encrypt.py

import string

letters = string.ascii_lowercase

message = input("\nEnter the message to be encrypted: ").lower()

plaintext = ""

for letter in message:
    if letter.isalpha():
        plaintext += letter

while True:
    try:
        fileName = input("\nEnter the name of the file that contains the key (exclude the .txt): ").strip() + ".txt"
        with open(fileName, "r") as f:
            key = f.read()
        break
    except (FileNotFoundError, OSError):
        print("This file doesn't exist. Try again.")
    
ciphertext = ""

for i, letter in enumerate(plaintext):
    ciphertext += letters[(letters.index(letter) + letters.index(key[i])) % 26]

print("\nThe used portion of the key has been automatically deleted.\nEncrypted message: " + ciphertext + "\n")

key = key[len(plaintext):]

with open(fileName, "w") as f:
    f.write(key)