DeprecationWarning if sync test requests async fixture

changelog/10839.deprecation.rst

@@ -0,0 +1 @@
+Requesting an asynchronous fixture without a `pytest_fixture_setup` hook that resolves it will now give a DeprecationWarning. This most commonly happens if a sync test requests an async fixture. This should have no effect on a majority of users with async tests or fixtures using async pytest plugins, but may affect non-standard hook setups or ``autouse=True``. For guidance on how to work around this warning see :ref:`sync-test-async-fixture`.

doc/en/deprecations.rst

@@ -15,6 +15,76 @@ Below is a complete list of all pytest features which are considered deprecated.
 :class:`~pytest.PytestWarning` or subclasses, which can be filtered using :ref:`standard warning filters <warnings>`.
 
 
+.. _sync-test-async-fixture:
+
+sync test depending on async fixture
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+.. deprecated:: 8.4
+
+Pytest has for a long time given an error when encountering an asynchronous test function, prompting the user to install
+a plugin that can handle it. It has not given any errors if you have an asynchronous fixture that's depended on by a
+synchronous test. If the fixture was an async function you did get an "unawaited coroutine" warning, but for async yield fixtures you didn't even get that.
+This is a problem even if you do have a plugin installed for handling async tests, as they may require
+special decorators for async fixtures to be handled, and some may not robustly handle if a user accidentally requests an
+async fixture from their sync tests. Fixture values being cached can make this even more unintuitive, where everything will
+"work" if the fixture is first requested by an async test, and then requested by a synchronous test.
+
+Unfortunately there is no 100% reliable method of identifying when a user has made a mistake, versus when they expect an
+unawaited object from their fixture that they will handle on their own. To suppress this warning
+when you in fact did intend to handle this you can wrap your async fixture in a synchronous fixture:
+
+.. code-block:: python
+
+    import asyncio
+    import pytest
+
+
+    @pytest.fixture
+    async def unawaited_fixture():
+        return 1
+
+
+    def test_foo(unawaited_fixture):
+        assert 1 == asyncio.run(unawaited_fixture)
+
+should be changed to
+
+
+.. code-block:: python
+
+    import asyncio
+    import pytest
+
+
+    @pytest.fixture
+    def unawaited_fixture():
+        async def inner_fixture():
+            return 1
+
+        return inner_fixture()
+
+
+    def test_foo(unawaited_fixture):
+        assert 1 == asyncio.run(unawaited_fixture)
+
+
+You can also make use of `pytest_fixture_setup` to handle the coroutine/asyncgen before pytest sees it - this is the way current async pytest plugins handle it.
+
+If a user has an async fixture with ``autouse=True`` in their ``conftest.py``, or in a file
+containing both synchronous tests and the fixture, they will receive this warning.
+Unless you're using a plugin that specifically handles async fixtures
+with synchronous tests, we strongly recommend against this practice.
+It can lead to unpredictable behavior (with larger scopes, it may appear to "work" if an async
+test is the first to request the fixture, due to value caching) and will generate
+unawaited-coroutine runtime warnings (but only for non-yield fixtures).
+Additionally, it creates ambiguity for other developers about whether the fixture is intended to perform
+setup for synchronous tests.
+
+The `anyio pytest plugin <https://anyio.readthedocs.io/en/stable/testing.html>`_ supports
+synchronous tests with async fixtures, though certain limitations apply.
+
+
 .. _import-or-skip-import-error:
 
 ``pytest.importorskip`` default behavior regarding :class:`ImportError`

pyproject.toml

@@ -336,7 +336,7 @@ disable = [
 ]
 
 [tool.codespell]
-ignore-words-list = "afile,asser,assertio,feld,hove,ned,noes,notin,paramete,parth,socio-economic,tesults,varius,wil"
+ignore-words-list = "afile,asend,asser,assertio,feld,hove,ned,noes,notin,paramete,parth,socio-economic,tesults,varius,wil"
 skip = "*/plugin_list.rst"
 write-changes = true
 

src/_pytest/fixtures.py

@@ -73,6 +73,7 @@
 from _pytest.scope import _ScopeName
 from _pytest.scope import HIGH_SCOPES
 from _pytest.scope import Scope
+from _pytest.warning_types import PytestRemovedIn9Warning
 
 
 if sys.version_info < (3, 11):
@@ -575,6 +576,7 @@
         # The are no fixtures with this name applicable for the function.
         if not fixturedefs:
             raise FixtureLookupError(argname, self)
+
         # A fixture may override another fixture with the same name, e.g. a
         # fixture in a module can override a fixture in a conftest, a fixture in
         # a class can override a fixture in the module, and so on.
@@ -959,6 +961,8 @@
         ids: tuple[object | None, ...] | Callable[[Any], object | None] | None = None,
         *,
         _ispytest: bool = False,
+        # only used in a deprecationwarning msg, can be removed in pytest9
+        _autouse: bool = False,
     ) -> None:
         check_ispytest(_ispytest)
         # The "base" node ID for the fixture.
@@ -1005,6 +1009,9 @@
         self.cached_result: _FixtureCachedResult[FixtureValue] | None = None
         self._finalizers: Final[list[Callable[[], object]]] = []
 
