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 - This will emit warning when creating event loop from policy, later this will become an alias of get_running_loop.
• asyncio.new_event_loop - This will emit warning when a custom event loop is used, later this will ignore custom policies.
• 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)"]