def test_startService(self):
        """
        When L{TimerService.startService} is called, it marks itself
        as running, creates a L{task.LoopingCall} and starts it.
        """
        self.timer.startService()
        self.assertTrue(self.timer.running, "Service is started")
        self.assertIsInstance(self.timer._loop, task.LoopingCall)
        self.assertIdentical(self.clock, self.timer._loop.clock)
        self.assertTrue(self.timer._loop.running, "LoopingCall is started")

        When L{TimerService.startService} is called, it marks itself
        as running, creates a L{task.LoopingCall} and starts it.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/application/test/test_internet.py