+        # only used to emit a deprecationwarning, can be removed in pytest9
+        self._autouse = _autouse
+
     @property
     def scope(self) -> _ScopeName:
         """Scope string, one of "function", "class", "module", "package", "session"."""
@@ -1136,6 +1143,25 @@
 
     fixturefunc = resolve_fixture_function(fixturedef, request)
     my_cache_key = fixturedef.cache_key(request)
+
+    if inspect.isasyncgenfunction(fixturefunc) or inspect.iscoroutinefunction(
+        fixturefunc
+    ):
+        auto_str = " with autouse=True" if fixturedef._autouse else ""
+
+        warnings.warn(
+            PytestRemovedIn9Warning(
+                f"{request.node.name!r} requested an async fixture "
+                f"{request.fixturename!r}{auto_str}, with no plugin or hook that "
+                "handled it. This is usually an error, as pytest does not natively "
+                "support it. "
+                "This will turn into an error in pytest 9.\n"
+                "See: https://docs.pytest.org/en/stable/deprecations.html#sync-test-depending-on-async-fixture"
+            ),
+            # no stacklevel will point at users code, so we just point here
+            stacklevel=1,
+        )
+
     try:
         result = call_fixture_func(fixturefunc, request, kwargs)
     except TEST_OUTCOME as e:
@@ -1666,6 +1692,7 @@
             params=params,
             ids=ids,
             _ispytest=True,
+            _autouse=autouse,
         )
 
         faclist = self._arg2fixturedefs.setdefault(name, [])

testing/acceptance_test.py

@@ -1286,6 +1286,104 @@ def test_3():
     result.assert_outcomes(failed=3)
 
 
+def test_warning_on_sync_test_async_fixture(pytester: Pytester) -> None:
+    pytester.makepyfile(
+        test_sync="""
+            import pytest
+
+            @pytest.fixture
+            async def async_fixture():
+                ...
+
+            def test_foo(async_fixture):
+                # suppress unawaited coroutine warning
+                try:
+                    async_fixture.send(None)
+                except StopIteration:
+                    pass
+        """
+    )
+    result = pytester.runpytest()
+    result.stdout.fnmatch_lines(
+        [
+            "*== warnings summary ==*",
+            (
+                "*PytestRemovedIn9Warning: 'test_foo' requested an async "
+                "fixture 'async_fixture', with no plugin or hook that handled it. "
+                "This is usually an error, as pytest does not natively support it. "
+                "This will turn into an error in pytest 9."
+            ),
+            "  See: https://docs.pytest.org/en/stable/deprecations.html#sync-test-depending-on-async-fixture",
+        ]
+    )
+    result.assert_outcomes(passed=1, warnings=1)
+
+
+def test_warning_on_sync_test_async_fixture_gen(pytester: Pytester) -> None:
+    pytester.makepyfile(
+        test_sync="""
+            import pytest
+
+            @pytest.fixture
+            async def async_fixture():
+                yield
+
+            def test_foo(async_fixture):
+                # async gens don't emit unawaited-coroutine
+                ...
+        """
+    )
+    result = pytester.runpytest()
+    result.stdout.fnmatch_lines(
+        [
+            "*== warnings summary ==*",
+            (
+                "*PytestRemovedIn9Warning: 'test_foo' requested an async "
+                "fixture 'async_fixture', with no plugin or hook that handled it. "
+                "This is usually an error, as pytest does not natively support it. "
+                "This will turn into an error in pytest 9."
+            ),
+            "  See: https://docs.pytest.org/en/stable/deprecations.html#sync-test-depending-on-async-fixture",
+        ]
+    )
+    result.assert_outcomes(passed=1, warnings=1)
+
+
+def test_warning_on_sync_test_async_autouse_fixture(pytester: Pytester) -> None:
+    pytester.makepyfile(
+        test_sync="""
+            import pytest
+
+            @pytest.fixture(autouse=True)
+            async def async_fixture():
+                ...
+
+            # We explicitly request the fixture to be able to
+            # suppress the RuntimeWarning for unawaited coroutine.
+            def test_foo(async_fixture):
+                try:
+                    async_fixture.send(None)
+                except StopIteration:
+                    pass
+        """
+    )
+    result = pytester.runpytest()
+    result.stdout.fnmatch_lines(
+        [
+            "*== warnings summary ==*",
+            (
+                "*PytestRemovedIn9Warning: 'test_foo' requested an async "
+                "fixture 'async_fixture' with autouse=True, with no plugin or hook "
+                "that handled it. "
+                "This is usually an error, as pytest does not natively support it. "
+                "This will turn into an error in pytest 9."
+            ),
+            "  See: https://docs.pytest.org/en/stable/deprecations.html#sync-test-depending-on-async-fixture",
+        ]
+    )
+    result.assert_outcomes(passed=1, warnings=1)
+
+
 def test_pdb_can_be_rewritten(pytester: Pytester) -> None:
     pytester.makepyfile(
         **{