def test_reactorSelectionMixinNonExistent(self):
        """
        Test that the usage mixin exits when trying to use a non existent
        reactor (the name not matching to any reactor), giving an error
        message.
        """
        class ReactorSelectionOptions(usage.Options, app.ReactorSelectionMixin):
            pass
        self.pluginResults = []

        options = ReactorSelectionOptions()
        options.messageOutput = NativeStringIO()
        e = self.assertRaises(usage.UsageError, options.parseOptions,
                              ['--reactor', 'fakereactortest', 'subcommand'])
        self.assertIn("fakereactortest", e.args[0])
        self.assertIn("help-reactors", e.args[0])

        Test that the usage mixin exits when trying to use a non existent
        reactor (the name not matching to any reactor), giving an error
        message.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/test/test_application.py