code: def query_string_attribute(self, target, display_mask, attr):
      
      reply = NVCtrlQueryStringAttributeReplyRequest(display=self.display,
                                                     opcode=self.display.get_extension_major(extname),
                                                     target_id=target.id(),
                                                     target_type=target.type(),
                                                     display_mask=display_mask,
                                                     attr=attr)
      if not reply._data.get('flags'):
          return None
      return str(reply._data.get('string')).strip('\0')
docs: Return the value of a string attribute