Re: [PATCH] python: Replace asyncio.get_event_loop for Python 3.14

["Daniel P. Berrangé" <berrange@redhat.com>]

On Tue, Jul 15, 2025 at 11:51:10AM +0100, Richard W.M. Jones wrote:
> In Python 3.14, no asyncio event loop gets generated automatically.
> Instead create one when we need it.  This should work with Python 3.13
> as well.  This is a mechanical change as suggested here:
> 
> https://bugzilla.redhat.com/show_bug.cgi?id=2375004#c4
> 
> See-also: https://docs.python.org/3.14/whatsnew/3.14.html#id7
> Thanks: Miro Hrončok
> Signed-off-by: Richard W.M. Jones <rjones@redhat.com>
> ---
>  python/qemu/qmp/legacy.py  | 5 ++++-
>  python/qemu/qmp/qmp_tui.py | 5 ++++-
>  python/qemu/qmp/util.py    | 5 ++++-
>  python/tests/protocol.py   | 5 ++++-
>  4 files changed, 16 insertions(+), 4 deletions(-)
> 
> diff --git a/python/qemu/qmp/legacy.py b/python/qemu/qmp/legacy.py
> index 22a2b5616e..e11d05afbd 100644
> --- a/python/qemu/qmp/legacy.py
> +++ b/python/qemu/qmp/legacy.py
> @@ -86,7 +86,10 @@ def __init__(self,
>                  "server argument should be False when passing a socket")
>  
>          self._qmp = QMPClient(nickname)
> -        self._aloop = asyncio.get_event_loop()
> +        try:
> +            self._aloop = asyncio.get_event_loop()
> +        except RuntimeError:
> +            self._aloop = asyncio.new_event_loop()
>          self._address = address
>          self._timeout: Optional[float] = None
>

The usage contract of QEMUMonitorProtocol is sufficiently ill
defined that we probably have to expect the caller might have
arranged for a specific event loop to be set against the
thread, so the try/except dance is likely required.


> diff --git a/python/qemu/qmp/qmp_tui.py b/python/qemu/qmp/qmp_tui.py
> index 2d9ebbd20b..84c5da4e66 100644
> --- a/python/qemu/qmp/qmp_tui.py
> +++ b/python/qemu/qmp/qmp_tui.py
> @@ -377,7 +377,10 @@ def run(self, debug: bool = False) -> None:
>          screen = urwid.raw_display.Screen()
>          screen.set_terminal_properties(256)
>  
> -        self.aloop = asyncio.get_event_loop()
> +        try:
> +            self.aloop = asyncio.get_event_loop()
> +        except RuntimeError:
> +            self.aloop = asyncio.new_event_loop()
>          self.aloop.set_debug(debug)
>  
>          # Gracefully handle SIGTERM and SIGINT signals

I'm somewhat inclined to say that this "App" class should
always use asyncio.new_event_loop, as the usge model is
for running a self contained QMP application. I don't
expect we should need to integrate with code that has
set an event loop against the thread.

> diff --git a/python/qemu/qmp/util.py b/python/qemu/qmp/util.py
> index ca6225e9cd..8f9f849a5f 100644
> --- a/python/qemu/qmp/util.py
> +++ b/python/qemu/qmp/util.py
> @@ -160,7 +160,10 @@ def asyncio_run(coro: Coroutine[Any, Any, T], *, debug: bool = False) -> T:
>          return asyncio.run(coro, debug=debug)
>  
>      # Python 3.6
> -    loop = asyncio.get_event_loop()
> +    try:
> +        loop = asyncio.get_event_loop()
> +    except RuntimeError:
> +        loop = asyncio.new_event_loop()
>      loop.set_debug(debug)
>      ret = loop.run_until_complete(coro)
>      loop.close()

This change is not requred as this code path only runs with
Python <= 3.6. In fact it is dead code given we only support
python >= 3.9.

I'm prepping a patch to kill all py 3.6 compat code.

> diff --git a/python/tests/protocol.py b/python/tests/protocol.py
> index 56c4d441f9..aa7c0e4397 100644
> --- a/python/tests/protocol.py
> +++ b/python/tests/protocol.py
> @@ -228,7 +228,10 @@ def async_test(async_test_method):
>          Decorator; adds SetUp and TearDown to async tests.
>          """
>          async def _wrapper(self, *args, **kwargs):
> -            loop = asyncio.get_event_loop()
> +            try:
> +                loop = asyncio.get_event_loop()
> +            except RuntimeError:
> +                loop = asyncio.new_event_loop()
>              loop.set_debug(True)

For the usage model here we should exclusively use
asyncio.new_event_loop(). This is a test decorator
so it is impossible for an existing event loop to be
set against the thread.

With regards,
Daniel
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