Help with getting sub-program to print text

[djwilson0495]

I'm trying to use the following sub-program to print a shift code:

def shift():
    message = input("Please enter a message:")
    number = int(input('Please enter the number that you would like to shift by (1-26):'))
    new_message = ""
    for letter in message:
        letter = letter.lower()
        if letter.isalpha():
            new_position = alphabet.index(letter) + number
            if new_position > 25:
                new_position = new_position - 26
            new_letter = alphabet[new_position]
        elif letter.isnumeric():
            letter = letter
            print(letter)
        elif letter == " " or letter == "," or letter == "." or letter == ";" or letter == ":":
            letter = letter
            print(letter)
        else:
            print("Error in message please try a different message")
    print(new_message)
    print()

But its just printing blank space, can someone help me with the reason it;s doing this?