def test_addObservers(self):
        """
        Any log system observers L{TrialRunner.run} adds are removed by the
        time it returns.
        """
        originalCount = len(self._getObservers())
        self.runner.run(self.test)
        newCount = len(self._getObservers())
        self.assertEqual(newCount, originalCount)

        Any log system observers L{TrialRunner.run} adds are removed by the
        time it returns.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/trial/test/test_runner.py