code: def _add_delete_key_pb(self):
          
          new_mutation = _datastore_pb2.Mutation()
          self._mutations.append(new_mutation)
          return new_mutation.delete
docs: Adds a new mutation for a key to be deleted.
  
          :rtype: :class:`.entity_pb2.Key`
          :returns: The newly created key protobuf that will be
                    deleted when sent with a commit.