credit.py

# Main function
def main():
    num = input("Number: ")
    type = getCardType(num)
    firstSum = getFirstSum(num)
    secondSum = getSecondSum(num)

    # Validate if total sum ends with 0
    if (firstSum + secondSum) % 10 == 0:
        print(type)
    else:
        print("INVALID")


# Get type of card based on
def getCardType(num):
    if len(num) == 13 and int(num[0]) == 4:
        return "VISA"
    elif len(num) == 15 and int(num[0:2]) in {34, 37}:
        return "AMEX"
    elif len(num) == 16 and int(num[0]) == 4:
        return "VISA"
    elif len(num) == 16 and int(num[0:2]) in range(51, 56):
        return "MASTERCARD"
    else:
        return "INVALID"


# Iterate backwards over input from second last number and multiply by two, adding numbers to sum
def getFirstSum(num):
    sum = 0
    for i in num[-2::-2]:
        if int(i) < 5:
            sum += int(i) * 2
        else:
            sum += (int(i) * 2 % 10) + 1
    return sum


# Iterate backwards over input from last number and adding numbers to sum
def getSecondSum(num):
    sum = 0
    for i in num[::-2]:
        sum += int(i)
    return sum


main()