Composable types [python-protocol] - Typescript to Python field guide

composition.py

from typing import Protocol


class Animal(Protocol):
    length: float

    def eat(self, food: str) -> None:
        ...

# Fish extends the protocol Animal and is itself a protocol
class Fish(Animal, Protocol):
    def swim(self, how_long: float) -> None:
        ...

# MyFish implements the Fish interface implicitly without inheritance
class MyFish:
    length: float = 12

    def eat(self, food: str) -> None:
        print(f"Eating {food}")

    def swim(self, howLong: float) -> None:
        print(f"Swimming for {howLong}")

# MyFish implements the Fish protocol but does not directly reference it
fish = MyFish()

def do_something_fishy(what: Fish):
    what.eat("worms")
    what.swim(what.length)


do_something_fishy(fish) # this is ok!