bpo-45156: Fixes inifite loop on unittest.mock.seal() (GH-28300)

Fixes infinite loop on unittest.mock.seal() of mocks created by
unittest.create_autospec().

Co-authored-by: Dong-hee Na <donghee.na92@gmail.com>

Lib/unittest/test/testmock/testsealable.py

@@ -171,6 +171,67 @@ class TestSealable(unittest.TestCase):
             m.test1().test2.test3().test4()
         self.assertIn("mock.test1().test2.test3().test4", str(cm.exception))
 
+    def test_seal_with_autospec(self):
+        # https://bugs.python.org/issue45156
+        class Foo:
+            foo = 0
+            def bar1(self):
+                return 1
+            def bar2(self):
+                return 2
+
+            class Baz:
+                baz = 3
+                def ban(self):
+                    return 4
+
+        for spec_set in (True, False):
+            with self.subTest(spec_set=spec_set):
+                foo = mock.create_autospec(Foo, spec_set=spec_set)
+                foo.bar1.return_value = 'a'
+                foo.Baz.ban.return_value = 'b'
+
+                mock.seal(foo)
+
+                self.assertIsInstance(foo.foo, mock.NonCallableMagicMock)
+                self.assertIsInstance(foo.bar1, mock.MagicMock)
+                self.assertIsInstance(foo.bar2, mock.MagicMock)
+                self.assertIsInstance(foo.Baz, mock.MagicMock)
+                self.assertIsInstance(foo.Baz.baz, mock.NonCallableMagicMock)
+                self.assertIsInstance(foo.Baz.ban, mock.MagicMock)
+
+                self.assertEqual(foo.bar1(), 'a')
+                foo.bar1.return_value = 'new_a'
+                self.assertEqual(foo.bar1(), 'new_a')
+                self.assertEqual(foo.Baz.ban(), 'b')
+                foo.Baz.ban.return_value = 'new_b'
+                self.assertEqual(foo.Baz.ban(), 'new_b')
+
+                with self.assertRaises(TypeError):
+                    foo.foo()
+                with self.assertRaises(AttributeError):
+                    foo.bar = 1
+                with self.assertRaises(AttributeError):
+                    foo.bar2()
+
+                foo.bar2.return_value = 'bar2'
+                self.assertEqual(foo.bar2(), 'bar2')
+
+                with self.assertRaises(AttributeError):
+                    foo.missing_attr
+                with self.assertRaises(AttributeError):
+                    foo.missing_attr = 1
+                with self.assertRaises(AttributeError):
+                    foo.missing_method()
+                with self.assertRaises(TypeError):
+                    foo.Baz.baz()
+                with self.assertRaises(AttributeError):
+                    foo.Baz.missing_attr
+                with self.assertRaises(AttributeError):
+                    foo.Baz.missing_attr = 1
+                with self.assertRaises(AttributeError):
+                    foo.Baz.missing_method()
+
 
 if __name__ == "__main__":
     unittest.main()