223 – Protocols

A protocol lets you define a type through the methods that it requires without impacting the class hierarchy directly. To create a protocol, you define a class that inherits from typing.Protocol:

from typing import Protocol

class Diver(Protocol):
    def dive(self) -> None: ...

The class Diver defines that any class that implements a method dive with the signature dive(self) -> None is also a Diver.

A protocol can be used as a type in function signatures, for example:

def go_for_a_dive(obj: Diver) -> None:
    # ...

Suppose that now you define the class Penguin:

class Penguin:
    def dive(self) -> None:
        # ...

The class Penguin does not inherit from Diver. But since the class Penguin implements the method dive, a Penguin is also a Diver. That’s why this code would type check:

go_for_a_dive(Penguin())