unittest: make `obj` work more like `Function`/`Class`

Previously, the `obj` of a `TestCaseFunction` (the unittest plugin item
type) was the unbound method. This is unlike regular `Class` where the
`obj` is a bound method to a fresh instance.

This difference necessitated several special cases in in places outside
of the unittest plugin, such as `FixtureDef` and `FixtureRequest`, and
made things a bit harder to understand.

Instead, match how the python plugin does it, including collecting
fixtures from a fresh instance.

The downside is that now this instance for fixture-collection is kept
around in memory, but it's the same as `Class` so nothing new. Users
should only initialize stuff in `setUp`/`setUpClass` and similar
methods, and not in `__init__` which is generally off-limits in
`TestCase` subclasses.

I am not sure why there was a difference in the first place, though I
will say the previous unittest approach is probably the preferable one,
but first let's get consistency.

src/_pytest/compat.py

@@ -86,7 +86,6 @@ def getfuncargnames(
     function: Callable[..., object],
     *,
     name: str = "",
-    is_method: bool = False,
     cls: type | None = None,
 ) -> tuple[str, ...]:
     """Return the names of a function's mandatory arguments.
@@ -97,9 +96,8 @@ def getfuncargnames(
     * Aren't bound with functools.partial.
     * Aren't replaced with mocks.
 
-    The is_method and cls arguments indicate that the function should
-    be treated as a bound method even though it's not unless, only in
-    the case of cls, the function is a static method.
+    The cls arguments indicate that the function should be treated as a bound
+    method even though it's not unless the function is a static method.
 
     The name parameter should be the original name in which the function was collected.
     """
@@ -137,7 +135,7 @@ def getfuncargnames(
     # If this function should be treated as a bound method even though
     # it's passed as an unbound method or function, remove the first
     # parameter name.
-    if is_method or (
+    if (
         # Not using `getattr` because we don't want to resolve the staticmethod.
         # Not using `cls.__dict__` because we want to check the entire MRO.
         cls

src/_pytest/fixtures.py

@@ -462,12 +462,8 @@ class FixtureRequest(abc.ABC):
     @property
     def instance(self):
         """Instance (can be None) on which test function was collected."""
-        # unittest support hack, see _pytest.unittest.TestCaseFunction.
-        try:
-            return self._pyfuncitem._testcase  # type: ignore[attr-defined]
-        except AttributeError:
-            function = getattr(self, "function", None)
-            return getattr(function, "__self__", None)
+        function = getattr(self, "function", None)
+        return getattr(function, "__self__", None)
 
     @property
     def module(self):
@@ -965,7 +961,6 @@ class FixtureDef(Generic[FixtureValue]):
         func: "_FixtureFunc[FixtureValue]",
         scope: Union[Scope, _ScopeName, Callable[[str, Config], _ScopeName], None],
         params: Optional[Sequence[object]],
-        unittest: bool = False,
         ids: Optional[
             Union[Tuple[Optional[object], ...], Callable[[Any], Optional[object]]]
         ] = None,
@@ -1011,9 +1006,7 @@ class FixtureDef(Generic[FixtureValue]):
         # a parameter value.
         self.ids: Final = ids
         # The names requested by the fixtures.
-        self.argnames: Final = getfuncargnames(func, name=argname, is_method=unittest)
-        # Whether the fixture was collected from a unittest TestCase class.
-        self.unittest: Final = unittest
+        self.argnames: Final = getfuncargnames(func, name=argname)
         # If the fixture was executed, the current value of the fixture.
         # Can change if the fixture is executed with different parameters.
         self.cached_result: Optional[_FixtureCachedResult[FixtureValue]] = None
@@ -1092,25 +1085,20 @@ def resolve_fixture_function(
     """Get the actual callable that can be called to obtain the fixture
     value, dealing with unittest-specific instances and bound methods."""
     fixturefunc = fixturedef.func
-    if fixturedef.unittest:
-        if request.instance is not None:
-            # Bind the unbound method to the TestCase instance.
-            fixturefunc = fixturedef.func.__get__(request.instance)  # type: ignore[union-attr]
-    else:
-        # The fixture function needs to be bound to the actual
-        # request.instance so that code working with "fixturedef" behaves
-        # as expected.
-        if request.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.
-            if hasattr(fixturefunc, "__self__") and not isinstance(
-                request.instance,
-                fixturefunc.__self__.__class__,  # type: ignore[union-attr]
-            ):
-                return fixturefunc
-            fixturefunc = getimfunc(fixturedef.func)
-            if fixturefunc != fixturedef.func:
-                fixturefunc = fixturefunc.__get__(request.instance)  # type: ignore[union-attr]
+    # The fixture function needs to be bound to the actual
+    # request.instance so that code working with "fixturedef" behaves
+    # as expected.
+    if request.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.
+        if hasattr(fixturefunc, "__self__") and not isinstance(
+            request.instance,
+            fixturefunc.__self__.__class__,  # type: ignore[union-attr]
+        ):
+            return fixturefunc
+        fixturefunc = getimfunc(fixturedef.func)
+        if fixturefunc != fixturedef.func:
+            fixturefunc = fixturefunc.__get__(request.instance)  # type: ignore[union-attr]
     return fixturefunc
 
