Why can't you await __aenter__? (Understanding trio's contracts around async context managers)

[yang]

Why can't you await __aenter__? (Understanding trio's contracts around async context managers)

[CoolCat467]

For line number debugging purposes, which exact version of Trio are you using? For example, on my machine, I can do this to get the version:

>>> import trio
>>> trio.__version__
'0.26.0'

[A5rocks]

Nevermind I misread this issue.

[yang]

0.22.2

[CoolCat467]

I've got this based off your minimal example:

from __future__ import annotations

import trio

from contextlib import asynccontextmanager, AbstractAsyncContextManager
from collections.abc import Callable, AsyncGenerator
from typing import TypeVar


T = TypeVar("T")


@asynccontextmanager
async def par_w(a: Callable[[], AbstractAsyncContextManager[T]]) -> AsyncGenerator[T, None]:
    ca = a()
    value = await ca.__aenter__()
    yield value
    await ca.__aexit__(None, None, None)


async def run() -> None:
    context_manager = trio.open_nursery
    async def do_three() -> None:
        print("3 start")
        await trio.sleep(1)
        print("3 end")
    async with par_w(context_manager) as nursery:
        print("1")
        await trio.sleep(1)
        print("2")
        nursery.start_soon(do_three)
        print("end")
    print("4")


if __name__ == "__main__":
    trio.run(run)

and I don't see any errors happening, so I'm guessing it's an issue with whatever async context manager you are passing in as a to your par_w function. If you could share what that is that would be great.

[TeamSpen210]

The issue here is similar to the async generator issue. Because you're calling with statement methods directly, you have to be incredibly careful to preserve the nesting order. Otherwise, Trio's internal cancel scope stack can get messed up, breaking everything.

It's probably best to avoid calling them directly, instead use ExitStack, or put things in different tasks for concurrency.