python runtime_error ai_generated true

Task exception was never retrieved future: <Task finished name='Task-5' coro=<worker() done, defined at app.py:42> exception=ValueError('bad payload')>

ID: python/task-exception-never-retrieved

Also available as: JSON · Markdown · 中文
80%Fix Rate
89%Confidence
0Evidence
2024-08-02First Seen

Version Compatibility

VersionStatusIntroducedDeprecatedNotes
3.8 active — — —
3.9 active — — —
3.10 active — — —
3.11 active — — —
3.12 active — — —

Root Cause

A task created with asyncio.create_task() raised an exception, but no one awaited the task or attached a done callback, so the exception is discarded.

generic

中文

通过 asyncio.create_task() 创建的任务抛出异常，但没有人 await 该任务或附加完成回调，因此异常被丢弃。

Workarounds

  1. 90% success 
    Attach a done callback that logs exceptions:

def _log(task):
    if (exc := task.exception()):
        logger.exception('task failed', exc_info=exc)

task = asyncio.create_task(worker())
task.add_done_callback(_log)
  2. 95% success 
    Keep strong references and await tasks in a TaskGroup (3.11+):

async with asyncio.TaskGroup() as tg:
    tg.create_task(worker())
  3. 85% success 
    For fire-and-forget, store tasks in a module-level set and remove in the callback to prevent GC.

Dead Ends

Common approaches that don't work:

  1. 70% fail

    Only adds more logging; the underlying exception is still lost.

  2. 80% fail

    Hides real bugs and still leaves the task result unretrieved.

  3. 85% fail

    Does not attach a handler to the task's exception.