How to properly type class abstract async methods and override them in child classes

[acartine]

I have an abstract class with the method

    @abstractmethod
    async def reap(self, ts: int) -> None:
        pass

the child class implements it as follows

    async def reap(self, ts_ms: int) -> None:
        await self.__reap(ts_ms)

pyright reports the following

/home/acartine/code/nifty/src/nifty_worker/trend/toplist/async_toplist.py
  /home/acartine/code/nifty/src/nifty_worker/trend/toplist/async_toplist.py:154:15 - error: Method "reap" overrides class "AbstractTopList" in an incompatible manner
    Return type mismatch: base method returns type "Coroutine[Any, Any, None]", override returns type "Awaitable[None]"
      "Awaitable[None]" is incompatible with "Coroutine[Any, Any, None]" (reportIncompatibleMethodOverride)
1 error, 0 warnings, 0 informations 

what is the proper way to achieve what i'm trying to acheive here? is it to define the child method without async, and have it return Coroutine[Any,Any,None] ?

[erictraut]

Could you provide a complete, minimal code sample that exhibits the problem? I tried to recreate the problem from your description, but I'm not receiving any error.

# pyright: strict
from abc import ABC, abstractmethod

class Base(ABC):
    @abstractmethod
    async def reap(self, ts: int) -> None:
        pass

class Child(Base):
    async def reap(self, ts: int) -> None:
        pass

[acartine]

hey i went to go do so and could not. it looks like i ran it against an an unsaved buffer.

however i was able to reproduce the error message with a function decorator above the reap implementation. The function decorator had the return value

Callable[..., Awaitable[_T]].

so i assume that was the problem and that the typing of the decorator was incorrect. When i changed it to

Callable[..., Coroutine[Any, Any, _T]]

pycheck was happy.

I was fiddling around with that and i must have had an unsaved buffer out of view when running my usual sanity checks.

[Kolpa]

It's been a while but i've just ran into what i think is the same issue running python 3.12.4 with pyright 1.1.369
the following sample exhibits the same issue for me, in my case it is important to have the AsyncGenerator return type annotated.

class Test(ABC):
    @abstractmethod
    async def test(self) -> AsyncGenerator[bool]:
        pass


class TestImpl(Test):
    async def test(self) -> AsyncGenerator[bool]:
        yield True

I've put this in the pyright playground as well: https://pyright-play.net/?code=GYJw9gtgBAhgRgYygSwgBzCALlAggIQGEAaWOAZyxBgSwgFMsALMAEwChRIosBPNZADsA5inSYcucr0EIA4vUH1qWTO3YIANjHLkoAFXqUAFAUIBKAFzsotqAAF4larQbM2NuzplJW9YDxGWMbk9JrA5lAAtAB8eNKyCkoqmADacGBgmgC61nb5UGg65Ooa2roGQQCS6JrGhpRWnrbeslB%2BAVhBIWER0XFSPknKMKog6Zk5eQW2vMhhrAYgAK707EA

[erictraut]

In Python, the presence of a yield statement changes the type of a function or method. Type checkers like pyright and mypy reflect this in their type evaluation logic.

There has been some recent discussion about clarifying and/or changing this in the typing spec, but no consensus has been reached. Unless/until the spec is modified, I don't plan to make any changes in pyright because the current behavior has been in place for a long time, and it matches mypy's behavior.

You can accommodate the current behavior by adding a yield statement to the abstract method implementation.

class Test(ABC):
    @abstractmethod
    async def test(self) -> AsyncGenerator[bool]:
        yield True

[kavinyao]

@erictraut Does it mean it's not possible to avoid type error at the moment if the return type include a typevar (like this)?

[erictraut]

@kavinyao, if you're able to modify Base.generate, you can add a yield statement after the raise statement. That will turn it into a coroutine, and Derived.generate will be compatible with it.

[kavinyao]

I thought I must yield a value but looks like it's not the case. Thanks!

[DimitriPapadopoulos]

In my case (zarr-developers/zarr-python#2381) I have to silence MyPy as it complains about the value-less yield:

class Test(ABC):
    @abstractmethod
    async def test(self) -> AsyncGenerator[bool]:
        yield  # type: ignore[misc]