Close #19880: Fix a reference leak in unittest.TestCase. Explicitly break

reference cycles between frames and the _Outcome instance.

Lib/unittest/case.py

@@ -69,6 +69,9 @@ def testPartExecutor(self, test_case, isTest=False):
             else:
                 self.success = False
                 self.errors.append((test_case, exc_info))
+            # explicitly break a reference cycle:
+            # exc_info -> frame -> exc_info
+            exc_info = None
         else:
             if self.result_supports_subtests and self.success:
                 self.errors.append((test_case, None))
@@ -559,8 +562,8 @@ def run(self, result=None):
             return
         expecting_failure = getattr(testMethod,
                                     "__unittest_expecting_failure__", False)
+        outcome = _Outcome(result)
         try:
-            outcome = _Outcome(result)
             self._outcome = outcome
 
             with outcome.testPartExecutor(self):
@@ -593,6 +596,15 @@ def run(self, result=None):
                 if stopTestRun is not None:
                     stopTestRun()
 
+            # explicitly break reference cycles:
+            # outcome.errors -> frame -> outcome -> outcome.errors
+            # outcome.expectedFailure -> frame -> outcome -> outcome.expectedFailure
+            outcome.errors.clear()
+            outcome.expectedFailure = None
+
+            # clear the outcome, no more needed
+            self._outcome = None
+
     def doCleanups(self):
         """Execute all cleanup functions. Normally called for you after
         tearDown."""

Lib/unittest/test/test_case.py

@@ -1533,6 +1533,32 @@ def testNoCycles(self):
             del case
             self.assertFalse(wr())
 
+    def test_no_exception_leak(self):
+        # Issue #19880: TestCase.run() should not keep a reference
+        # to the exception
+        class MyException(Exception):
+            ninstance = 0
+
+            def __init__(self):
+                MyException.ninstance += 1
+                Exception.__init__(self)
+
+            def __del__(self):
+                MyException.ninstance -= 1
+
+        class TestCase(unittest.TestCase):
+            def test1(self):
+                raise MyException()
+
+            @unittest.expectedFailure
+            def test2(self):
+                raise MyException()
+
+        for method_name in ('test1', 'test2'):
+            testcase = TestCase(method_name)
+            testcase.run()
+            self.assertEqual(MyException.ninstance, 0)
+
 
 if __name__ == "__main__":
     unittest.main()

Misc/NEWS

@@ -21,6 +21,9 @@ Core and Builtins
 Library
 -------
 
+- Issue #19880: Fix a reference leak in unittest.TestCase. Explicitly break
+  reference cycles between frames and the _Outcome instance.
+
 - Issue #17429: platform.linux_distribution() now decodes files from the UTF-8
   encoding with the surrogateescape error handler, instead of decoding from the
   locale encoding in strict mode. It fixes the function on Fedora 19 which is