code: def remove_observer(self, callback):
          
          if callback not in self._observers:
              raise ValueError('{} is not an observer of {}'
                               .format(callback, self))
          self._observers.remove(callback)
docs: Remove an observer from this event.
  
          Args:
              callback: A function or coroutine callback to remove from this
                  event.
  
          Raises:
              ValueError: If the callback is not an observer of this event.