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The Luhn algorithm, also known as the "modulus 10" algorithm, is a checksum formula used to validate a variety of identification numbers, such as credit card numbers, IMEI numbers, National Provider Identifier numbers in the United States, Canadian Social Insurance Numbers and Israel ID Numbers.

Generating check digit:

- Lets assume you have a number as: 3 - 7 - 5 - 6 - 2 - 1 - 9 - 8 - 6 - 7 - X where X is the check digit.
- Now starting from the right most digit i.e. check digit, double the every second digit. New number will be: 3 - 14 - 5 - 12 - 2 - 2 - 9 - 16 - 6 - 14 - X
- Now if double of a digit is more then 9, add the digits. So the number will become: 3 - 5 - 5 - 3 - 2 - 2 - 9 - 7 - 6 - 5 - X
- Now add all digits. 47 + X
- Multiply the non-check part by 9. So it will be 47 * 9 = 423
- Unit digit in the multiplication result is the check digit. X = 3
- Valid number would be 37562198673.

""" Python script to check validity of credit card numbers Author : https://www.pythoncircle.Com """ import sys def usage(): msg = """ usage: python3 credit_card_validator credit_card_number example: python3 credit_card_validator 34678253793 """ print(msg) def get_cc_number(): if len(sys.argv) < 2: usage() sys.exit(1) return sys.argv[1] def sum_digits(digit): if digit < 10: return digit else: sum = (digit % 10) + (digit // 10) return sum def validate(cc_num): # reverse the credit card number cc_num = cc_num[::-1] # convert to integer cc_num = [int(x) for x in cc_num] # double every second digit doubled_second_digit_list = list() digits = list(enumerate(cc_num, start=1)) for index, digit in digits: if index % 2 == 0: doubled_second_digit_list.append(digit * 2) else: doubled_second_digit_list.append(digit) # add the digits if any number is more than 9 doubled_second_digit_list = [sum_digits(x) for x in doubled_second_digit_list] # sum all digits sum_of_digits = sum(doubled_second_digit_list) # return True or False return sum_of_digits % 10 == 0 if __name__ == "__main__": print(validate(get_cc_number()))

Code is available on github.

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