code: def listeners_iter(self):
          
          topics = set(six.iterkeys(self._topics))
          while topics:
              event_type = topics.pop()
              try:
                  yield event_type, self._topics[event_type]
              except KeyError:
                  pass
docs: Return an iterator over the mapping of event => listeners bound.
  
          The listener list(s) returned should **not** be mutated.
  
          NOTE(harlowja): Each listener in the yielded (event, listeners)
          tuple is an instance of the :py:class:`~.Listener`  type, which
          itself wraps a provided callback (and its details filter
          callback, if any).