Python - If "a" or "b"

I have this (simplified):

    foo = input("Test")
    if foo is "a" or "b":
        print("Test")

This returns Test for everything that the user inputs, while those work and only return when a or b is given.

    foo = input("Test")
    if foo is "a" or foo is "b":
        print("Test")

-

    foo = input("Test")
    if foo in ("a" or "b"):
        print("Test")

Why does the first one not work?

I can only guess that the first one actually checks if foo == "a", but not if foo == "b", and that the or "b" part always returns True - but why?