[3.13] gh-118899: Add tests for NotImplemented attribute access (GH…

…-118902) (#118968)

gh-118899: Add tests for `NotImplemented` attribute access (GH-118902)
(cherry picked from commit ec1398e)

Co-authored-by: Nikita Sobolev <mail@sobolevn.me>

Lib/test/test_builtin.py

@@ -2139,6 +2139,24 @@ def test_warning_notimplemented(self):
         with self.assertWarns(DeprecationWarning):
             self.assertFalse(not NotImplemented)
 
+    def test_singleton_attribute_access(self):
+        for singleton in (NotImplemented, Ellipsis):
+            with self.subTest(singleton):
+                self.assertIs(type(singleton), singleton.__class__)
+                self.assertIs(type(singleton).__class__, type)
+
+                # Missing instance attributes:
+                with self.assertRaises(AttributeError):
+                    singleton.prop = 1
+                with self.assertRaises(AttributeError):
+                    singleton.prop
+
+                # Missing class attributes:
+                with self.assertRaises(TypeError):
+                    type(singleton).prop = 1
+                with self.assertRaises(AttributeError):
+                    type(singleton).prop
+
 
 class TestBreakpoint(unittest.TestCase):
     def setUp(self):