Caeser Cypher Help

[Ararisvalerian]

Back story: I was asked to teach some technology classes this year. Little did I know I would be asked to teach python. Now I am learning python with the kids. One of their assignments is to write a script that inputs a line of plaintext and a distance value and outputs an encrypted text using a Caesar cipher. The script should work for any printable characters. The code I wrote works well except it does not work for characters. Can anyone help me tell me what I am doing incorrectly?

# Caesar Cipher

MAX_KEY_SIZE = 26

def getMode():
    while True:
       print('Do you wish to encrypt or decrypt a message?')
       mode = input().lower()
       if mode in 'encrypt e decrypt d'.split():
           return mode
       else:
           print('Enter either "encrypt" or "e" or "decrypt" or "d".')
            
def getMessage():
    print('Enter your message:')
    return input()

def getKey():
    key = 0
    while True:
        print('Enter the key number (1-%s)' % (MAX_KEY_SIZE))
        key = int(input())
        if (key >= 1 and key <= MAX_KEY_SIZE):
            return key
        
def getTranslatedMessage(mode, message, key):
    if mode[0] == 'd':
        key = -key
    translated = ''
    
    for symbol in message:
        if symbol.isalpha():
            num = ord(symbol)
            num += key
            
            if symbol.isupper():
                if num > ord('Z'):
                    num -= 26
                elif num < ord('A'):
                    num += 26
            elif symbol.islower():
                if num > ord('z'):
                    num -= 26
                elif num < ord('a'):
                    num += 26
                    
            translated += chr(num)
        else:
           translated += symbol
    return translated

mode = getMode()
message = getMessage()
key = getKey()

print('Your translated text is:')
print(getTranslatedMessage(mode, message, key))