code: def _AddAttribute(self, attribute):
      
      if attribute.identifier in self._attributes:
        raise KeyError((
            'Volume attribute object already set for volume attribute '
            'identifier: {0:s}.').format(attribute.identifier))
  
      self._attributes[attribute.identifier] = attribute
docs: Adds an attribute.
  
      Args:
        attribute (VolumeAttribute): a volume attribute.
  
      Raises:
        KeyError: if volume attribute is already set for the corresponding volume
            attribute identifier.