[issue44665] asyncio.create_task() documentation should mention user needs to keep reference to the task

[Alexander Hartl]

Alexander Hartl  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

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[issue44665] asyncio.create_task() documentation should mention user needs to keep reference to the task

[Vincent Bernat]

Vincent Bernat  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.

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[issue44665] asyncio.create_task() documentation should mention user needs to keep reference to the task

[Chris Meyer]

Chris Meyer  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()

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nosy: +cmeyer

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[issue44665] asyncio.create_task() documentation should mention user needs to keep reference to the task

[Joannah Nanjekye]

Joannah Nanjekye  added the comment:

@bernat and ncoghlan, please see if the wording I have used in the linked PR 
helps to clarify this.

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stage: patch review -> 

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[issue44665] asyncio.create_task() documentation should mention user needs to keep reference to the task

[Joannah Nanjekye]

Change by Joannah Nanjekye :


--
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

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[issue44665] asyncio.create_task() documentation should mention user needs to keep reference to the task

[Vincent Bernat]

New submission from Vincent Bernat :

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

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