The event loop warns only on Windows 3.14+ if it isn't closed before garbage collection

Bug report

Bug description:

This program reports ResourceWarning: unclosed event loop <ProactorEventLoop running=False closed=False debug=False> only on Windows and only since 3.14:

import asyncio
asyncio.set_event_loop(loop := asyncio.new_event_loop())
loop.run_until_complete(asyncio.sleep(1))

Other platforms and versions in the 3.9-3.15 Linux-MacOS-Windows matrix don't do that.

See jobs.
I've reproduced this at main too.

Key things I'm investigating:

Is it mandatory to call .close() manually, if we don't use a runner?
- If yes, why does only Windows 3.14+ proactor loop raise this warning, and loops on other platforms and versions do not?
- If no (since most platforms and versions seemingly do not require this), why does the loop raise the warning?
In general, could and should the asyncio REPL use a runner?

I've found this while working on the asyncio REPL. This issue blocks me from completing #140287. Note that the asyncio REPL does call .stop(); it only doesn't call .close(). .stop() is not meant to stop the loop for good (the loop is re-entrant), .close() is irreversible and idempotent.

CPython versions tested on:

3.9, 3.10, 3.11, 3.12, 3.13, 3.14, 3.15, CPython main branch

Operating systems tested on:

Windows, macOS, Linux

@ZeroIntensity

CC @ZeroIntensity labels: stdlib, topic-asyncio, 3.13, 3.14, 3.15

All of them should raise the warning, if not then it is a bug which should be fixed.

If yes, why does only Windows 3.14+ proactor loop raise this warning, and loops on other platforms and versions do not?

The implementation on Windows and on Unix is different. The Unix implementation is based on BaseSelectorEventLoop which uses a "self pipe" which does:

self._add_reader(self._ssock.fileno(), self._read_from_self)

This line creates a reference cycle. It prevents the event loop to be deallocated by the GC.

The Windows implementation also uses a "self pipe" but with a different implementation which doesn't seem to suffer from the same reference cycle issue.

Thanks. I'll see what I can do.

This line creates a reference cycle. It prevents the event loop to be deallocated by the GC.

That's not the actual issue here though. The issue is that the code calls asyncio.set_event_loop which stores the loop inside the policy object which is a module global so that global variable keeps the loop alive until the module is cleared.

The code raises the warning if the set_event_loop is removed.

import asyncio
loop = asyncio.new_event_loop()
loop.run_until_complete(asyncio.sleep(1))

❯ ./python.exe main.py
/Users/kumaraditya/work/cpython/Lib/asyncio/base_events.py:758: ResourceWarning: unclosed event loop <_UnixSelectorEventLoop running=False closed=False debug=False>

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The event loop warns only on Windows 3.14+ if it isn't closed before garbage collection #141262

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The event loop warns only on Windows 3.14+ if it isn't closed before garbage collection #141262

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Bug report

Bug description:

CPython versions tested on:

Operating systems tested on:

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johnslavik commented on Nov 8, 2025

kumaraditya303 commented on Nov 9, 2025

vstinner commented on Nov 13, 2025

johnslavik commented on Nov 15, 2025

kumaraditya303 commented on Nov 16, 2025

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