code: def filler(self):
      
      if not self.filled:
        raise SlotNotFilledError('Slot with name "%s", key "%s" not yet filled.'
                                 % (self.name, self.key))
      return self._filler_pipeline_key.name()
docs: Returns the pipeline ID that filled this slot's value.
  
      Returns:
        A string that is the pipeline ID.
  
      Raises:
        SlotNotFilledError if the value hasn't been filled yet.