def opt_help_reactors(self):
        """
        Display a list of possibly available reactor names.
        """
        rcts = sorted(self._getReactorTypes(), key=attrgetter('shortName'))
        notWorkingReactors = ""
        for r in rcts:
            try:
                namedModule(r.moduleName)
                self.messageOutput.write('    %-4s\t%s\n' %
                                         (r.shortName, r.description))
            except ImportError as e:
                notWorkingReactors += ('    !%-4s\t%s (%s)\n' %
                                       (r.shortName, r.description, e.args[0]))

        if notWorkingReactors:
            self.messageOutput.write('\n')
            self.messageOutput.write('    reactors not available '
                                     'on this platform:\n\n')
            self.messageOutput.write(notWorkingReactors)
        raise SystemExit(0)

        Display a list of possibly available reactor names.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/application/app.py