CS50 2017 vigenere.py Issues "TypeError: not all arguments converted during string formatting"

Question

As the title suggests, I have a "TypeError: not all arguments converted during string formatting" error in vigenere.py for Pset6.

Here's the code:

import cs50
import sys

if len(sys.argv) == 1 or len(sys.argv) > 2:
    print("Usage: python vigenere.py <key>")
    exit(1)

key = sys.argv[1]
for i in range(len(key)):
    if key[i].isalpha() is not True:
        print("Keyword must only contain letters A-Z or a-z")
        exit(2)

print("plaintext: ", end="")
plaintext = cs50.get_string()
plaintext_size = len(plaintext)
key_size = len(key)
ciphertext = []
for i in range(plaintext_size):
    j = 0
    if j >= key_size:
        j = 0
    if plaintext[i].isalpha():
        k = key[j].upper() % 65
        c = plaintext[i] + k
        if ord(plaintext[i].upper()) + k > ord('Z') or ord(plaintext[i].lower() + k > ord('z')):
            c = c - 26
        ciphertext.append(chr(c))
        j = j + 1
    else:
        ciphertext[i] = plaintext[i]

print("ciphertext: ", end="")
print("".join(ciphertext))
exit(0)

Any help would be appreciated. Thanks.

Answer 1

On this line:

k = key[j].upper() % 65

The % operator in Python does different things depending on whether the left operand is a number or a string. If it's a number, then % does modulo division as in C (with the added feature that Python's modulo also works with floats). If it's a string, however, % performs string formatting, filling a similar role as the sprintf function from C. Details on this formatting operation can be found here.

Additionally, whereas a char in C is effectively a kind of number, and can be used in arithmetic, there's no such implicit equivalence between ints and single-character strings in Python. Instead, you'll want to use the ord function.

P.S. When asking for help with an error message, it's helpful to point out which line Python says is causing the error.