Cfrtjryk

I am doing some projects so i can teach myself programming. The one that i finished states:
"Credit Card Validator - Takes in a credit card number from a common credit card vendor and validates it to make sure that it is a valid number (look into how credit cards use a checksum)."
Would be glad if i can get a constructive feedback on this one.

import sys



cardnumb_input = input('Enter your credit card number: ')





def list_tweak():

    try:

        cardnumb = [int(i) for i in cardnumb_input]

        cardnumb[0::2] = [x * 2 for x in cardnumb[0::2]]

        cardnumb = [str(i) for i in cardnumb]

        cardnumb = list(''.join(cardnumb))

        cardnumb = [int(i) for i in cardnumb]

        return cardnumb



    except ValueError:

        print('Please enter only number in format: xxxxxxxxxxxxxxxx')

        sys.exit(1)





def valid_or_not():

    total = sum(list_tweak())

    if total % 10 != 0:

        print('It seems that this credit card number is not valid.')

    else:

        print('This credit card number is valid. ')





valid_or_not()