def test_allUnsetWithInvariant(self):
        """
        In an empty L{base.PositionError} with invariant testing, all
        dilutions of positions are L{None}.
        """
        positionError = base.PositionError(testInvariant=True)
        self.assertIsNone(positionError.pdop)
        self.assertIsNone(positionError.hdop)
        self.assertIsNone(positionError.vdop)

        In an empty L{base.PositionError} with invariant testing, all
        dilutions of positions are L{None}.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/positioning/test/test_base.py