Fix ref cycles in TestCase.assertRaises() (#193)

bpo-23890: unittest.TestCase.assertRaises() now manually breaks a
reference cycle to not keep objects alive longer than expected.

Lib/unittest/case.py

@@ -153,6 +153,7 @@ class _AssertRaisesBaseContext(_BaseTestCaseContext):
         If args is not empty, call a callable passing positional and keyword
         arguments.
         """
+        try:
             if not _is_subtype(self.expected, self._base_type):
                 raise TypeError('%s() arg 1 must be %s' %
                                 (name, self._base_type_str))
@@ -175,6 +176,9 @@ class _AssertRaisesBaseContext(_BaseTestCaseContext):
                 self.obj_name = str(callable_obj)
             with self:
                 callable_obj(*args, **kwargs)
+        finally:
+            # bpo-23890: manually break a reference cycle
+            self = None
 
 
 class _AssertRaisesContext(_AssertRaisesBaseContext):
@@ -725,7 +729,11 @@ class TestCase(object):
                self.assertEqual(the_exception.error_code, 3)
         """
         context = _AssertRaisesContext(expected_exception, self)
+        try:
             return context.handle('assertRaises', args, kwargs)
+        finally:
+            # bpo-23890: manually break a reference cycle
+            context = None
 
     def assertWarns(self, expected_warning, *args, **kwargs):
         """Fail unless a warning of class warnClass is triggered

Lib/unittest/test/test_case.py

@@ -1273,6 +1273,19 @@ test case
         with self.assertRaises(TypeError):
             self.assertRaises((ValueError, object))
 
+    def testAssertRaisesRefcount(self):
+        # bpo-23890: assertRaises() must not keep objects alive longer
+        # than expected
+        def func() :
+            try:
+                raise ValueError
+            except ValueError:
+                raise ValueError
+
+        refcount = sys.getrefcount(func)
+        self.assertRaises(ValueError, func)
+        self.assertEqual(refcount, sys.getrefcount(func))
+
     def testAssertRaisesRegex(self):
         class ExceptionMock(Exception):
             pass

Misc/NEWS

@@ -291,6 +291,9 @@ Extension Modules
 Library
 -------
 
+- bpo-23890: unittest.TestCase.assertRaises() now manually breaks a reference
+  cycle to not keep objects alive longer than expected.
+
 - bpo-29901: The zipapp module now supports general path-like objects, not
   just pathlib.Path.