def test_singleAsynchronousSuccess(self):
        """
        When given a callable that returns a successful Deferred, include the
        result of the Deferred in the results list, tagged with a SUCCESS flag.
        """
        d = util._runSequentially([lambda: defer.succeed(None)])
        self.assertDeferredResult(d, self.assertEqual, [(defer.SUCCESS, None)])

        When given a callable that returns a successful Deferred, include the
        result of the Deferred in the results list, tagged with a SUCCESS flag.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/trial/test/test_util.py