decrypt.py

import string

letters = string.ascii_lowercase

while True:
    ciphertext = input("\nEnter the message to be decrypted: ").lower()
    if ciphertext.isalpha:
        break
    else:
        print("Make sure you only include alphabet letters. Try again.")

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.")

plaintext = ""

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

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

key = key[len(ciphertext):]

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