code: def off(self, event=None, listener=None):
          
          if event is None:
              events = self._listeners.keys()
          else:
              events = [event]
          for event in events:
              if listener is None:
                  self._listeners[event] = []
              else:
                  self._listeners[event] = [
                      l for l in self._listeners[event]
                      if l.callback != listener]
docs: Remove a ``listener`` that was to be called on ``event``.
  
          If ``listener`` is :class:`None`, all listeners for the given ``event``
          will be removed.
  
          If ``event`` is :class:`None`, all listeners for all events on this
          object will be removed.