def opt_reactor(self, shortName):
        """
        Which reactor to use (see --help-reactors for a list of possibilities)
        """
        # Actually actually actually install the reactor right at this very
        # moment, before any other code (for example, a sub-command plugin)
        # runs and accidentally imports and installs the default reactor.
        #
        # This could probably be improved somehow.
        try:
            installReactor(shortName)
        except NoSuchReactor:
            msg = ("The specified reactor does not exist: '%s'.\n"
                   "See the list of available reactors with "
                   "--help-reactors" % (shortName,))
            raise usage.UsageError(msg)
        except Exception as e:
            msg = ("The specified reactor cannot be used, failed with error: "
                   "%s.\nSee the list of available reactors with "
                   "--help-reactors" % (e,))
            raise usage.UsageError(msg)
        else:
            self["reactor"] = shortName

        Which reactor to use (see --help-reactors for a list of possibilities)
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/application/app.py