def test_onlySupportedReactors(self):
        """
        --help-reactors with only supported reactors
        """
        def getReactorTypes():
            yield twisted_reactors.default

        options = app.ReactorSelectionMixin()
        options._getReactorTypes = getReactorTypes
        options.messageOutput = NativeStringIO()
        self.assertRaises(SystemExit, options.opt_help_reactors)
        message = options.messageOutput.getvalue()
        self.assertNotIn("reactors not available", message)

        --help-reactors with only supported reactors
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/test/test_application.py