Why can I put a cat in a list of animals?

Question

If lists are invariant, how come it's legal to append a Cat to a list[Animal]?

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

class Cat(Animal):
    def speak(self) -> None:
        super().speak()
        print("also: meow")

animals: list[Animal] = [Animal(), Animal()]
animals.append(Cat())

Doesn't this require list to be covariant?

A list being invariant only means that you can't put a list of cats into a variable which stores a list of animals (or vice versa).

It's still legal to put a cat into a variable which stores an animal:

cat: Cat = Cat()
animal: Animal = cat

and this is roughly what happens when you call animals.append(Cat()): animals.append expects an animal, and a cat is a perfectly fine animal:

cat: Cat = Cat()
animal: Animal = cat

animals.append(animal)

Does this break anything?

list is invariant because if it weren't, bad things could happen:

def add_pets(animals: list[Animal]) -> None:
    animals.append(Dog())
    animals.append(Dragon())

cats: list[Cat] = []
animals: list[Animal] = cats  # this is illegal!
add_pets(animals)  # ...because this would make the cats very uncomfortable

But if you put a Cat into a list of Animals, everything will be fine. You can only extract an Animal from the list, and a Cat will work whenever an Animal works (at least it should).