Class U

Lib/test/test_functools.py:2593–2595

        c.Container.register(T)
        self.assertEqual(h(t), "sized")   # because it's explicitly in the MRO
        class U:
            def __len__(self):
                return 0
        u = U()
        self.assertEqual(h(u), "sized")   # implicit Sized subclass inferred
                                          # from the existence of __len__()