I want to specify "X or any subclass of X"

Question

class Animal:
    def speak(self) -> None:
        print("*generic animal noises*")

def choose_speaker(speaker1: Animal, speaker2: Animal) -> None:
    if random.random() >= 0.5:
        speaker1.speak()
    else:
        speaker2.speak()

How do I specify that choose_speaker accepts Animal or any subclass of Animal?

It already does!

If a function works with objects of class A, it also works with subclasses of A. (At least it should!) This is called "substitutability".

Now, in Python land, this is not the only view on inheritance. Inheritance is also a mechanism for code reuse, see this talk from Raymond Hettinger for an explanation.

What does it look like? Consider a function like this:

def contains_foo(things: dict[str, int]) -> bool:
    try:
        things["foo"]
    except KeyError:
        return False
    else:
        return True

and then you might refactor it like this:

def contains_foo(things: dict[str, int]) -> bool:
    return "foo" in things

This refactoring relies on some assumptions about how dicts work.

The first version will always return True for a defaultdict, but the second one will not. defaultdict violates some implicit assumptions about dictionaries, which makes it harder to know if a function that accepts a dictionary (or a Mapping) will work correctly with a defaultdict.

Nevertheless, in the typing world, a child class is considered appropriate whenever a parent class is.