 
@@ -1614,7 +1602,6 @@ class FixtureManager:
             Union[Tuple[Optional[object], ...], Callable[[Any], Optional[object]]]
         ] = None,
         autouse: bool = False,
-        unittest: bool = False,
     ) -> None:
         """Register a fixture
 
@@ -1635,8 +1622,6 @@ class FixtureManager:
             The fixture's IDs.
         :param autouse:
             Whether this is an autouse fixture.
-        :param unittest:
-            Set this if this is a unittest fixture.
         """
         fixture_def = FixtureDef(
             config=self.config,
@@ -1645,7 +1630,6 @@ class FixtureManager:
             func=func,
             scope=scope,
             params=params,
-            unittest=unittest,
             ids=ids,
             _ispytest=True,
         )
@@ -1667,8 +1651,6 @@ class FixtureManager:
     def parsefactories(
         self,
         node_or_obj: nodes.Node,
-        *,
-        unittest: bool = ...,
     ) -> None:
         raise NotImplementedError()
 
@@ -1677,8 +1659,6 @@ class FixtureManager:
         self,
         node_or_obj: object,
         nodeid: Optional[str],
-        *,
-        unittest: bool = ...,
     ) -> None:
         raise NotImplementedError()
 
@@ -1686,8 +1666,6 @@ class FixtureManager:
         self,
         node_or_obj: Union[nodes.Node, object],
         nodeid: Union[str, NotSetType, None] = NOTSET,
-        *,
-        unittest: bool = False,
     ) -> None:
         """Collect fixtures from a collection node or object.
 
@@ -1739,7 +1717,6 @@ class FixtureManager:
                 func=func,
                 scope=marker.scope,
                 params=marker.params,
-                unittest=unittest,
                 ids=marker.ids,
                 autouse=marker.autouse,
             )

src/_pytest/python.py

@@ -1314,7 +1314,6 @@ class Metafunc:
                     func=get_direct_param_fixture_func,
                     scope=scope_,
                     params=None,
-                    unittest=False,
                     ids=None,
                     _ispytest=True,
                 )

src/_pytest/unittest.py

@@ -15,7 +15,6 @@ from typing import TYPE_CHECKING
 from typing import Union
 
 import _pytest._code
-from _pytest.compat import getimfunc
 from _pytest.compat import is_async_function
 from _pytest.config import hookimpl
 from _pytest.fixtures import FixtureRequest
@@ -63,6 +62,14 @@ class UnitTestCase(Class):
     # to declare that our children do not support funcargs.
     nofuncargs = True
 
+    def newinstance(self):
+        # TestCase __init__ takes the method (test) name. The TestCase
+        # constructor treats the name "runTest" as a special no-op, so it can be
+        # used when a dummy instance is needed. While unittest.TestCase has a
+        # default, some subclasses omit the default (#9610), so always supply
+        # it.
+        return self.obj("runTest")
+
     def collect(self) -> Iterable[Union[Item, Collector]]:
         from unittest import TestLoader
 
@@ -76,15 +83,15 @@ class UnitTestCase(Class):
             self._register_unittest_setup_class_fixture(cls)
             self._register_setup_class_fixture()
 
