python: fix instance handling in static and class method tests

and also fixes a regression in pytest 8.0.0 where `setup_method` crashes
if the class has static or class method tests.

It is allowed to have a test class with static/class methods which
request non-static/class method fixtures (including `setup_method`
xunit-fixture). I take it as a given that we need to support this
somewhat odd scenario (stdlib unittest also supports it).

This raises a question -- when a staticmethod test requests a bound
fixture, what is that fixture's `self`?

stdlib unittest says - a fresh instance for the test.

Previously, pytest said - some instance that is shared by all
static/class methods. This is definitely broken since it breaks test
isolation.

Change pytest to behave like stdlib unittest here.

In practice, this means stopping to rely on `self.obj.__self__` to get
to the instance from the test function's binding. This doesn't work
because staticmethods are not bound to anything.

Instead, keep the instance explicitly and use that.

BTW, I think this will allow us to change `Class`'s fixture collection
(`parsefactories`) to happen on the class itself instead of a class
instance, allowing us to avoid one class instantiation. But needs more
work.

Fixes #12065.

changelog/12065.bugfix.rst

@@ -0,0 +1,4 @@
+Fixed a regression in pytest 8.0.0 where test classes containing ``setup_method`` and tests using ``@staticmethod`` or ``@classmethod`` would crash with ``AttributeError: 'NoneType' object has no attribute 'setup_method'``.
+
+Now the :attr:`request.instance <pytest.FixtureRequest.instance>` attribute of tests using ``@staticmethod`` and ``@classmethod`` is no longer ``None``, but a fresh instance of the class, like in non-static methods.
+Previously it was ``None``, and all fixtures of such tests would share a single ``self``.

src/_pytest/fixtures.py

@@ -470,8 +470,9 @@ class FixtureRequest(abc.ABC):
     @property
     def instance(self):
         """Instance (can be None) on which test function was collected."""
-        function = getattr(self, "function", None)
-        return getattr(function, "__self__", None)
+        if self.scope != "function":
+            return None
+        return getattr(self._pyfuncitem, "instance", None)
 
     @property
     def module(self):

src/_pytest/python.py

@@ -302,10 +302,10 @@ class PyobjMixin(nodes.Node):
         """Python instance object the function is bound to.
 
         Returns None if not a test method, e.g. for a standalone test function,
-        a staticmethod, a class or a module.
+        a class or a module.
         """
-        node = self.getparent(Function)
-        return getattr(node.obj, "__self__", None) if node is not None else None
+        # Overridden by Function.
+        return None
 
     @property
     def obj(self):
@@ -1702,7 +1702,8 @@ class Function(PyobjMixin, nodes.Item):
         super().__init__(name, parent, config=config, session=session)
 
         if callobj is not NOTSET:
-            self.obj = callobj
+            self._obj = callobj
+            self._instance = getattr(callobj, "__self__", None)
 
         #: Original function name, without any decorations (for example
         #: parametrization adds a ``"[...]"`` suffix to function names), used to access
@@ -1752,12 +1753,31 @@ class Function(PyobjMixin, nodes.Item):
         """Underlying python 'function' object."""
         return getimfunc(self.obj)
 
-    def _getobj(self):
-        assert self.parent is not None
+    @property
+    def instance(self):
+        try:
+            return self._instance
+        except AttributeError:
             if isinstance(self.parent, Class):
                 # Each Function gets a fresh class instance.
-            parent_obj = self.parent.newinstance()
+                self._instance = self._getinstance()
             else:
+                self._instance = None
+        return self._instance
+
+    def _getinstance(self):
+        if isinstance(self.parent, Class):
+            # Each Function gets a fresh class instance.
+            return self.parent.newinstance()
+        else:
+            return None
+
+    def _getobj(self):
+        instance = self.instance
+        if instance is not None:
+            parent_obj = instance
+        else:
+            assert self.parent is not None
             parent_obj = self.parent.obj  # type: ignore[attr-defined]
         return getattr(parent_obj, self.originalname)
 

src/_pytest/unittest.py

@@ -177,16 +177,15 @@ class TestCaseFunction(Function):
     nofuncargs = True
     _excinfo: Optional[List[_pytest._code.ExceptionInfo[BaseException]]] = None
 
