translatemessageQWERTYUIOPASDFGHJKLZXCVBNM,Hello World,encrypt 'Pcssi Bidsm' loop through each symbol in the message encryptdecrypt the symbol symbol is not in LETTERS, just add it  encryptmessageQWERTYUIOPASDFGHJKLZXCVBNM, Hello World 'Pcssi Bidsm'  decryptmessageQWERTYUIOPASDFGHJKLZXCVBNM, Hello World 'Itssg Vgksr'
from typing import Literal

LETTERS = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"


def translate_message(
    key: str, message: str, mode: Literal["encrypt", "decrypt"]
) -> str:
    """
    >>> translate_message("QWERTYUIOPASDFGHJKLZXCVBNM","Hello World","encrypt")
    'Pcssi Bidsm'
    """
    chars_a = LETTERS if mode == "decrypt" else key
    chars_b = key if mode == "decrypt" else LETTERS
    translated = ""
    # loop through each symbol in the message
    for symbol in message:
        if symbol.upper() in chars_a:
            # encrypt/decrypt the symbol
            sym_index = chars_a.find(symbol.upper())
            if symbol.isupper():
                translated += chars_b[sym_index].upper()
            else:
                translated += chars_b[sym_index].lower()
        else:
            # symbol is not in LETTERS, just add it
            translated += symbol
    return translated


def encrypt_message(key: str, message: str) -> str:
    """
    >>> encrypt_message("QWERTYUIOPASDFGHJKLZXCVBNM", "Hello World")
    'Pcssi Bidsm'
    """
    return translate_message(key, message, "encrypt")


def decrypt_message(key: str, message: str) -> str:
    """
    >>> decrypt_message("QWERTYUIOPASDFGHJKLZXCVBNM", "Hello World")
    'Itssg Vgksr'
    """
    return translate_message(key, message, "decrypt")


def main() -> None:
    message = "Hello World"
    key = "QWERTYUIOPASDFGHJKLZXCVBNM"
    mode = "decrypt"  # set to 'encrypt' or 'decrypt'

    if mode == "encrypt":
        translated = encrypt_message(key, message)
    elif mode == "decrypt":
        translated = decrypt_message(key, message)
    print(f"Using the key {key}, the {mode}ed message is: {translated}")


if __name__ == "__main__":
    import doctest

    doctest.testmod()
    main()
