How to use "None" in list when i'm using .index() and object wight not exist in list

I want to write a function that encrypt text using caesar cipher. But I want to let non-letters characters to be the same.
I have list with alphabet and a "questions for user"

alphabet = ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z']

direction = input("Type 'encode' to encrypt, type 'decode' to decrypt:\n").lower()
text = input("Type your message:\n").lower()
shift = int(input("Type the shift number:\n"))

This is function which should let non-letters to be in code non changed

def encrypt(text, shift):
    text_encrypted = []  # blank text

    for letter in text:  # check every letter
        indeksik = alphabet.index(letter)
        if indeksik == None:
            text_encrypted.append(letter)
        else:
            text_encrypted.append(alphabet[indeksik + shift])

But then I’m getting this error:

Tracebac k (most recent call last):
  File "C:\Users\mateu\PycharmProjects\Cipher\main.py", line 25, in <module>
    encrypt(text, shift)
  File "C:\Users\mateu\PycharmProjects\Cipher\main.py", line 16, in encrypt
    indeksik = alphabet.index(letter)
ValueError: ' ' is not in list

I know that ' ' is not in list. That’s the point – how I can still append to another list these spaces and other non-alphabetical characters?
(Yes, I know that in this moment it will crash when I will shift letter "beyond z" – but I will work with this later)

>Solution :

index() raises a ValueError exception if there is no such item. if the value is not in the list. You can do something like:

if letter in alphabet:
    # Found the letter
else:
    # Not found

The other possible solution is to handle the exception itself, but I’d probably go with the first one.