[8.2.x] unittest: fix assertion errors on unittest reruns (#12438)

Co-authored-by: Ran Benita <ran@unusedvar.com>

changelog/12424.bugfix.rst

@@ -0,0 +1 @@
+Fix crash with `assert testcase is not None` assertion failure when re-running unittest tests using plugins like pytest-rerunfailures. Regressed in 8.2.2.

src/_pytest/unittest.py

@@ -216,7 +216,7 @@ class TestCaseFunction(Function):
             self._explicit_tearDown()
             self._explicit_tearDown = None
         self._obj = None
-        self._instance = None
+        del self._instance
         super().teardown()
 
     def startTest(self, testcase: "unittest.TestCase") -> None:

testing/plugins_integration/pytest_rerunfailures_integration.py

@@ -0,0 +1,11 @@
+import unittest
+
+
+class MyTestCase(unittest.TestCase):
+    first_time = True
+
+    def test_fail_the_first_time(self) -> None:
+        """Regression test for issue #12424."""
+        if self.first_time:
+            type(self).first_time = False
+            self.fail()

testing/plugins_integration/requirements.txt

@@ -1,9 +1,7 @@
-anyio[curio,trio]==4.3.0
-django==5.0.4
-pytest-asyncio==0.23.6
-# Temporarily not installed until pytest-bdd is fixed:
-# https://github.com/pytest-dev/pytest/pull/11785
-# pytest-bdd==7.0.1
+anyio[curio,trio]==4.4.0
+django==5.0.6
+pytest-asyncio==0.23.7
+pytest-bdd==7.1.2
 pytest-cov==5.0.0
 pytest-django==4.8.0
 pytest-flakes==4.0.5
@@ -11,7 +9,7 @@ pytest-html==4.1.1
 pytest-mock==3.14.0
 pytest-rerunfailures==14.0
 pytest-sugar==1.0.0
-pytest-trio==0.7.0
+pytest-trio==0.8.0
 pytest-twisted==1.14.1
 twisted==24.3.0
 pytest-xvfb==3.0.0

tox.ini

@@ -143,7 +143,7 @@ commands =
     pytest --cov=. simple_integration.py
     pytest --ds=django_settings simple_integration.py
     pytest --html=simple.html simple_integration.py
-    pytest --reruns 5 simple_integration.py
+    pytest --reruns 5 simple_integration.py pytest_rerunfailures_integration.py
     pytest pytest_anyio_integration.py
     pytest pytest_asyncio_integration.py
     pytest pytest_mock_integration.py