Bug report

Bug description:

async with asyncio.timeout(0) will catch and process a prior unrelated cancellation of the enclosing task.

import asyncio

async def test(timeout):
    task = asyncio.current_task()
    task.cancel()

    try:
        async with asyncio.timeout(timeout):
            await asyncio.sleep(1)
    except TimeoutError:
        print("timeout caught")
    except asyncio.CancelledError:
        print("CancelledError caught")
        raise

    await asyncio.sleep(2)
    print("not cancelled")

asyncio.run(test(timeout=0))  # prints "timeout caught" and "not_cancelled"

This code prints timeout caught and not_cancelled, which means that asyncio.timeout(0) swallows the preceding task.cancel(), so the task pretty much ignores cancellation and proceeds.

Notably, that doesn't happen with a positive timeout. In the above example, asyncio.run(test(timeout=0.001)) will only print CancelledError caught, which I believe is the correct behavior.
In this case, asyncio.Timeout's internal timeout handler (_on_timeout) never gets to run, so the context manager doesn't interfere at all - it doesn't cancel/uncancel the task and simply passes CancelledError through.

Possible solution

I believe this behavior was unintentedly introduced in #102815, where the logic in __aexit__ was changed to only consider new cancel requests when deciding if it should raise a TimeoutError. That was necessary for Timeout to work correctly if used while handling a CancelledError, but now it can get confused between "internal" and "external" cancellations that happened on the same loop iteration.

I think we could capture task's _must_cancel flag when entering the context manager. If it was set, it means that CancelledError was supposed to be delivered and Timeout shouldn't mess with that attempt.

Draft PR to illustrate the idea: #134472

Additional context

I've hit this issue in production with redis-py, which uses asyncio.timeout(0) internally in the connection parser:

# reduced for brevity
from redis.asyncio import Redis

async def test(timeout):
    redis: Redis = await get_redis()
    task = asyncio.current_task()
    task.cancel()
    await redis.set("foo", "bar")
    assert False, "not cancelled"

CPython versions tested on:

3.12, 3.13, CPython main branch, 3.14, 3.11

Operating systems tested on:

macOS

Linked PRs

• gh-134471: Fix asyncio.timeout(0) swallowing an unrelated prior cancellation. #134472