AsyncIterator output type for abstract method

[giokara-work]

(venv) ➜  pytype --version
2023.07.12
(venv) ➜  python --version
Python 3.10.12

Consider following example:

import abc
import asyncio
from typing import AsyncIterator

class BaseTestClass(abc.ABC):

    @abc.abstractmethod
    async def count(self) -> AsyncIterator[int]:
        raise NotImplementedError()

    async def follow(self) -> None:
        async for message in self.count():
            print(message)

class TestClass(BaseTestClass):
    async def count(self) -> AsyncIterator[int]:
        for i in range(5):
            yield i

if __name__ == '__main__':
    asyncio.run(TestClass().follow())

Output of pytype is:

(venv) ➜  pytype minimal_async_iterator.py
Computing dependencies
Analyzing 1 sources with 0 local dependencies
ninja: Entering directory `.pytype'
[1/1] check minimal_async_iterator
FAILED: <dir>/.pytype/pyi/minimal_async_iterator.pyi 
<dir>/venv/bin/python3.10 -m pytype.single --imports_info <dir>/.pytype/imports/minimal_async_iterator.imports --module-name minimal_async_iterator --platform darwin -V 3.10 -o <dir>/.pytype/pyi/minimal_async_iterator.pyi --analyze-annotated --nofail --quick <dir>/minimal_async_iterator.py
File "<dir>/minimal_async_iterator.py", line 17, in follow: No attribute '__aiter__' on Coroutine[Any, Any, AsyncIterator[int]] [attribute-error]
File "<dir>/minimal_async_iterator.py", line 21, in TestClass: Overriding method signature mismatch [signature-mismatch]
  Base signature: 'def BaseTestClass.count(self) -> Coroutine[Any, Any, AsyncIterator[int]]'.
  Subclass signature: 'def TestClass.count(self) -> AsyncIterator[int]'.
  Return type mismatch.

For more details, see https://google.github.io/pytype/errors.html
ninja: build stopped: subcommand failed.
Leaving directory '.pytype'

Simply removing the async keyword from the definition of BaseTestClass.count makes pytype run successfully. This is unexpected, since now the derived function TestClass.count does not have the same signature as its base class. Running pylint on the file will now instead fail.

[eltoder]

This is actually correct and works exactly the same way in mypy:

async_iter.py:12: error: "Coroutine[Any, Any, AsyncIterator[int]]" has no attribute "__aiter__" (not async iterable)  [attr-defined]
async_iter.py:12: note: Maybe you forgot to use "await"?
async_iter.py:16: error: Return type "AsyncIterator[int]" of "count" incompatible with return type "Coroutine[Any, Any, AsyncIterator[int]]" in supertype "BaseTestClass"  [override]

See here for the explanation: https://stackoverflow.com/a/68911014
Or these mypy issues for more of the same: python/mypy#5385 python/mypy#5070

If removing async makes pylint fail, this is probably a bug in pylint.

[eltoder]

This is also now explained in mypy docs. See the last example on this page: https://mypy.readthedocs.io/en/latest/more_types.html#typing-async-await

[giokara-work]

One common confusion is that the presence of a yield statement in an async def function has an effect on the type of the function

That was indeed my confusion. Many thanks for the references!