<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def get_uid(self, idx):
 """ 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 """

        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