[Python] How can I use a decorator to flag classes as passing a certain Protocol?

I would like to be able to define a decorator for a class in python adding a class attribute which I can then check for in a Protocol, to be used in type checking elsewhere.

I tried the following code:

from dataclasses import dataclass
from typing import Protocol, Any

class Canine(Protocol):
__canine__: Any

def make_canine(cls):
setattr(cls, "__canine__", True)
return cls

@make_canine
@dataclass
class Dog:
name: str
weight: float
colour: str
good_dog: bool = True


@dataclass
class Cat:
name: str
weight: float
colour: str
good_cat: bool = True


def play_fetch(animal: Canine):
print(f"Playing fetch with {animal}...")

clifford = Dog('Clifford', 1000, 'Red')
garfield = Cat('Garfield', 10, 'Orange')

play_fetch(Dog) # I expected this to pass type checking
play_fetch(Cat) # I expected this to fail type checking

play_fetch(clifford) # I thought this would also pass, but I'm not sure?
play_fetch(garfield) # I expect this to fail

print(Dog.__canine__) # print(True)
print(Cat.__canine__) # AttributeError


None of the lines that I thought would pass type checking did. Can someone explain why it doesn't behave as I expected?

Continue reading...