-    def _getobj(self):
+    def _getinstance(self):
         assert isinstance(self.parent, UnitTestCase)
-        testcase = self.parent.obj(self.name)
-        return getattr(testcase, self.name)
+        return self.parent.obj(self.name)
 
     # Backward compat for pytest-django; can be removed after pytest-django
     # updates + some slack.
     @property
     def _testcase(self):
-        return self._obj.__self__
+        return self.instance
 
     def setup(self) -> None:
         # A bound method to be called during teardown() if set (see 'runtest()').
@@ -296,7 +295,8 @@ class TestCaseFunction(Function):
     def runtest(self) -> None:
         from _pytest.debugging import maybe_wrap_pytest_function_for_tracing
 
-        testcase = self.obj.__self__
+        testcase = self.instance
+        assert testcase is not None
 
         maybe_wrap_pytest_function_for_tracing(self)
 

testing/python/fixtures.py

@@ -4577,3 +4577,48 @@ def test_deduplicate_names() -> None:
     assert items == ("a", "b", "c", "d")
     items = deduplicate_names((*items, "g", "f", "g", "e", "b"))
     assert items == ("a", "b", "c", "d", "g", "f", "e")
+
+
+def test_staticmethod_classmethod_fixture_instance(pytester: Pytester) -> None:
+    """Ensure that static and class methods get and have access to a fresh
+    instance.
+
+    This also ensures `setup_method` works well with static and class methods.
+
+    Regression test for #12065.
+    """
+    pytester.makepyfile(
+        """
+        import pytest
+
+        class Test:
+            ran_setup_method = False
+            ran_fixture = False
+
+            def setup_method(self):
+                assert not self.ran_setup_method
+                self.ran_setup_method = True
+
+            @pytest.fixture(autouse=True)
+            def fixture(self):
+                assert not self.ran_fixture
+                self.ran_fixture = True
+
+            def test_method(self):
+                assert self.ran_setup_method
+                assert self.ran_fixture
+
+            @staticmethod
+            def test_1(request):
+                assert request.instance.ran_setup_method
+                assert request.instance.ran_fixture
+
+            @classmethod
+            def test_2(cls, request):
+                assert request.instance.ran_setup_method
+                assert request.instance.ran_fixture
+        """
+    )
+    result = pytester.runpytest()
+    assert result.ret == ExitCode.OK
+    result.assert_outcomes(passed=3)

testing/python/integration.py

@@ -410,22 +410,37 @@ def test_function_instance(pytester: Pytester) -> None:
     items = pytester.getitems(
         """
         def test_func(): pass
+
         class TestIt:
             def test_method(self): pass
+
             @classmethod
             def test_class(cls): pass
+
             @staticmethod
             def test_static(): pass
         """
     )
     assert len(items) == 4
+
     assert isinstance(items[0], Function)
     assert items[0].name == "test_func"
     assert items[0].instance is None
+
     assert isinstance(items[1], Function)
     assert items[1].name == "test_method"
     assert items[1].instance is not None
     assert items[1].instance.__class__.__name__ == "TestIt"
+
+    # Even class and static methods get an instance!
+    # This is the instance used for bound fixture methods, which
+    # class/staticmethod tests are perfectly able to request.
+    assert isinstance(items[2], Function)
+    assert items[2].name == "test_class"
+    assert items[2].instance is not None
+
     assert isinstance(items[3], Function)
     assert items[3].name == "test_static"
-    assert items[3].instance is None
+    assert items[3].instance is not None
+
+    assert items[1].instance is not items[2].instance is not items[3].instance