118 – Safely overriding methods

The decorator typing.overrides can be used to mark that a method is overriding a method from a parent class. If it isn’t, type checkers will complain.

Here’s an example super class that implements two methods sound and sleep:

class Animal:
    def sound(self) -> str:
        ...

    def sleep(self) -> None:
        ...

Suppose you want to override this class to create a class Cat:

from typing import overrides

class Cat(Animal):
    @overrides
    def sound(self) -> str:
        ...
        
    @overrides
    def seep(self) -> None
        ...

A type checker will see that Cat.sound and Cat.seep are supposed to be overrides of methods from the parent class(es). Since a type checker won’t find a method seep in a parent class (because it’s supposed to be “sleep”, with an extra L), you will get an error and you’ll easily spot your typo.

Using overrides is also useful if you’re inheriting from parent classes from other packages, since it will let you know if a method name changes; for example, if the API changes in a version upgrade.