<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def notify_batch_pending(self, batch):
 """Adds a Batch id to the pending cache, with its transaction ids. Args: batch (str):
 The id of the pending batch """

        txn_ids = {t.header_signature for t in batch.transactions}
        with self._lock:
            self._pending.add(batch.header_signature)
            self._batch_info[batch.header_signature] = txn_ids
            self._update_observers(batch.header_signature,
                                   ClientBatchStatus.PENDING)