type hints

sample.py

from typing import Callable, TypeVar, cast, Generic


class SomeRequest:
    pass


class SomeResponse:
    pass


T = TypeVar('T')


class Result(Generic[T]):

    def __inti__(self):
        self._result = None

    def set(self, result):
        self._result = result

    def get(self) -> T:
        return self._result


Req = TypeVar('Req')
Resp = TypeVar('Resp')


def lazy_result(fn: Callable[..., Resp]) -> Callable[..., Result[Resp]]:

    def wrapper(self, request) -> Result[Resp]:
        r = Result()
        resp = fn(self, request)
        r.set(resp)
        return r

    return wrapper


class SomeService:

    @lazy_result
    def foo(self, request: SomeRequest) -> SomeResponse:
        return SomeResponse()


service = SomeService()

req = service.foo(SomeRequest())
response = req.get()    # type inference does not work in PyCharm

assert type(response) is SomeResponse, response