How to Validate Credit Card Using Python?

I'm a beginner Python learner and I'm currently working on Luhn Algorithm to check credit card validation . I wrote most of the code, but I'm stuck with 2 errors I get 1st one is num is referenced before assignment. 2nd one I'm getting is object of type '_io.TextIOWrapper' has no len(). Further help/ guidance will be greatly appreciated.

def checkSecondDigits(num):
    length = len(num)
    sum =  0
    for i in range(length-2,-1,-2):
      number = eval(num[i])
      number = number * 2
      if number > 9:
          strNumber = str(number)
          number = eval(strNumber[0]) + eval(strNumber[1])
          sum += number
      return sum

def odd_digits(num):
    length = len(num)
    sumOdd = 0
    for i in range(length-1,-1,-2):
        num += eval(num[i])
    return sumOdd

def c_length(num):
    length = len(num)
    if num >= 13 and num <= 16:
    if num [0] == "4" or num [0] == "5" or num [0] == "6" or (num [0] == "3" and num [1] == "7"):
        return True
    else:
        return False


def main():
    filename = input("What is the name of your input file? ")
    infile= open(filename,"r")
    cc = (infile.readline().strip())
    print(format("Card Number", "20s"), ("Valid / Invalid"))
    print("------------------------------------")
    while cc!= "EXIT":
        even = checkSecondDigits(num)
        odd = odd_digits(num)
        c_len = c_length(num)
        tot = even + odd

        if c_len == True and tot % 10 == 0:
            print(format(cc, "20s"), format("Valid", "20s"))
        else:
            print(format(cc, "20s"), format("Invalid", "20s"))
        num = (infile.readline().strip())