template class

example.py

from typing import ClassVar, Protocol


class APIKey(Protocol):  # provided by some framework
    header_name: ClassVar[str]
    key: str

    def __init__(self, key: str) -> None:  # so that framework can construct the subclass
        self.key = key


class MyAPIKey(APIKey):
    header_name = "foo"  # user fills in the blanks or type checker complains



def endpoint(auth: MyAPIKey) -> None:  # framework knows the right class to inject from the signature
    assert auth.key == "123"


def framework() -> None:
    # get key from request
    endpoint(MyAPIKey("123"))  # confusing error here due to how Protocol overrides __init__ from __init_subclass__