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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def batch_update(self, statements):
 """Perform a batch of DML statements via an ``ExecuteBatchDml`` request. :type statements: Sequence[Union[ str, Tuple[str, Dict[str, Any], Dict[str, Union[dict, .types.Type]]]]] :param statements: List of DML statements, with optional params / param types. If passed, 'params' is a dict mapping names to the values for parameter replacement. Keys must match the names used in the corresponding DML statement. If 'params' is passed, 'param_types' must also be passed, as a dict mapping names to the type of value passed in 'params'. :rtype: Tuple(status, Sequence[int]) :returns: Status code, plus counts of rows affected by each completed DML statement. Note that if the staus code is not ``OK``, the statement triggering the error will not have an entry in the list, nor will any statements following that one. """

        parsed = []
        for statement in statements:
            if isinstance(statement, str):
                parsed.append({"sql": statement})
            else:
                dml, params, param_types = statement
                params_pb = self._make_params_pb(params, param_types)
                parsed.append(
                    {"sql": dml, "params": params_pb, "param_types": param_types}
                )

        database = self._session._database
        metadata = _metadata_with_prefix(database.name)
        transaction = self._make_txn_selector()
        api = database.spanner_api

        response = api.execute_batch_dml(
            session=self._session.name,
            transaction=transaction,
            statements=parsed,
            seqno=self._execute_sql_count,
            metadata=metadata,
        )

        self._execute_sql_count += 1
        row_counts = [
            result_set.stats.row_count_exact for result_set in response.result_sets
        ]
        return response.status, row_counts