def check_digit ():

        users_gtin=raw_input("enter first seven digits of gtin ")

        gtin_seven_digits=unicode(users_gtin)


        if len(gtin_seven_digits) == 7 and gtin_seven_digits.isnumeric():
            ck = ((((int(gtin_seven_digits[0])) + (int(gtin_seven_digits[2])) + (int(gtin_seven_digits[4])) + (int(gtin_seven_digits[6])))*3) + ((int(gtin_seven_digits[1])) + (int(gtin_seven_digits[3])) + (int(gtin_seven_digits[5])))) %10
            final_ck = 10-ck

            if final_ck == 10:
                final_ck=0
                print "Is your check digit",final_ck,"?"

            else:
                print "Is your check digit",final_ck,"?"

        else:
            print "please try typing an seven digit number"
            check_digit()


        choice=raw_input("enter (a) to restart or press anything other than the letter  a  to end this program <<<< ").upper()


        if choice == "A":
            check_digit()

check_digit ()

, вероятно, не самый эффективный, но надеюсь, что это поможет ..