-        self.session._fixturemanager.parsefactories(self, unittest=True)
+        self.session._fixturemanager.parsefactories(self.newinstance(), self.nodeid)
+
         loader = TestLoader()
         foundsomething = False
         for name in loader.getTestCaseNames(self.obj):
             x = getattr(self.obj, name)
             if not getattr(x, "__test__", True):
                 continue
-            funcobj = getimfunc(x)
-            yield TestCaseFunction.from_parent(self, name=name, callobj=funcobj)
+            yield TestCaseFunction.from_parent(self, name=name)
             foundsomething = True
 
         if not foundsomething:
@@ -169,23 +176,21 @@ class UnitTestCase(Class):
 class TestCaseFunction(Function):
     nofuncargs = True
     _excinfo: Optional[List[_pytest._code.ExceptionInfo[BaseException]]] = None
-    _testcase: Optional["unittest.TestCase"] = None
 
     def _getobj(self):
-        assert self.parent is not None
-        # Unlike a regular Function in a Class, where `item.obj` returns
-        # a *bound* method (attached to an instance), TestCaseFunction's
-        # `obj` returns an *unbound* method (not attached to an instance).
-        # This inconsistency is probably not desirable, but needs some
-        # consideration before changing.
-        return getattr(self.parent.obj, self.originalname)  # type: ignore[attr-defined]
+        assert isinstance(self.parent, UnitTestCase)
+        testcase = self.parent.obj(self.name)
+        return getattr(testcase, self.name)
+
+    # Backward compat for pytest-django; can be removed after pytest-django
+    # updates + some slack.
+    @property
+    def _testcase(self):
+        return self._obj.__self__
 
     def setup(self) -> None:
         # A bound method to be called during teardown() if set (see 'runtest()').
         self._explicit_tearDown: Optional[Callable[[], None]] = None
-        assert self.parent is not None
-        self._testcase = self.parent.obj(self.name)  # type: ignore[attr-defined]
-        self._obj = getattr(self._testcase, self.name)
         super().setup()
 
     def teardown(self) -> None:
@@ -193,7 +198,6 @@ class TestCaseFunction(Function):
         if self._explicit_tearDown is not None:
             self._explicit_tearDown()
             self._explicit_tearDown = None
-        self._testcase = None
         self._obj = None
 
     def startTest(self, testcase: "unittest.TestCase") -> None:
@@ -292,14 +296,14 @@ class TestCaseFunction(Function):
     def runtest(self) -> None:
         from _pytest.debugging import maybe_wrap_pytest_function_for_tracing
 
-        assert self._testcase is not None
+        testcase = self.obj.__self__
 
         maybe_wrap_pytest_function_for_tracing(self)
 
         # Let the unittest framework handle async functions.
         if is_async_function(self.obj):
             # Type ignored because self acts as the TestResult, but is not actually one.
-            self._testcase(result=self)  # type: ignore[arg-type]
+            testcase(result=self)  # type: ignore[arg-type]
         else:
             # When --pdb is given, we want to postpone calling tearDown() otherwise
             # when entering the pdb prompt, tearDown() would have probably cleaned up
@@ -311,16 +315,16 @@ class TestCaseFunction(Function):
             assert isinstance(self.parent, UnitTestCase)
             skipped = _is_skipped(self.obj) or _is_skipped(self.parent.obj)
             if self.config.getoption("usepdb") and not skipped:
-                self._explicit_tearDown = self._testcase.tearDown
-                setattr(self._testcase, "tearDown", lambda *args: None)
+                self._explicit_tearDown = testcase.tearDown
+                setattr(testcase, "tearDown", lambda *args: None)
 
             # We need to update the actual bound method with self.obj, because
             # wrap_pytest_function_for_tracing replaces self.obj by a wrapper.
-            setattr(self._testcase, self.name, self.obj)
+            setattr(testcase, self.name, self.obj)
             try:
-                self._testcase(result=self)  # type: ignore[arg-type]
+                testcase(result=self)  # type: ignore[arg-type]
             finally:
-                delattr(self._testcase, self.name)
+                delattr(testcase, self.name)
 
     def _traceback_filter(
         self, excinfo: _pytest._code.ExceptionInfo[BaseException]

testing/test_unittest.py

@@ -208,10 +208,14 @@ def test_teardown_issue1649(pytester: Pytester) -> None:
 
     """
     )
+
     pytester.inline_run("-s", testpath)
     gc.collect()
+
+    # Either already destroyed, or didn't run setUp.
     for obj in gc.get_objects():
-        assert type(obj).__name__ != "TestCaseObjectsShouldBeCleanedUp"
+        if type(obj).__name__ == "TestCaseObjectsShouldBeCleanedUp":
+            assert not hasattr(obj, "an_expensive_obj")
 
 
 def test_unittest_skip_issue148(pytester: Pytester) -> None: