code: def start(self):
          
          assert not self.has_started(), "called start() on an active GeventLoop"
          self._stop_event = Event()
          # note that we don't use safe_greenlets.spawn because we take care of it in _loop by ourselves
          self._greenlet = gevent.spawn(self._loop)
docs: Starts the loop. Calling a running loop is an error.