Contextvars + async generator + break + finally

[sylvainmouquet]

Bug report

Bug description:

import asyncio
import contextvars

async def gen():
    var = contextvars.ContextVar('one')

    token = var.set(1)

    ctx = contextvars.copy_context()
    print(list(ctx.items()))

    try:
        yield 1
        yield 2
    finally:
        ctx = contextvars.copy_context()
        print(list(ctx.items()))

        try:
            var.reset(token)
        except Exception as e:
            print(e.args)

async def main():
    async for i in gen():
        print(i)
        break

asyncio.run(main())

result :

[(<ContextVar name='one' at 0x107312930>, 1)]
1
[(<ContextVar name='one' at 0x107312930>, 1)]
("<Token var=<ContextVar name='one' at 0x107312930> at 0x107303480> was created in a different Context",)

In finally block it's not possible to manually reset the contextvars

CPython versions tested on:

3.12

Operating systems tested on:

macOS

[graingert]

Yes this is expected, because the async generator finalization is triggered by the garbage collector, where asyncio uses sys.set_asyncgen_hooks() to aclose the async generator in a new task. See https://peps.python.org/pep-0525/#finalization

If you want to ensure finalization in the same context use

async def main():
    async with contextlib.aclosing(gen()) as agen:
        async for i in agen:
            print(i)
            break()

[graingert]

See also https://docs.python.org/3/reference/expressions.html#asynchronous-generator-functions

If an asynchronous generator happens to exit early by break, the caller task being cancelled, or other exceptions, the generator’s async cleanup code will run and possibly raise exceptions or access context variables in an unexpected context–perhaps after the lifetime of tasks it depends, or during the event loop shutdown when the async-generator garbage collection hook is called. To prevent this, the caller must explicitly close the async generator by calling aclose() method to finalize the generator and ultimately detach it from the event loop.

And https://docs.python.org/3/library/contextlib.html#contextlib.aclosing

This pattern ensures that the generator’s async exit code is executed in the same context as its iterations (so that exceptions and context variables work as expected, and the exit code isn’t run after the lifetime of some task it depends on).

[graingert]

I'm going to close this, as working as documented