def test_frozenset(self):
        """
        Check that a L{frozenset} can contain a circular reference and be
        serialized and unserialized without losing the reference.
        """
        a = SimpleJellyTest(None, None)
        s = frozenset([a])
        a.x = s
        res = jelly.unjelly(jelly.jelly(a))
        self.assertIsInstance(res.x, frozenset)
        self.assertEqual(list(res.x), [res])

        Check that a L{frozenset} can contain a circular reference and be
        serialized and unserialized without losing the reference.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/spread/test/test_jelly.py