IsolatedAsyncioTestCase and asyncio.run no-longer call asyncio.set_event_loop

[graingert]

in https://github.com/python/cpython/pull/31799/files#diff-1f2ae0f6c6010caf9d5f1c80cd6033a796ffe2b60554f5df84f554f4a08e622b the calls to asyncio.set_event_loop() were removed which breaks aiohttp aio-libs/aiohttp#6757 and asyncio.SafeChildWatcher

this looks like an intentional breaking change - and if it is should be documented

[graingert]

import sys
import asyncio


async def run_subprocess():
    proc = await asyncio.create_subprocess_shell(
        "exit 0",
        stdin=asyncio.subprocess.DEVNULL,
        stdout=asyncio.subprocess.DEVNULL,
        stderr=asyncio.subprocess.DEVNULL,
    )
    await proc.wait()
    print("success!")


async def amain():
    await asyncio.to_thread(asyncio.run, run_subprocess())


def main():
    asyncio.get_event_loop_policy().set_child_watcher(asyncio.SafeChildWatcher())
    asyncio.run(amain())


if __name__ == "__main__":
    sys.exit(main())

 graingert@superjacent  ~/projects  python3.10 demo_subprocess.py
success!
 graingert@superjacent  ~/projects  python3.11 demo_subprocess.py 
Traceback (most recent call last):
  File "/home/graingert/projects/demo_subprocess.py", line 26, in <module>
    sys.exit(main())
             ^^^^^^
  File "/home/graingert/projects/demo_subprocess.py", line 22, in main
    asyncio.run(amain())
    ^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/asyncio/runners.py", line 181, in run
    return runner.run(main)
           ^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/asyncio/runners.py", line 115, in run
    return self._loop.run_until_complete(task)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/asyncio/base_events.py", line 650, in run_until_complete
    return future.result()
           ^^^^^^^^^^^^^^^
  File "/home/graingert/projects/demo_subprocess.py", line 17, in amain
    await asyncio.to_thread(asyncio.run, run_subprocess())
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/asyncio/threads.py", line 25, in to_thread
    return await loop.run_in_executor(None, func_call)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/concurrent/futures/thread.py", line 58, in run
    result = self.fn(*self.args, **self.kwargs)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/asyncio/runners.py", line 181, in run
    return runner.run(main)
           ^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/asyncio/runners.py", line 115, in run
    return self._loop.run_until_complete(task)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/asyncio/base_events.py", line 650, in run_until_complete
    return future.result()
           ^^^^^^^^^^^^^^^
  File "/home/graingert/projects/demo_subprocess.py", line 6, in run_subprocess
    proc = await asyncio.create_subprocess_shell(
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/asyncio/subprocess.py", line 205, in create_subprocess_shell
    transport, protocol = await loop.subprocess_shell(
                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/asyncio/base_events.py", line 1647, in subprocess_shell
    transport = await self._make_subprocess_transport(
                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/asyncio/unix_events.py", line 204, in _make_subprocess_transport
    raise RuntimeError("asyncio.get_child_watcher() is not activated, "
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
RuntimeError: asyncio.get_child_watcher() is not activated, subprocess support is not installed.

[graingert]

I know there were some efforts to deprecate get/set_event_loop_policy and child watchers #82772 so I think those Deprecations should be applied before asyncio.run/IsolatedAsyncioTestCase stops calling asyncio.set_event_loop()

[graingert]

alternatively is there still time to deprecate the child watchers system and the policy system in favor of
asyncio.Runner(loop_factory=asyncio.ProactorEventLoop/asyncio.SelectorEventLoop/uvloop.uvloop.new_event_loop) before the 3.11 release?

that would be deprecating:

asyncio.get_event_loop() # already deprecated unless the loop is running 
asyncio.set_event_loop()  # asyncio.set_event_loop(None) should probably be exempt
asyncio.get_event_loop_policy()
asyncio.set_event_loop_policy()  # asyncio.set_event_loop_policy(None) should probably be exempt
asyncio.set_child_watcher()

asyncio.get_child_watcher() will remain and will always be asyncio.PidfdChildWatcher on old kernels and asyncio.ThreadedChildWatcher on very old kernels

asyncio.new_event_loop() will issue a DeprecationWarning if the current policy is not the default policy, and then become an alias of

if sys.platform == "win32":
    new_event_loop = ProactorEventLoop
else:
    new_event_loop = SelectorEventLoop

[graingert]

this is also broken with the PidfdChildWatcher