Learning the ropes: understanding Python generics

I've come across this EuroPython Conference talk on Python type system Generics. The presenter gave explanatory code snippets to cover a few concepts: - how generics work and how the type system interacts with inheritance - variance of generic types (how generics work and how the type system interacts with inheritance) - a case study

Also, a select code snippet to highlight use case of Generic and `TypeVar``

from typing import TypeVar, Generic


class Food:
    pass


T = TypeVar("T", bound=Food)


class CatFood(Food):
    pass


class Animal(Generic[T]):

    def feed(self, food: T):
        print("Yum!")


class Cat(Animal[CatFood]):

    def feed(self, food: CatFood):
        print("Yum!")


class Dog(Animal[Food]):

    def feed(self, food: Food):
        ...

def feed(self, food: CatFood): will break without Generic and TypeVar, because by default callable behaves contravariant in types of arguments.

Contravariant means that a data type can be substituted with a more general type. A more general type of Animal is object so def feed(self, food: object would be expected.