code: def add_listener(self, callback, mask=EVENT_ALL):
          
          self._scheduler.add_listener(callback, mask)
docs: Add a listener for scheduler events.
  
          When a matching event  occurs, ``callback`` is executed with the event object as its
          sole argument. If the ``mask`` parameter is not provided, the callback will receive events
          of all types.
  
          For further info: https://apscheduler.readthedocs.io/en/latest/userguide.html#scheduler-events
  
          :param callback: any callable that takes one argument
          :param int mask: bitmask that indicates which events should be listened to