deprecate: warn on class-scoped fixture as instance method (#10819) (#14011)

Author: yastcher

changelog/10819.bugfix.rst

@@ -0,0 +1,3 @@
+Fixed class-scoped fixtures defined in base classes not binding to the correct test instance when inherited by child test classes -- by :user:`yastcher`.
+
+Fixes :issue:`10819` and :issue:`14011`.

src/_pytest/deprecated.py

@@ -35,6 +35,14 @@
     "Use @pytest.fixture instead; they are the same."
 )
 
+CLASS_FIXTURE_INSTANCE_METHOD = PytestRemovedIn10Warning(
+    "Class-scoped fixture defined as instance method is deprecated.\n"
+    "Instance attributes set in this fixture will NOT be visible to test methods,\n"
+    "as each test gets a new instance while the fixture runs only once per class.\n"
+    "Use @classmethod decorator and set attributes on cls instead.\n"
+    "See https://docs.pytest.org/en/stable/deprecations.html#class-scoped-fixture-as-instance-method"
+)
+
 # This deprecation is never really meant to be removed.
 PRIVATE = PytestDeprecationWarning("A private pytest class or function was used.")
 

src/_pytest/fixtures.py

@@ -54,6 +54,7 @@
 from _pytest.config import ExitCode
 from _pytest.config.argparsing import Parser
 from _pytest.deprecated import check_ispytest
+from _pytest.deprecated import CLASS_FIXTURE_INSTANCE_METHOD
 from _pytest.deprecated import YIELD_FIXTURE
 from _pytest.main import Session
 from _pytest.mark import ParameterSet
@@ -1148,6 +1149,16 @@ def resolve_fixture_function(
     # request.instance so that code working with "fixturedef" behaves
     # as expected.
     instance = request.instance
+
+    if fixturedef._scope is Scope.Class:
+        # Check if fixture is an instance method (bound to instance, not class)
+        if hasattr(fixturefunc, "__self__"):
+            bound_to = fixturefunc.__self__
+            # classmethod: bound_to is the class itself (a type)
+            # instance method: bound_to is an instance (not a type)
+            if not isinstance(bound_to, type):
+                warnings.warn(CLASS_FIXTURE_INSTANCE_METHOD, stacklevel=2)
+
     if instance is not None:
         # Handle the case where fixture is defined not in a test class, but some other class
         # (for example a plugin class with a fixture), see #2270.

testing/deprecated_test.py

@@ -107,3 +107,24 @@ def collect(self):
             parent=mod.parent,
             fspath=legacy_path("bla"),
         )
+
+
+def test_class_scope_instance_method_is_deprecated(pytester: Pytester) -> None:
+    pytester.makepyfile(
+        """
+        import pytest
+
+        class TestClass:
+            @pytest.fixture(scope="class")
+            def fix(self):
+                self.attr = True
+
+            def test_foo(self, fix):
+                pass
+        """
+    )
+    result = pytester.runpytest("-Werror::pytest.PytestRemovedIn10Warning")
+    result.assert_outcomes(errors=1)
+    result.stdout.fnmatch_lines(
+        ["*PytestRemovedIn10Warning: Class-scoped fixture defined as instance method*"]
+    )

testing/test_inherit_class_fixture.py

@@ -0,0 +1,92 @@
+from __future__ import annotations
+
+import typing
+
+import pytest
+
+
+class ParentBase:
+    """from issue #14011"""
+
+    name = ""
+    variable = ""
+    flag: bool
+
+    @classmethod
+    def setup(cls) -> None:
+        cls.variable = cls.name
+
+    @classmethod
+    def teardown(cls) -> None:
+        pass
+
+    @pytest.fixture(scope="class")
+    @classmethod
+    def fix(cls) -> typing.Generator[None]:
+        cls.setup()
+        yield
+        cls.teardown()
+
+    @pytest.fixture(scope="class", autouse=True)
+    @classmethod
+    def base_autouse(cls) -> None:
+        cls.flag = True
+
+
+@pytest.mark.usefixtures("fix")
+class Test1(ParentBase):
+    name = "test1"
+
+    def test_a(self) -> None:
+        assert self.variable == self.name
+
+
+@pytest.mark.usefixtures("fix")
+class Test2(ParentBase):
+    name = "test2"
+
+    def test_a(self) -> None:
+        assert self.variable == self.name
+
+
+class TestChild(ParentBase):
+    def test_flag(self) -> None:
+        assert self.flag
+
+
+class BaseTestClass:
+    """from issue #10819"""
+
+    test_func_scope_set = None
+    test_class_scope_set = None
+
+    @pytest.fixture(scope="class", autouse=True)
+    @classmethod
+    def dummy_class_fixture(cls) -> None:
+        cls.test_class_scope_set = True
+
+    @pytest.fixture(scope="function", autouse=True)
+    def dummy_func_fixture(self) -> None:
+        self.test_func_scope_set = True
+
+
+class TestDummy(BaseTestClass):
+    def test_dummy(self) -> None:
+        assert self.test_func_scope_set is True
+        assert self.test_class_scope_set is True
+
+
+@pytest.mark.usefixtures("fix")
+class TestMultipleMethods(ParentBase):
+    """check class-scoped fixture with multiple test methods"""
+
+    name = "multi"
+
+    def test_a(self) -> None:
+        assert self.variable == self.name
+
+    def test_b(self) -> None:
+        assert self.variable == self.name
+
+    def test_c(self) -> None:
+        assert self.variable == self.name