Remember a key to be deleted during :meth:`commit`.

        :type key: :class:`google.cloud.datastore.key.Key`
        :param key: the key to be deleted.

        :raises: :class:`~exceptions.ValueError` if the batch is not in
                 progress, if key is not complete, or if the key's
                 ``project`` does not match ours.

def delete(self, key):
        """Remember a key to be deleted during :meth:`commit`.

        :type key: :class:`google.cloud.datastore.key.Key`
        :param key: the key to be deleted.

        :raises: :class:`~exceptions.ValueError` if the batch is not in
                 progress, if key is not complete, or if the key's
                 ``project`` does not match ours.
        """
        if self._status != self._IN_PROGRESS:
            raise ValueError("Batch must be in progress to delete()")

        if key.is_partial:
            raise ValueError("Key must be complete")

        if self.project != key.project:
            raise ValueError("Key must be from same project as batch")

        key_pb = key.to_protobuf()
        self._add_delete_key_pb().CopyFrom(key_pb)