def test_runnerDebugger(self):
        """
        Trial uses specified debugger if the debugger is available.
        """
        self.parseOptions([
            '--reporter', 'capturing',
            '--debugger',
            'twisted.trial.test.test_runner.RunnerTests.cdebugger',
            '--debug',
            'twisted.trial.test.sample',
        ])
        my_runner = self.getRunner()
        result = self.runSampleSuite(my_runner)
        self.assertEqual(self.standardReport, result._calls)
        self.assertEqual(['runcall'], my_runner.debugger._calls)

        Trial uses specified debugger if the debugger is available.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/trial/test/test_runner.py