[issue44665] asyncio.create_task() documentation should mention user needs to keep reference to the task
New submission from Vincent Bernat <python@vincent.bernat.ch>: asyncio will only keep weak references to alive tasks (in `_all_tasks`). If a user does not keep a reference to a task and the task is not currently executing or sleeping, the user may get "Task was destroyed but it is pending!". I would suggest adding the following paragraph to `create_task()` documentation: Python only keeps weak references to the scheduled tasks. To avoid the task being destroyed by the garbage collector while still pending, a reference to it should be kept until the task is done. And maybe an example in case a user wants something "fire and forget"? ```python running_tasks = set() # [...] task = asyncio.create_task(some_background_function()) running_tasks.add(task) task.add_done_callback(lambda t: running_tasks.remove(t)) ``` The same applies to ensure_future as it now uses create_task, so maybe a "See create_task()". ---------- assignee: docs@python components: Documentation messages: 397741 nosy: bernat, docs@python priority: normal severity: normal status: open title: asyncio.create_task() documentation should mention user needs to keep reference to the task type: enhancement versions: Python 3.10, Python 3.11, Python 3.9 _______________________________________ Python tracker <report@bugs.python.org> <https://bugs.python.org/issue44665> _______________________________________
Change by Joannah Nanjekye <nanjekyejoannah@gmail.com>: ---------- keywords: +patch nosy: +nanjekyejoannah nosy_count: 2.0 -> 3.0 pull_requests: +27437 stage: -> patch review pull_request: https://github.com/python/cpython/pull/29163 _______________________________________ Python tracker <report@bugs.python.org> <https://bugs.python.org/issue44665> _______________________________________
Joannah Nanjekye <nanjekyejoannah@gmail.com> added the comment: @bernat and ncoghlan, please see if the wording I have used in the linked PR helps to clarify this. ---------- stage: patch review -> _______________________________________ Python tracker <report@bugs.python.org> <https://bugs.python.org/issue44665> _______________________________________
Chris Meyer <cmeyer1969@gmail.com> added the comment: Is there a way to reproduce this issue? I run the following code in Python 3.9 and it works as expected (prints "xyz" twice). import asyncio import gc async def xyz(): print("xyz") event_loop = asyncio.get_event_loop() event_loop.create_task(xyz()) t = event_loop.create_task(xyz()) del t gc.collect() event_loop.stop() event_loop.run_forever() ---------- nosy: +cmeyer _______________________________________ Python tracker <report@bugs.python.org> <https://bugs.python.org/issue44665> _______________________________________
Vincent Bernat <python@vincent.bernat.ch> added the comment: Hummm, I have a hard time finding a short example when `__del__` is not called until the task is finished. Sorry. I did run into this several time, for example here: https://github.com/ldo/dbussy/pull/45 (and some internal projects as well). So, it happens from time to time, but it is hard to find a simple reproducer. ---------- _______________________________________ Python tracker <report@bugs.python.org> <https://bugs.python.org/issue44665> _______________________________________
Alexander Hartl <alexander.hartl@tuwien.ac.at> added the comment: I just found this PR when a task of mine spontaneously crashed with a "Task was destroyed but it is pending" in the middle of program execution. I think the warning should be added to `loop.create_task()`, too. Not sure if `loop.call_later()` and `loop.call_at()` are also affected? I think it would be a good idea to add the fire-and-forget example that @bernat gave. At the moment, stackoverflow is full of suggestions to just use `create_task()` in this case, ignoring the return value. Actually, I think it is a true shortcoming that asyncio doesn't provide a fire-and forget functionality by itself. ---------- nosy: +alexhartl _______________________________________ Python tracker <report@bugs.python.org> <https://bugs.python.org/issue44665> _______________________________________
participants (4)
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Alexander Hartl -
Chris Meyer
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Joannah Nanjekye
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Vincent Bernat