Automatic conversion from ReturnType to Coroutine[Any, Any, ReturnType] for functions with "pass"

[Tayum]

I guess there is a bug when checking async def functions with "empty" body, which are typed to return AsyncIterable.

Please see the snippet:

import asyncio
from abc import ABC, abstractmethod

from typing import AsyncIterable, List


class NumGetter(ABC):
    @abstractmethod
    async def get_nums(self) -> AsyncIterable[int]:
        pass

    async def get_nums_by_batches(self, batch_size: int) -> AsyncIterable[List[int]]:
        batch = []

        async for num in self.get_nums():
            batch.append(num)

            if len(batch) == batch_size:
                yield batch
                batch = []

        if batch:
            yield batch


class FiveNumGetter(NumGetter):
    async def get_nums(self) -> AsyncIterable[int]:
        for i in range(5):
            yield i
            await asyncio.sleep(0.01)


async def main() -> None:
    num_getter = FiveNumGetter()

    async for batch in num_getter.get_nums_by_batches(batch_size=2):
        print(batch)


if __name__ == '__main__':
    asyncio.run(main())

mypy produces errors, but it shouldn't because function decorated with @abstractmethod commonly has only pass, ..., raise NotImplementedError statements or docstrings in it.

Actual behavior

mypy gives errors:

scratch_1.py:15: error: "Coroutine[Any, Any, AsyncIterable[int]]" has no attribute "__aiter__" (not async iterable)
scratch_1.py:27: error: Return type "AsyncIterable[int]" of "get_nums" incompatible with return type "Coroutine[Any, Any, AsyncIterable[int]]" in supertype "NumGetter"

Expected behavior

No errors.

Versions

$ python --version
Python 3.7.5

$ mypy --version
mypy 0.761

Mypy configuration

python_version = 3.7
ignore_missing_imports = True

---

The easiest way I got myself escaped was to change:

    @abstractmethod
    async def get_nums(self) -> AsyncIterable[int]:
        pass

to

    @abstractmethod
    async def get_nums(self) -> AsyncIterable[int]:
        yield

But that gives mypy error:

scratch_1.py:10: error: Yield value expected

Surely I can write:

    @abstractmethod
    async def get_nums(self) -> AsyncIterable[int]:
        yield 0

and that will produce no errors, but in real application I do not have an int return type there, but rather a complex class which requires some mandatory fields to be instantiated in __init__, and it would look quite weird.

That's why to overcome this I wrote:

    @abstractmethod
    async def get_nums(self) -> AsyncIterable[int]:
        yield  # type: ignore

But that still does not feel as it is the way it should work.

[ilevkivskyi]

This is a duplicate of #5070