Fixed #28358 -- Prevented LazyObject from mimicking nonexistent attributes.

Thanks Sergey Fedoseev for the initial patch.

tests/utils_tests/test_lazyobject.py

@@ -32,6 +32,28 @@ class LazyObjectTestCase(TestCase):
 
         return AdHocLazyObject()
 
+    def test_getattribute(self):
+        """
+        Proxy methods don't exist on wrapped objects unless they're set.
+        """
+        attrs = [
+            "__getitem__",
+            "__setitem__",
+            "__delitem__",
+            "__iter__",
+            "__len__",
+            "__contains__",
+        ]
+        foo = Foo()
+        obj = self.lazy_wrap(foo)
+        for attr in attrs:
+            with self.subTest(attr):
+                self.assertFalse(hasattr(obj, attr))
+                setattr(foo, attr, attr)
+                obj_with_attr = self.lazy_wrap(foo)
+                self.assertTrue(hasattr(obj_with_attr, attr))
+                self.assertEqual(getattr(obj_with_attr, attr), attr)
+
     def test_getattr(self):
         obj = self.lazy_wrap(Foo())
         self.assertEqual(obj.foo, "bar")