def test_runnerDebuggerDefaultsToPdb(self):
        """
        Trial uses pdb if no debugger is specified by `--debugger`
        """
        self.parseOptions(['--debug', 'twisted.trial.test.sample'])
        pdbrcFile = FilePath("pdbrc")
        pdbrcFile.touch()

        self.runcall_called = False
        def runcall(pdb, suite, result):
            self.runcall_called = True
        self.patch(pdb.Pdb, "runcall", runcall)

        self.runSampleSuite(self.getRunner())

        self.assertTrue(self.runcall_called)

        Trial uses pdb if no debugger is specified by `--debugger`
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/trial/test/test_runner.py