asyncio.TaskGroup swallows cancellations coming from outside of it

Bug report

Bug description:

After tg.cancel() taskgroup silently swallows any cancellation requested from outside it

Repro:

import asyncio

async def worker():
    try:
        await asyncio.sleep(3600)
    finally:
        await asyncio.sleep(1)

async def main():
    async with asyncio.timeout(0.5):
        async with asyncio.TaskGroup() as tg:
            tg.create_task(worker())
            await asyncio.sleep(0.1)
            tg.cancel()
    print("no error")

asyncio.run(main())

Expected output:

raise TimeoutError from exc_val
TimeoutError

But actually:

no error

Proposed fix - add else branch: if uncancel() leaves requests pending, cancellation was not ours, so we need to propogate it

        if self._parent_cancel_requested:
            # If this flag is set we *must* call uncancel().
            if self._parent_task.uncancel() == 0:
                # If there are no pending cancellations left,
                # don't propagate CancelledError.
                propagate_cancellation_error = None
            else:
                propagate_cancellation_error = exceptions.CancelledError()

Have a fix ready

CPython versions tested on:

CPython main branch

Operating systems tested on:

macOS

Linked PRs

I think there might be issues if the parent task is already cancelling though I haven't verified it, do you want to look into that?

I think there might be issues if the parent task is already cancelling though I haven't verified it, do you want to look into that?

Yeah, i'll take a look today

I think there might be issues if the parent task is already cancelling though I haven't verified it, do you want to look into that?

I just checked this, and if i understand you well - no, there is no issues:

import asyncio

async def main():
    t = asyncio.current_task()
    async with asyncio.TaskGroup() as tg:
        tg.create_task(asyncio.sleep(3600))
        t.cancel()
        tg.cancel()
    print("swallowed, still pending:", t.cancelling())

asyncio.run(main())

On main this code prints "swallowed, still pending: 1", in the fix branch it raises CancelledError

gh-155433: Fix TaskGroup losing outside cancellation after cancel() (#…

Drop test for pythongh-155433

[3.15] gh-155433: Fix TaskGroup losing outside cancellation after can…

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`asyncio.TaskGroup` swallows cancellations coming from outside of it #155433

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asyncio.TaskGroup swallows cancellations coming from outside of it #155433

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Bug report

Bug description:

CPython versions tested on:

Operating systems tested on:

Linked PRs

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kumaraditya303 commented on Aug 20, 2026

deadlovelll commented on Aug 20, 2026

deadlovelll commented on Aug 20, 2026

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`asyncio.TaskGroup` swallows cancellations coming from outside of it #155433