credit.py

from cs50 import get_string

def main():
    card_number = get_string("Number: ")
    length = len(card_number)
    if not is_valid(card_number):
        print("INVALID")
    else:
        if length == 15 and (card_number[0:2] == "34" or card_number[0:2] == "37"):
            print("AMEX")
        elif length == 16 and (int(card_number[0:2]) >= 51 and int(card_number[0:2]) <= 55):
            print("MASTERCARD")
        elif (length == 13 or length == 16) and (card_number[0:1] == "4"):
            print("VISA")

def is_valid(number):
    index = 0
    sum_of_even_numbers = 0
    sum_of_odd_numbers = 0
    for digit in number:
        index += 1
        digit = int(digit)
        if (len(number) - index) % 2 == 0:
            # Even numbers: Add to total
            sum_of_even_numbers += digit
        else:
            # Odd number: Multiply by 2, sum product's individual digits, add to total
            digit *= 2
            if digit > 9:
                digit = int(str(digit)[0:1]) + int(str(digit)[1:2])
            sum_of_odd_numbers += digit

    if str(sum_of_even_numbers + sum_of_odd_numbers)[1:2] == "0":
        return True

    return False

main()