信用中的 NameError - pset6 CS50x

Question

I am new to Python. My only background in Python is CS50(Week 6). I am trying to implement Credit from pset6. In this, we have to implement Luhn's Algorithm.

But, I am getting the undermentioned error and I am not able to understand why:

Traceback (most recent call last):
  File "credit.py", line 57, in <module>
    main()
  File "credit.py", line 9, in main
    if IfAmex(arr_number):
  File "credit.py", line 27, in IfAmex
    if Luhn(card_n):
  File "credit.py", line 50, in Luhn
    sum2 = sum(int(c) for c in str(2 * card_n[i]) for i in range(1, len(card_n), 2))
NameError: name 'i' is not defined

Here is my code:

from sys import exit
from cs50 import get_int

def main():
    number = get_int("Number: ")
    # Converting the number into a list
    arr_number = [int(x) for x in str(number)]

    if IfAmex(arr_number):
        print("AMEX")
        exit(0)
    elif IfMasCard(arr_number):
        print("MASTERCARD")
        exit(0)
    elif IfVisa(arr_number):
        print("VISA")
        exit(0)
    else:
        print("INVALID")
        exit(0)

def IfAmex(card_n):
    if not len(card_n) == 15:
        return False
    if not (card_n[0] * 10 + card_n[1]) in [34, 37]:
        return False
    if Luhn(card_n):
        return True

def IfMasCas(card_n):
    if not len(card_n) == 16:
        return False
    if not (card_n[0] * 10 + card_n[1]) in range(51, 56):
        return False
    if Luhn(card_n):
        return True

def IfVisa(card_n):
    if not len(card_n) in [13, 16]:
        return False
    if not card_n[0] == 4:
        return False
    if Luhn(card_n):
        return True

def Luhn(card_n):
    # Reversing the digits
    card_n = card_n[::-1] 

    sum1 = sum(card_n[::2])
    sum2 = sum(int(c) for c in str(2 * card_n[i]) for i in range(1, len(card_n), 2)) # The Error Line

    luhnsum = sum1 + sum2

    return luhnsum % 10 == 0

if __name__ == '__main__':
    main()

And the test case I used was(along with the correct output):

$ python credit.py
Number: 378282246310005
AMEX

Also, please tell me whether I am using exit() correctly or not.

Answer 1

You use i here as an index of card_n array. But you forgot to define it.

Define it and assign it to a value before using.

sum2 = sum(int(c) for c in str(2 * card_n[i])