def test_segmentsFromNegative(self):
        """
        Verify that segmentsFrom notices when the ancestor isn't an ancestor.
        """
        self.assertRaises(
            ValueError,
            self.path.child(b"a").child(b"b").child(b"c").segmentsFrom,
                self.path.child(b"d").child(b"c").child(b"e"))

        Verify that segmentsFrom notices when the ancestor isn't an ancestor.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/test/test_paths.py