Deprecate asyncio policy system

[kumaraditya303]

asyncio's policy system deprecation

asyncio's policy system¹ has been a source of confusion and problems in asyncio for a very long time. The policies no longer serve a real purpose. Loops are always per thread, there is no need to have a "current loop" when no loop is currently running. This issue discusses the changes to deprecate it in Python 3.14 and schedule its removal in 3.16 or later.

The usual user applications would use the runner APIs (see flowchart) while those who want more control like Jupyter project can create an event loop and manage it themselves, the difference would be that instead of them first getting the policy then event loop then can directly create it like

loop = MyCustomEventLoop()
loop.run_until_complete(task)

rather than currently

asyncio.set_event_loop_policy(MyPolicy())
policy = asyncio.get_event_loop_policy()
loop = policy.new_event_loop()
loop.run_until_complete(task)

See these discussions for more background:

• Deprecate get_event_loop() #83710 (comment)
• Finish deprecation in asyncio.get_event_loop() #93453 (comment)
• deprecate the asyncio child watchers system #94597 (comment)
• deprecate the asyncio child watchers system #94597

Functions and classes to be deprecated and later removed

• asyncio.get_event_loop_policy
• asyncio.set_event_loop_policy
• asyncio.AbstractEventLoopPolicy
• asyncio.DefaultEventLoopPolicy
• asyncio.WindowsSelectorEventLoopPolicy
• asyncio.WindowsProactorEventLoopPolicy
• asyncio.set_event_loop

Functions to be modified

• asyncio.get_event_loop - In 3.16 or later this will become an alias to get_running_loop.
• asyncio.new_event_loop - In 3.16 or later this will ignore custom policies and will be an alias to asyncio.EventLoop
• asyncio.run & asyncio.Runner - In 3.16 or later this will be modified to not use policy system as that will be gone and rely solely on loop_factory.

The Grand Plan

• To minimize changes, all the deprecated functions will be underscored i.e. set_event_loop -> _set_event_loop and set_event_loop will emit the warning then call _set_event_loop as its underlying implementation. This way internally asyncio can still call these functions until they are removed without need of many ignore warnings and the tests too can easily be adapted.
• The deprecated classes will emit warnings when they are subclassed as it was done for child watchers.
• The runner APIs will be remain unmodified but making sure that correct warnings are emitted internally when policy system is used.

The Future

---
title: Flowchart for asyncio.run
---
flowchart TD
    A["asyncio.run(coro, loop_factory=...)"] --> B{loop_factory}
    B -->|loop_factory is None| D{platform}
    B -->|loop_factory is not None| E["loop = loop_factory()"]
    D --> |Unix| F["loop = SelectorEventLoop()"]
    D --> |Windows| G["loop = ProactorEventLoop()"]
    E --> H
    F --> H
    G --> H 
    H["loop.run_until_complete(coro)"]

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Linked PRs

• gh-127949: deprecate asyncio.set_event_loop_policy #128024
• gh-127949: deprecate asyncio.get_event_loop_policy #128053
• gh-127949: fix resource warnings in test_tasks.py due to avoiding a call to get_event_loop() #128172
• gh-127949: fix DeprecationWarning in test_inspect.py #128215
• gh-127949: deprecate asyncio policy classes #128216
• gh-127949: deprecate asyncio.set_event_loop #128218
• gh-127949: add docs for asyncio policy deprecation #128269
• gh-127949: make deprecation of policy system more prominent #128290
• [3.13] gh-127949: fix resource warnings in test_tasks.py (GH-128172) #131805
• [3.12] gh-127949: fix resource warnings in test_tasks.py (GH-128172) #131806

Footnotes

1. https://docs.python.org/3.14/library/asyncio-policy.html ↩

[graingert]

asyncio.new_event_loop could just be deprecated in favour of asyncio.EventLoop

[asvetlov]

I support the deprecation in general, details are the subject for future discussion.
For example, policies are used by very many libraries. It's better for, e.g., maintainers of pytest-asyncio if we keep the deprecation for 5 Python releases (the issue proposes 2 years).
Let me sleep on other proposed details.
But anyway, I agree we should deprecate policies at least in a very soft form. If we agree on the 5 year deprecation period we can increase the pressure release-to-release

[graingert]

I support the deprecation in general, details are the subject for future discussion. For example, policies are used by very many libraries. It's better for, e.g., maintainers of pytest-asyncio if we keep the deprecation for 5 Python releases (the issue proposes 2 years). Let me sleep on other proposed details. But anyway, I agree we should deprecate policies at least in a very soft form. If we agree on the 5 year deprecation period we can increase the pressure release-to-release

how about in 2 years (2 Python releases so 3.16 or 3.27) we swap the default loop_factory for asyncio.Runner and asyncio.run to loop_factory=asyncio.EventLoop This would allow policy users like pytest-asyncio to continue to use asyncio.get_event_loop() with the policy system, but would move most users off the system

[kumaraditya303]

FTR, asyncio.get_event_loop ¹ in Python versions 3.10.0–3.10.8 and 3.11.0, an incorrect deprecation was added for this but it was backed out later because things like asyncio.set_event_loop_policy weren't deprecated at that time.

With the current plan all of this would be deprecated at the same time so there won't be any issue like that.

Footnotes

1. https://docs.python.org/3.11/library/asyncio-eventloop.html#asyncio.get_event_loop check the note ↩