CleanIt | source Autonomy Software in Rust-lang with gRPC | Robotics library

import re

alphabet = "abcdefghijklmnopqrstuvwxyz"

def cleanIt(clean):
    global alphabet
    s = re.sub('[^a-z]+', '?', str(clean))
    return s

def symbol_add(symbol, key):
    position = alphabet.find(symbol)
    newPosition = (position + key) % 26
    newLetter = alphabet[newPosition]
    return newLetter

def cipher(data,key):
    text = ""
    for letter in data:
        if letter in alphabet:
            text += symbol_add(symbol=letter,key=key)
    return text

def main():
    try:
        msg = (input("Write the message you would like to encrypt\n"))
        key = int(input("which key would you like to use?\n"))
        # Note: you need to assign msg to be equal to cleanIt(clean=msg). 
        #       Just calling cleanIt(clean=msg) won't work, as strings 
        #       are immutable in Python
        msg = cleanIt(clean=msg)
        print(cipher(data=msg, key=key))
    except ValueError:
        print("Write a number!")

main()