Error with Protocols and AsyncContextManager

[junqed]

• Are you reporting a bug, or opening a feature request?
  A bug

• Please insert below the code you are checking with mypy,

from __future__ import annotations

import asyncio
import typing as t
from contextlib import asynccontextmanager

import typing_extensions as te


class LockProto(te.Protocol):
    async def with_lock(self) -> t.AsyncContextManager[str]: ...


class Consumer:
    def __init__(self, locker: LockProto) -> None:
        self.locker = locker

    def run(self) -> None:
        async with self.locker.with_lock() as name:
            print(name)


class SockLock:
    @asynccontextmanager
    async def with_lock(self) -> t.AsyncGenerator[str, None]:
        yield 'some-string'


async def main() -> None:
    cons = Consumer(SockLock())
    await cons.run()


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

• What is the actual behavior/output?

bug.py:18: error: "Coroutine[Any, Any, AsyncContextManager[str]]" has no attribute "__enter__"
bug.py:18: error: "Coroutine[Any, Any, AsyncContextManager[str]]" has no attribute "__exit__"
bug.py:29: error: Argument 1 to "Consumer" has incompatible type "SockLock"; expected "LockProto"
bug.py:29: note: Following member(s) of "SockLock" have conflicts:
bug.py:29: note:     Expected:
bug.py:29: note:         def with_lock(self) -> Coroutine[Any, Any, AsyncContextManager[str]]
bug.py:29: note:     Got:
bug.py:29: note:         def with_lock(*Any, **Any) -> AsyncContextManager[Any]

• What is the behavior/output you expect?
  No errors

• What are the versions of mypy and Python you are using?
  0.761

• What are the mypy flags you are using? (For example --strict-optional)
  No flags, just mypy bug.py

[Michael0x2a]

Not sure about the error on line 29, but the error from line 18 at least seems to be a legitimate one -- you need to use async with (and consequently make your run function an async def). Otherwise you get the following runtime error:

Traceback (most recent call last):
  File "test.py", line 34, in <module>
    main()
  File "test.py", line 30, in main
    cons.run()
  File "test.py", line 18, in run
    with self.locker.with_lock() as name:
AttributeError: __enter__

[junqed]

Not sure about the error on line 29, but the error from line 18 at least seems to be a legitimate one -- you need to use async with (and consequently make your run function an async def). Otherwise you get the following runtime error:

Sorry, I forgot about it, fixed in my example

[JukkaL]

It looks like the type annotations were incorrect and the errors were legitimate. This version passes without errors (I'm not 100% sure if the annotations are correct here, though):

import asyncio
from contextlib import asynccontextmanager

from typing import AsyncGenerator, AsyncContextManager
from typing_extensions import Protocol


class LockProto(Protocol):
    def with_lock(self) -> AsyncContextManager[str]: ...


class Consumer:
    def __init__(self, locker: LockProto) -> None:
        self.locker = locker

    async def run(self) -> None:
        async with self.locker.with_lock() as name:
            print(name)


class SockLock:
    @asynccontextmanager
    async def with_lock(self) -> AsyncGenerator[str, None]:
        yield 'some-string'


async def main() -> None:
    cons = Consumer(SockLock())
    await cons.run()


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

Changes I made:

• Made with_lock a normal def
• Made run an async def

I wonder if some error messages could be improved here?

[junqed]

Thanks, moving away async from the protocol fixed the error. But why? Do I understand correctly that the decorator asynccontextmanager changes the function signature implicitly and mypy understands it? Is it a good idea to add such cases to the docs?

[JukkaL]

Yeah, asynccontextmanager changes the signature and mypy understands (to a certain extent, at least). Mentioning this in the documentation may be a good idea, if we find a good place to put it. contextmanager is another typical example and much more common, so it should be documented as well (or at first).

[junqed]

https://mypy.readthedocs.io/en/stable/kinds_of_types.html is it a good place for such kind of examples? Or at least just to put them in the examples https://mypy.readthedocs.io/en/stable/cheat_sheet_py3.html#miscellaneous

[JukkaL]

I'd say that appending to https://mypy.readthedocs.io/en/stable/kinds_of_types.html is a good place for contextmanager. asynccontextmanager could be documented in https://mypy.readthedocs.io/en/stable/more_types.html#typing-async-await. These don't feel important enough to be included in the cheat sheet.

[joybh98]

I'm taking this issue

[joybh98]

@JukkaL where should I add contextmanager in https://mypy.readthedocs.io/en/stable/kinds_of_types.html ?

[ethanhs]

@joybhallaa I would create a new section called "Context Managers".