What is the logic behind these if statements? [closed]

I'm new to coding so I don't really understand the logic behind some of this stuff. I do understand a basic if statement. Could someone please eli5 the logic below and why it doesn't work?

import argparse
parser = argparse.ArgumentParser()
parser.add_argument("mode", help="Use encode/decode")
args = parser.parse_args()
mode = args.mode
encode = 'encode'
decode = 'decode'

Now for my if statements. This one works fine but I want the inverse.

if mode == encode or mode == decode:
   print("Mode is supported!")

This one works fine and figured out after some searching

if not (mode == encode or mode == decode):
   print("Mode isn't supported")
   exit()

This was my first choice because I thought it should operate like the first one but opposite which it doesn't.

if mode != encode or mode != decode:
   print("Mode isn't supported")
   exit()