inject at the runner instead however there was a side-effect so I made a comment explaining it.

Author: Vizonex

pytest_asyncio/plugin.py

@@ -135,7 +135,7 @@ def fixture(
     *,
     scope: _ScopeName | Callable[[str, Config], _ScopeName] = ...,
     loop_scope: _ScopeName | None = ...,
-    loop_factory: _ScopeName | Callable[[], AbstractEventLoop] = ...,
+    loop_factory: Callable[[], AbstractEventLoop] | None = ...,
     params: Iterable[object] | None = ...,
     autouse: bool = ...,
     ids: (
@@ -153,7 +153,7 @@ def fixture(
     *,
     scope: _ScopeName | Callable[[str, Config], _ScopeName] = ...,
     loop_scope: _ScopeName | None = ...,
-    loop_factory: _ScopeName | Callable[[], AbstractEventLoop] = ...,
+    loop_factory: Callable[[], AbstractEventLoop] | None = ...,
     params: Iterable[object] | None = ...,
     autouse: bool = ...,
     ids: (
@@ -168,7 +168,7 @@ def fixture(
 def fixture(
     fixture_function: FixtureFunction[_P, _R] | None = None,
     loop_scope: _ScopeName | None = None,
-    loop_factory: _ScopeName | Callable[[], AbstractEventLoop] = ...,
+    loop_factory: Callable[[], AbstractEventLoop] | None = None,
     **kwargs: Any,
 ) -> (
     FixtureFunction[_P, _R]
@@ -192,7 +192,11 @@ def _is_asyncio_fixture_function(obj: Any) -> bool:
     return getattr(obj, "_force_asyncio_fixture", False)
 
 
-def _make_asyncio_fixture_function(obj: Any, loop_scope: _ScopeName | None, loop_factory: _ScopeName | None) -> None:
+def _make_asyncio_fixture_function(
+    obj: Any,
+    loop_scope: _ScopeName | None,
+    loop_factory: Callable[[], AbstractEventLoop] | None,
+) -> None:
     if hasattr(obj, "__func__"):
         # instance method, check the function object
         obj = obj.__func__
@@ -285,14 +289,16 @@ def pytest_report_header(config: Config) -> list[str]:
 
 
 def _fixture_synchronizer(
-    fixturedef: FixtureDef, runner: Runner, request: FixtureRequest, loop_factory: Callable[[], AbstractEventLoop]
+    fixturedef: FixtureDef,
+    runner: Runner,
+    request: FixtureRequest,
 ) -> Callable:
     """Returns a synchronous function evaluating the specified fixture."""
     fixture_function = resolve_fixture_function(fixturedef, request)
     if inspect.isasyncgenfunction(fixturedef.func):
-        return _wrap_asyncgen_fixture(fixture_function, runner, request, loop_factory)  # type: ignore[arg-type]
+        return _wrap_asyncgen_fixture(fixture_function, runner, request)  # type: ignore[arg-type]
     elif inspect.iscoroutinefunction(fixturedef.func):
-        return _wrap_async_fixture(fixture_function, runner, request, loop_factory)  # type: ignore[arg-type]
+        return _wrap_async_fixture(fixture_function, runner, request)  # type: ignore[arg-type]
     else:
         return fixturedef.func
 
@@ -307,7 +313,6 @@ def _wrap_asyncgen_fixture(
     ],
     runner: Runner,
     request: FixtureRequest,
-    loop_factory:Callable[[], AbstractEventLoop]
 ) -> Callable[AsyncGenFixtureParams, AsyncGenFixtureYieldType]:
     @functools.wraps(fixture_function)
     def _asyncgen_fixture_wrapper(
@@ -337,9 +342,6 @@ async def async_finalizer() -> None:
                     msg = "Async generator fixture didn't stop."
                     msg += "Yield only once."
                     raise ValueError(msg)
-            if loop_factory:
-                _loop = loop_factory()
-                asyncio.set_event_loop(_loop)
 
             runner.run(async_finalizer(), context=context)
             if reset_contextvars is not None:
@@ -361,9 +363,8 @@ def _wrap_async_fixture(
     ],
     runner: Runner,
     request: FixtureRequest,
-    loop_factory: Callable[[], AbstractEventLoop] | None = None
 ) -> Callable[AsyncFixtureParams, AsyncFixtureReturnType]:
-    @functools.wraps(fixture_function)
+    @functools.wraps(fixture_function)  # type: ignore[arg-type]
     def _async_fixture_wrapper(
         *args: AsyncFixtureParams.args,
         **kwargs: AsyncFixtureParams.kwargs,
@@ -374,10 +375,6 @@ async def setup():
 
         context = contextvars.copy_context()
 
-        # ensure loop_factory gets ran before we start running...
-        if loop_factory:
-            asyncio.set_event_loop(loop_factory())
-
         result = runner.run(setup(), context=context)
         # Copy the context vars modified by the setup task into the current
         # context, and (if needed) add a finalizer to reset them.
@@ -522,16 +519,6 @@ def _can_substitute(item: Function) -> bool:
         func = item.obj
         return inspect.iscoroutinefunction(func)
 
