Shift for uppercase letters

I am writing a program on shifting a word. My desired output should be a:f b:g c:h ... y:d z:e A:F B:G C:H ... Y:D Z:E

import string

letters = string.ascii_letters #contains 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ'

#create the Caesar cypher
offset = 5 #choose your shift
totalLetters = 26
keys = {} #use dictionary for letter mapping
invkeys = {} #use dictionary for inverse letter mapping, you could use inverse search from original dict

for index, letter in enumerate(letters):
    # cypher setup
    if index < totalLetters: #lowercase
        letter = letters[index]
        keys[letter] = letters[(index + offset) % 26]
        print(letters[index] + ":" + keys[letter])
    else: #uppercase
        letter = letters.isupper()
        keys[letter] = letters[(index + offset) % 26]
        print(letters[index] + ":" + keys[letter])
       

But after running this code, my output is a:f b:g c:h ... y:d z:e A:f B:g C:h ... Y:d Z:e Seems isupper() function didn’t work here. Could you help with this based on my code structure. Thanks in advance!

Answer

in uppercase condition,

keys[letter] = letters[(index + offset) % 26 + 26]