python string if x != y is not working as expected

I am studying python 3. I know I should probably use argparse, but I wanted a simple test of a string in an if statement, getting data from the input() function.

If I test for 2 different strings with == things work as expected But if I reverse my if statement and test with != it does not seem to work

here is the code that works as expected

import sys
import os

tdown = input("topdown true? type t or f: ")
print("tdown", type(tdown), tdown)

if  tdown == 't' or tdown == 'f':
    pass
else:
    print(" need to type 't' or 'f', exiting")
    sys.exit(1)
print("passed arg test, tdown is: ", tdown)

running the code, and here are the results

./os_walk.py

topdown true? type t or f: t
tdown <class 'str'> t./os_walk.py
passed arg test, tdown is:  t

./os_walk.py

topdown true? type t or f: f
tdown <class 'str'> f
passed arg test, tdown is:  f

./os_walk.py

topdown true? type t or f: x
tdown <class 'str'> x
 need to type 't' or 'f', exiting

here is the code using != which does not work as expected

import sys
import os

tdown = input("topdown true? type t or f: ")
print("tdown", type(tdown), tdown)

if  tdown != 't' or tdown != 'f':
    print(" need to type 't' or 'f', exiting")
    sys.exit(1)
else:
    pass
print("passed arg test, tdown is: ", tdown)

if I run this we get these results

./os_walk.py

topdown true? type t or f: t
tdown <class 'str'> t
 need to type 't' or 'f', exiting

./os_walk.py

topdown true? type t or f: x
tdown <class 'str'> x
 need to type 't' or 'f', exiting

I dont understand why the != is giving me an unexpected result, can anyone explain please?