-<<<<<<< HEAD
-=======
-    def runtest(self) -> None:
-        # print(self.obj.pytestmark[0].__dict__)
-        synchronized_obj = wrap_in_sync(self.obj, self.obj.pytestmark[0].kwargs.get('loop_factory', None))
-        with MonkeyPatch.context() as c:
-            c.setattr(self, "obj", synchronized_obj)
-            super().runtest()
-
->>>>>>> edfbfef (figured out loop_factory :))
 
 class AsyncGenerator(PytestAsyncioFunction):
     """Pytest item created by an asynchronous generator"""
@@ -641,16 +628,32 @@ def pytest_pycollect_makeitem_convert_async_functions_to_subclass(
 
 
 @contextlib.contextmanager
-def _temporary_event_loop_policy(policy: AbstractEventLoopPolicy) -> Iterator[None]:
+def _temporary_event_loop_policy(
+    policy: AbstractEventLoopPolicy,
+    loop_facotry: Callable[..., AbstractEventLoop] | None,
+) -> Iterator[None]:
+
     old_loop_policy = _get_event_loop_policy()
     try:
         old_loop = _get_event_loop_no_warn()
     except RuntimeError:
         old_loop = None
+    # XXX: For some reason this function can override runner's
+    # _loop_factory (At least observed on backported versions of Runner)
+    # so we need to re-override if existing...
+    if loop_facotry:
+        _loop = loop_facotry()
+        _set_event_loop(_loop)
+    else:
+        _loop = None
+
     _set_event_loop_policy(policy)
     try:
         yield
     finally:
+        if _loop:
+            # Do not let BaseEventLoop.__del__ complain!
+            _loop.close()
         _set_event_loop_policy(old_loop_policy)
         _set_event_loop(old_loop)
 
@@ -742,8 +745,6 @@ def _synchronize_coroutine(
     def inner(*args, **kwargs):
         coro = func(*args, **kwargs)
         runner.run(coro, context=context)
-
-        asyncio.set_event_loop(_last_loop)
     return inner
 
 
@@ -767,7 +768,7 @@ def pytest_fixture_setup(fixturedef: FixtureDef, request) -> object | None:
 
     runner_fixture_id = f"_{loop_scope}_scoped_runner"
     runner = request.getfixturevalue(runner_fixture_id)
-    synchronizer = _fixture_synchronizer(fixturedef, runner, request, loop_factory)
+    synchronizer = _fixture_synchronizer(fixturedef, runner, request)
     _make_asyncio_fixture_function(synchronizer, loop_scope, loop_factory)
     with MonkeyPatch.context() as c:
         c.setattr(fixturedef, "func", synchronizer)
@@ -822,19 +823,32 @@ def _get_default_test_loop_scope(config: Config) -> Any:
 """
 
 
+def _get_loop_facotry(
+    request: FixtureRequest,
+) -> Callable[[], AbstractEventLoop] | None:
+    if asyncio_mark := request._pyfuncitem.get_closest_marker("asyncio"):
+        factory = asyncio_mark.kwargs.get("loop_factory", None)
+        print(f"FACTORY {factory}")
+        return factory
+    else:
+        return request.obj.__dict__.get("_loop_factory", None)  # type: ignore[attr-defined]
+
+
 def _create_scoped_runner_fixture(scope: _ScopeName) -> Callable:
     @pytest.fixture(
         scope=scope,
         name=f"_{scope}_scoped_runner",
     )
     def _scoped_runner(
-        event_loop_policy,
-        request: FixtureRequest,
+        event_loop_policy: AbstractEventLoopPolicy, request: FixtureRequest
     ) -> Iterator[Runner]:
         new_loop_policy = event_loop_policy
-        debug_mode = _get_asyncio_debug(request.config)
-        with _temporary_event_loop_policy(new_loop_policy):
-            runner = Runner(debug=debug_mode).__enter__()
+
+        # We need to get the factory now because
+        # _temporary_event_loop_policy can override the Runner
+        factory = _get_loop_facotry(request)
+        with _temporary_event_loop_policy(new_loop_policy, factory):
+            runner = Runner(loop_factory=factory).__enter__()
             try:
                 yield runner
             except Exception as e:

tests/test_asyncio_mark.py

@@ -249,6 +249,16 @@ class CustomEventLoop(asyncio.SelectorEventLoop):
             @pytest.mark.asyncio(loop_factory=CustomEventLoop)
             async def test_has_different_event_loop():
                 assert type(asyncio.get_running_loop()).__name__ == "CustomEventLoop"
+
+            @pytest_asyncio.fixture(loop_factory=CustomEventLoop)
+            async def custom_fixture():
+                yield asyncio.get_running_loop()
+
+            async def test_with_fixture(custom_fixture):
+                # Both of these should be the same...
+                type(asyncio.get_running_loop()).__name__ == "CustomEventLoop"
+                type(custom_fixture).__name__ == "CustomEventLoop"
+
             """
         )
     )