Python - Message Encryption Returns `NoneType` Error

Question

I am trying to use the even and odd definitions to encrypt my message. The function def swap_letters(message) is what I am using for that:

• I loop through the lists of odd and even letters with `"message = message + 'x'"';
• Then, I add the next odd letter to the final message with "for counter in range(0, int(len(message)/2)): ...;
• Finally, I add the next even letter to the final message using "letter_1.append(even_letters[counter])...

I make my new_message by joining the letter_1 and then return it

What mistake do I make and how should I fix this code to have it run correctly?

import tkinter as Tk 
from tkinter import messagebox, simpledialog, Tk

def is_even(number):
    return number % 2 == 0

def get_even_letters(message):
    even_letters = []
    for counter in range(0, len(message)):
        if is_even(counter):
            even_letters.append(message[counter])


def get_odd_letters(message):
    odd_letters = []
    for counter in range(0, len(message)):
        if not is_even(counter):
            odd_letters.append(message[counter])
    return odd_letters

def swap_letters(message):
    letter_1 = []
    if not is_even(len(message)):
        message = message + 'x'
    even_letters = get_even_letters(message)
    odd_letters = get_odd_letters(message)
    for counter in range(0, int(len(message)/2)):
        letter_1.append(odd_letters[counter])
        letter_1.append(even_letters[counter])
    new_message = ''.join(letter_1)
    return new_message

def get_message():
    message = simpledialog.askstring('Message', 'Enter the secret here: ')
    return message

def get_task():
    task = simpledialog.askstring('Task', 'Do you want to encrypt or decrypt?')
    return task

root = Tk()

while True:
    task = get_task()
    if task == 'encrypt':
        message = get_message()
        encrypted = swap_letters(message)
        messagebox.showinfo('Ciphertext of the secret message is:', encrypted)
    elif task == 'decrypt':
        message = get_message()
        decrypted = swap_letters(message)
        messagebox.showinfo('Plaintext of the secret message is:', decrypted)
    else:
        break
root.mainloop()

This is the error I get:

    Traceback (most recent call last):
  File "T:\Sublime Text 3\shhhhh.py", line 47, in <module>
    encrypted = swap_letters(message)
  File "T:\Sublime Text 3\shhhhh.py", line 29, in swap_letters
    letter_1.append(even_letters[counter])

TypeError: 'NoneType' object is not subscriptable
[Finished in 21.2s with exit code 1]
[shell_cmd: python -u "T:\Sublime Text 3\shhhhh.py"]
[dir: T:\Sublime Text 3]
[path: C:\Program Files (x86)\Python36-32\Scripts\;C:\Program Files `(x86)\Python36-32\;C:\WINDOWS\system32;C:\WINDOWS;C:\WINDOWS\System32\Wbem;C:\WINDOWS\System32\WindowsPowerShell\v1.0\;C:\Program Files (x86)\NVIDIA Corporation\PhysX\Common;C:\Program Files\dotnet\;C:\Program Files\Microsoft SQL Server\130\Tools\Binn\;C:\Program Files (x86)\Xoreax\IncrediBuild;C:\Users\mwliv\AppData\Local\Microsoft\WindowsApps;]`

Answer 1

Your get_even_letters function doesn't return anything explicitly, so it implicitly returns None, and thus the error you're seeing later on. Adding a return statement should fix the problem:

def get_even_letters(message):
    even_letters = []
    for counter in range(0, len(message)):
        if is_even(counter):
            even_letters.append(message[counter])

    return even_letters # Here!