def test_dontPutCircularReferencesInDictionaryKeys(self):
        """
        If a dictionary key contains a circular reference (which is probably a
        bad practice anyway) it will be resolved by a
        L{crefutil._DictKeyAndValue}, not by placing a L{crefutil.NotKnown}
        into a dictionary key.
        """
        self.assertRaises(AssertionError,
                          dict().__setitem__, crefutil.NotKnown(), "value")

        If a dictionary key contains a circular reference (which is probably a
        bad practice anyway) it will be resolved by a
        L{crefutil._DictKeyAndValue}, not by placing a L{crefutil.NotKnown}
        into a dictionary key.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/test/test_persisted.py