code: def get_uid(self, idx):
          
          assert idx is not None
  
          if isinstance(idx, (int, float, str)):
              return self.uid[idx]
  
          ret = []
          for i in idx:
              tmp = self.uid.get(i, None)
              assert tmp is not None, (
                  'Model <{}> does not have element <{}>'.format(self._name, i))
              ret.append(self.uid[i])
  
          return ret
docs: Return the `uid` of the elements with the given `idx`
  
          :param list, matrix idx: external indices
          :type idx: list, matrix
          :return: a matrix of uid