Class MMB

Lib/test/test_typing.py:7775–7785

        self.assertIsInstance(MMC(), typing.Mapping)

        class MMB(typing.MutableMapping[KT, VT]):
            def __getitem__(self, k):
                return None
            def __setitem__(self, k, v):
                pass
            def __delitem__(self, k):
                pass
            def __iter__(self):
                return iter(())
            def __len__(self):
                return 0

        self.assertEqual(len(MMB()), 0)
        self.assertEqual(len(MMB[str, str]()), 0)