bpo-35330: Don't call the wrapped object if side_effect is set (GH10973)

* tests: Further validate `wraps` functionality in `unittest.mock.Mock`

Add more tests to validate how `wraps` interacts with other features of
mocks.

* Don't call the wrapped object if `side_effect` is set

When a object is wrapped using `Mock(wraps=...)`, if an user sets a
`side_effect` in one of their methods, return the value of `side_effect`
and don't call the original object.

* Refactor what to be called on `mock_call`

When a `Mock` is called, it should return looking up in the following
order: `side_effect`, `return_value`, `wraps`. If any of the first two
return `mock.DEFAULT`, lookup in the next option.

It makes no sense to check for `wraps` returning default, as it is
supposed to be the original implementation and there is nothing to
fallback to.

Lib/unittest/test/testmock/testmock.py

@@ -558,6 +558,16 @@ class MockTest(unittest.TestCase):
         real.assert_called_with(1, 2, fish=3)
 
 
+    def test_wraps_prevents_automatic_creation_of_mocks(self):
+        class Real(object):
+            pass
+
+        real = Real()
+        mock = Mock(wraps=real)
+
+        self.assertRaises(AttributeError, lambda: mock.new_attr())
+
+
     def test_wraps_call_with_nondefault_return_value(self):
         real = Mock()
 
@@ -584,6 +594,118 @@ class MockTest(unittest.TestCase):
         self.assertEqual(result, Real.attribute.frog())
 
 
+    def test_customize_wrapped_object_with_side_effect_iterable_with_default(self):
+        class Real(object):
+            def method(self):
+                return sentinel.ORIGINAL_VALUE
+
+        real = Real()
+        mock = Mock(wraps=real)
+        mock.method.side_effect = [sentinel.VALUE1, DEFAULT]
+
+        self.assertEqual(mock.method(), sentinel.VALUE1)
+        self.assertEqual(mock.method(), sentinel.ORIGINAL_VALUE)
+        self.assertRaises(StopIteration, mock.method)
+
+
+    def test_customize_wrapped_object_with_side_effect_iterable(self):
+        class Real(object):
+            def method(self):
+                raise NotImplementedError()
+
+        real = Real()
+        mock = Mock(wraps=real)
+        mock.method.side_effect = [sentinel.VALUE1, sentinel.VALUE2]
+
+        self.assertEqual(mock.method(), sentinel.VALUE1)
+        self.assertEqual(mock.method(), sentinel.VALUE2)
+        self.assertRaises(StopIteration, mock.method)
+
+
+    def test_customize_wrapped_object_with_side_effect_exception(self):
+        class Real(object):
+            def method(self):
+                raise NotImplementedError()
+
+        real = Real()
+        mock = Mock(wraps=real)
+        mock.method.side_effect = RuntimeError
+
+        self.assertRaises(RuntimeError, mock.method)
+
+
+    def test_customize_wrapped_object_with_side_effect_function(self):
+        class Real(object):
+            def method(self):
+                raise NotImplementedError()
+
+        def side_effect():
+            return sentinel.VALUE
+
+        real = Real()
+        mock = Mock(wraps=real)
+        mock.method.side_effect = side_effect
+
+        self.assertEqual(mock.method(), sentinel.VALUE)
+
+
+    def test_customize_wrapped_object_with_return_value(self):
+        class Real(object):
+            def method(self):
+                raise NotImplementedError()
+
+        real = Real()
+        mock = Mock(wraps=real)
+        mock.method.return_value = sentinel.VALUE
+
+        self.assertEqual(mock.method(), sentinel.VALUE)
+
+
+    def test_customize_wrapped_object_with_return_value_and_side_effect(self):
+        # side_effect should always take precedence over return_value.
+        class Real(object):
+            def method(self):
+                raise NotImplementedError()
+
+        real = Real()
+        mock = Mock(wraps=real)
+        mock.method.side_effect = [sentinel.VALUE1, sentinel.VALUE2]
+        mock.method.return_value = sentinel.WRONG_VALUE
+
+        self.assertEqual(mock.method(), sentinel.VALUE1)
+        self.assertEqual(mock.method(), sentinel.VALUE2)
+        self.assertRaises(StopIteration, mock.method)
+
+
+    def test_customize_wrapped_object_with_return_value_and_side_effect2(self):
+        # side_effect can return DEFAULT to default to return_value
+        class Real(object):
+            def method(self):
+                raise NotImplementedError()
+
+        real = Real()
+        mock = Mock(wraps=real)
+        mock.method.side_effect = lambda: DEFAULT
+        mock.method.return_value = sentinel.VALUE
+
+        self.assertEqual(mock.method(), sentinel.VALUE)
+
+
+    def test_customize_wrapped_object_with_return_value_and_side_effect_default(self):
+        class Real(object):
+            def method(self):
+                raise NotImplementedError()
+
+        real = Real()
+        mock = Mock(wraps=real)
+        mock.method.side_effect = [sentinel.VALUE1, DEFAULT]
+        mock.method.return_value = sentinel.RETURN
+
+        self.assertEqual(mock.method(), sentinel.VALUE1)
+        self.assertEqual(mock.method(), sentinel.RETURN)
+        self.assertRaises(StopIteration, mock.method)
+
+
     def test_exceptional_side_effect(self):
         mock = Mock(side_effect=AttributeError)
         self.assertRaises(AttributeError, mock)