ComVisible

The Luhn algorithm or Luhn formula, is a simple formula used to validate a variety of identification numbers, such as credit card numbers. Here is my implementation written in Python:

def checkDigit(digits):
    digitList   = []
    digitOdd    = []
    checksumOdd = 0
    sumOdd      = 0
    sum_        = 0
    sumRounded  = 0
    difference  = 0

    #Support for creditcard numbers that are passed in as argument of
    #type string or list
    if isinstance(digits, str):
        for char in digits:
            digitList.append(int(char))
    elif isinstance(digits, list):
        digitList = digits

    checkDigit  = digitList[len(digitList) -1]

    #Remove the checkdigit from the list so we can easily iterate over
    #the digitList array without worrying to much about the checkdigit itself
    digitList.pop()

    #Step 1: Multiplication of all digits in odd index position of digits array
    i = 0
    for digit in digitList:
        if i % 2 == 0:
            digitProduct = digitList[i] * 2
            digitOdd.append(digitProduct)
        i += 1

    #Step 2: Built checksums of all products from step 1 and do addition of
    #all checksums
    for digit in digitOdd:
        if digit >= 10:
            digit = (digit - 10) + 1
        checksumOdd += digit

    #Step 3: Addition of all digits in even index position of digits array
    i = 0
    for digit in digitList:
        if i % 2 != 0:
            sumOdd += digit
        i += 1

    #Step 4: Addition of all sums from step 2 and step 3
    sum_ = sumOdd + checksumOdd

    #Step 5: Round result from step 4 to next largest, through 10 divideable number
    sumRounded = int((sum_ / 10) * 10 + 10)

    #Step 6: Calculate difference of result from step 5 and step 4
    difference = sumRounded - sum_

    #Step 7: Check if calculated number is the same as the check digit and
    #return either True or False
    if difference == checkDigit:
        return True
    else:
        return False

if __name__ == "__main__":
    #checkDigit method accepts Lists or Strings as parameter

    digitList  = [2,7,1,8,2,8,1,8,2,8,4,5,8,5,6,7]
    #digitList   = "2718281828458566"
    if checkDigit(digitList):
        print "Creditcard is valid!"
    else:
        print "Creditcard is not valid!"

This will either return True or False depending if the check digit is valid or not.