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asyncio.TaskGroup swallows cancellations coming from outside of it #155433
Description
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
Activity
kumaraditya303 commentedon Aug 20, 2026
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?
deadlovelll commentedon Aug 20, 2026
Yeah, i'll take a look today
deadlovelll commentedon Aug 20, 2026
I just checked this, and if i understand you well - no, there is no issues:
On main this code prints "swallowed, still pending: 1", in the fix branch it raises
CancelledErrorgh-155433: Fix TaskGroup losing outside cancellation after cancel() (#…
Drop test for pythongh-155433
[3.15] gh-155433: Fix TaskGroup losing outside cancellation after can…