code: def fill_datetime(self):
      
      if not self.filled:
        raise SlotNotFilledError('Slot with name "%s", key "%s" not yet filled.'
                                 % (self.name, self.key))
      return self._fill_datetime
docs: Returns when the slot was filled.
  
      Returns:
        A datetime.datetime.
  
      Raises:
        SlotNotFilledError if the value hasn't been filled yet.