gh-94597: deprecate asyncio.set_event_loop_policy

[graingert]

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📚 Documentation preview 📚: https://cpython-previews--110728.org.readthedocs.build/

• Issue: deprecate the asyncio child watchers system and policy system #94597

[cjw296]

No objection to the unittest or AsyncMock changes!

[gvanrossum]

Just a few nits, basically LG. That tearDownModule() pattern is annoying! :-)

I assume that deprecating get_event_loop_policy() will follow? Are there other APIs? I'm guessing we also want to avoid instantiating or subclassing the standard event loop policy classes (I expect this will turn up some significant uses of them).

Before we merge this we may need to bring this up on discuss.python.org (in the asyncio category) to see if there are significant objections to the proposed deprecation schedule.

[graingert]

Just a few nits, basically LG. That tearDownModule() pattern is annoying! :-)

I think we can start getting rid of those tearDownModules using #110774

I assume that deprecating get_event_loop_policy() will follow? Are there other APIs? I'm guessing we also want to avoid instantiating or subclassing the standard event loop policy classes (I expect this will turn up some significant uses of them).

Yep currently that's a bit more complicated because get/set_event_loop calls get_event_loop_policy and afaik we don't want to make get_event_loop an alias of get_running_loop anymore

[willingc]

Thanks for the PR. I've left a few questions for you.

[hugovk]

Please also mention the deprecation in What's New in Python 3.13.

Because it's to be removed in 3.15 or later it could be listed under either Pending Removal in Python 3.15 or Pending Removal in Future Versions.

I suggest the former, as it's more likely to be noticed and action taken, and include the "3.15 or later" wording. We can update later if needed.

[graingert]

Before we merge this we may need to bring this up on discuss.python.org (in the asyncio category) to see if there are significant objections to the proposed deprecation schedule.

discuss post posted here: https://discuss.python.org/t/removing-the-asyncio-policy-system-asyncio-set-event-loop-policy-in-python-3-15/37553

[gvanrossum]

Okay, I'm happy with this. Let's wait a bit if anyone brings up substantial objections in the Discourse thread.

[willingc]

Bravo @graingert for simplifying. ✨

[kumaraditya303]

Please also add a notice under asyncio changes, most users like myself won't look under 3.15 removals for this change. Thanks

[bedevere-app]

A Python core developer has requested some changes be made to your pull request before we can consider merging it. If you could please address their requests along with any other requests in other reviews from core developers that would be appreciated.

Once you have made the requested changes, please leave a comment on this pull request containing the phrase I have made the requested changes; please review again. I will then notify any core developers who have left a review that you're ready for them to take another look at this pull request.

And if you don't make the requested changes, you will be poked with soft cushions!

[graingert]

Please also add a notice under asyncio changes, most users like myself won't look under 3.15 removals for this change. Thanks

Can you link me to the location in the docs that you mean please?

[kumaraditya303]

Can you link me to the location in the docs that you mean please?

Add asyncio entry in https://docs.python.org/3.13/whatsnew/3.13.html#deprecated

[hugovk]

That sounds fine, then let's move it from the "Pending Removal in Python 3.15" section to this one.

[graingert]

I have made the requested changes; please review again

[gvanrossum]

LGTM, but let's wait until the dust in the Discourse thread settles.