globuous

    class Animal {}

    class Dog extends Animal {
        woof() {}
    }

    class Cat extends Animal {
        meow() {}
    }

    let append_animals = (animals: Animal[], animal: Animal) => animals.push(animal)

    let dogs = [new Dog()]
    append_animals(dogs, new Cat())

    dogs.map(dog => dog.woof())

    Uncaught TypeError: dog.woof is not a function

    class Animal:
        pass

    class Dog(Animal):
        pass


    def append_animals(animals: List[Animal], animal: Animal) -> None:
        animals.append(animal)


    dogs = [Dog()]
    append_animals(dogs, Dog())

    $ mypy types.py 
    types.py:16: error: Argument 1 to "append_animals" has incompatible type "List[Dog]"; expected "List[Animal]"
    types.py:16: note: "List" is invariant -- see https://mypy.readthedocs.io/en/stable/common_issues.html#variance
    types.py:16: note: Consider using "Sequence" instead, which is covariant