'Which reactor to use (see --help-reactors for a list of possibilities)'

def opt_reactor(self, shortName):

    try:
        installReactor(shortName)
    except NoSuchReactor:
        msg = ("The    specified    reactor    does    not    exist:    '%s'.\nSee    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
