bins = [ bin('Visa', '4', '4'), bin('AMEX', '34', '37'), bin('Diner's Club', '36', '36'), bin('MasterCard', '51', '55'), bin('Discover Card', '6011', '6011'), bin('Discover Card', '622126', '622925'), bin('Discover Card', '644', '649'), bin('Discover Card', '65', '65'), ] for a_bin in bins: if a_bin.matches(number): return a_bin.brand return None

Main Program

numbers = [ '378282246310005', '371449635398431', '378734493671000', '5610591081018250', '30569309025904', '38520000023237', '6011111111111117', '6011000990139424', '3530111333300000', '3566002020360505', '5555555555554444', '5105105105105100', '4111111111111111', '4012888888881881', '4222222222222', '76009244561', # This one seems to be wrong. '5019717010103742', '6331101999990016', ] for number in numbers: sum = checksum(number) is_valid = " valid " if card_is_valid(number) else "INVALID" brand = find_brand(number) print(f'{number:<16} ({sum}) is {is_valid}, {brand}')

Download the example to see additional details.