Optionally prevent child tasks from being cancelled in `asyncio.TaskGroup`

Feature or enhancement

Add a flag to asyncio.TaskGroup to control whether, if a child task crashes, other should be cancelled or not.

Pitch

Currently asyncio.TaskGroup always cancels all child tasks if one fails. Even if that's the most common use case, I'd like to be able to switch from the current behaviour to prevent child tasks cancellation when one failure occurs and raise an ExceptionGroup with all exceptions raised (only after other tasks completed).

• Excpetions raised in the async with block still have to cause child tasks to be canceled.
• SystemExit and KeyboardInterrupt raised in a child task still cancel other tasks.

Example usage:

async def zero_div():
    1 / 0

async def wait_some(d):
    await asyncio.sleep(d)
    print(f'finished after {d}')

async def main():
    async with asyncio.TaskGroup(abort_on_first_exception=False) as tg:
        tg.create_task(wait_some(2))
        tg.create_task(wait_some(3))
        tg.create_task(zero_div())

asyncio.run(main())
# Should print out:
# finished after 2
# finished after 3
# and then raise an `ExceptionGroup` with a `ZeroDivisionError`

Looking at asyncio.TaskGroup source code, it seems that it could be achieved by adding the abort_on_first_exception (or whatever it should be named) flag to the __init__ method and then modify _on_task_done as follow:

class TaskGroup:

    def __init__(self, abort_on_first_exception=True):
        ...
        self._abort_on_first_exception = abort_on_first_exception

    ...

    def _on_task_done(self, task):
        ...
        self._errors.append(exc)
        is_base_error = self._is_base_error(exc)
        if is_base_error and self._base_error is None:
            self._base_error = exc

        if self._parent_task.done():
            # Not sure if this case is possible, but we want to handle
            # it anyways.
            self._loop.call_exception_handler({
                'message': f'Task {task!r} has errored out but its parent '
                           f'task {self._parent_task} is already completed',
                'exception': exc,
                'task': task,
            })
            return

        if not self._aborting and not self._parent_cancel_requested:
            #comment skipped for brevity
            if is_base_error or self._abort_on_first_exception:
                self._abort()
                self._parent_cancel_requested = True
                self._parent_task.cancel()

If it's reasonable to have it in asyncio, I'll be glad to submit a PR for it.

Previous discussion

Post on discuss

Linked PRs

• gh-101648
• gh-105011

從 asyncio.TaskGroup 及其 _on_task_done 路徑開始，接著檢視連結的 PR gh-101648 和 gh-105011，以及先前的討論，再決定此 API 是否屬於 asyncio。該 issue 定義了一般例外、async-with 主體中的例外、SystemExit 和 KeyboardInterrupt 的預期行為；完成條件是達成共識的設計以及相應的實作測試。