Behaviour of AsyncExitStack in strict mode

Bug Report

To Reproduce

from contextlib import AsyncExitStack

import httpx


async def main() -> None:
    async with AsyncExitStack() as stack:
        http_client = await stack.enter_async_context(httpx.AsyncClient())
        reveal_type(http_client)

Output with strict = false:

repro.py:9: note: Revealed type is "httpx._client.AsyncClient*"

Output with strict = true:

repro.py:8: error: Need type annotation for "http_client"  [var-annotated]
repro.py:8: error: Argument 1 to "enter_async_context" of "AsyncExitStack" has incompatible type "AsyncClient"; expected "AsyncContextManager[<nothing>]"  [arg-type]
repro.py:9: note: Revealed type is "Any"
Found 2 errors in 1 file (checked 1 source file)

Expected Behavior

I would expect the type of http_client to not need annotated and to be inferred from the return type of enter_async_context. Is this behaviour correct?

Your Environment

Mypy version used: 0.931
Mypy configuration options from mypy.ini (and other config files):

[tool.mypy]
strict = true
show_error_codes = true

Python version used: 3.9.0
Operating system and version: Windows 10

The problem is in the annotations for the httpx library: it has declared overly permissive types for the AsyncClient.__aexit__ method. If you change the code mypy tells you more precisely:

% python -m mypy --strict samples/asynccli.py
samples/asynccli.py:4: error: Incompatible types in assignment (expression has type "AsyncClient", variable has type "AsyncContextManager[Any]")
samples/asynccli.py:4: note: Following member(s) of "AsyncClient" have conflicts:
samples/asynccli.py:4: note:     Expected:
samples/asynccli.py:4: note:         def __aexit__(self, Optional[Type[BaseException]], Optional[BaseException], Optional[TracebackType]) -> Awaitable[Optional[bool]]
samples/asynccli.py:4: note:     Got:
samples/asynccli.py:4: note:         def __aexit__(self, exc_type: Type[BaseException] = ..., exc_value: BaseException = ..., traceback: TracebackType = ...) -> Coroutine[Any, Any, None]
Found 1 error in 1 file (checked 1 source file)
% cat samples/asynccli.py 
import httpx
from typing import Any, AsyncContextManager

x: AsyncContextManager[Any] = httpx.AsyncClient()

The annotations at https://github.com/encode/httpx/blob/7883c98556eaa7630454d5b62426a5c497542408/httpx/_client.py#L1970 need to be changed to Optional.

Thank you, I will create a report in the httpx repo.

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Behaviour of `AsyncExitStack` in strict mode #12226

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Behaviour of AsyncExitStack in strict mode #12226

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JelleZijlstra commented on Feb 21, 2022

michaeloliverx commented on Feb 21, 2022

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Behaviour of `AsyncExitStack` in strict mode #12226