code: def begin(self):
          
          if self._status != self._INITIAL:
              raise ValueError("Batch already started previously.")
          self._status = self._IN_PROGRESS
docs: Begins a batch.
  
          This method is called automatically when entering a with
          statement, however it can be called explicitly if you don't want
          to use a context manager.
  
          Overridden by :class:`google.cloud.datastore.transaction.Transaction`.
  
          :raises: :class:`ValueError` if the batch has already begun.