python runtime_error ai_generated true

asyncio.exceptions.CancelledError: CancelledError not caught in gather

ID: python/asyncio-gather-exception-handling

Also available as: JSON · Markdown · 中文
80%Fix Rate
86%Confidence
0Evidence
2024-08-18First Seen

Version Compatibility

VersionStatusIntroducedDeprecatedNotes
3.8 active — — —
3.9 active — — —

Root Cause

When using asyncio.gather() with return_exceptions=False (default), an exception in one task cancels others, and CancelledError may propagate unexpectedly.

generic

中文

当使用 asyncio.gather() 且 return_exceptions=False（默认）时，一个任务中的异常会取消其他任务，并且 CancelledError 可能意外传播。

Workarounds

  1. 90% success Use return_exceptions=True to handle exceptions gracefully 
    results = await asyncio.gather(task1, task2, return_exceptions=True)
for r in results:
    if isinstance(r, Exception):
        # handle
  2. 85% success Catch CancelledError explicitly in each coroutine 
    async def safe_coro():
    try:
        await something()
    except asyncio.CancelledError:
        # cleanup and re-raise if needed
        raise

Dead Ends

Common approaches that don't work:

  1. Wrapping each task in try-except without re-raising 50% fail

    If the task is cancelled externally, CancelledError may still be raised and not caught by a general except.

  2. Using asyncio.shield() on all tasks 40% fail

    shield() prevents cancellation but may cause tasks to run indefinitely if not managed.