Perform a ``ParitionRead`` API request for rows in a table.

        :type table: str
        :param table: name of the table from which to fetch data

        :type columns: list of str
        :param columns: names of columns to be retrieved

        :type keyset: :class:`~google.cloud.spanner_v1.keyset.KeySet`
        :param keyset: keys / ranges identifying rows to be retrieved

        :type index: str
        :param index: (Optional) name of index to use, rather than the
                      table's primary key

        :type partition_size_bytes: int
        :param partition_size_bytes:
            (Optional) desired size for each partition generated.  The service
            uses this as a hint, the actual partition size may differ.

        :type max_partitions: int
        :param max_partitions:
            (Optional) desired maximum number of partitions generated. The
            service uses this as a hint, the actual number of partitions may
            differ.

        :rtype: iterable of bytes
        :returns: a sequence of partition tokens

        :raises ValueError:
            for single-use snapshots, or if a transaction ID is
            already associtated with the snapshot.

def partition_read(
        self,
        table,
        columns,
        keyset,
        index="",
        partition_size_bytes=None,
        max_partitions=None,
    ):
        """Perform a ``ParitionRead`` API request for rows in a table.

        :type table: str
        :param table: name of the table from which to fetch data

        :type columns: list of str
        :param columns: names of columns to be retrieved

        :type keyset: :class:`~google.cloud.spanner_v1.keyset.KeySet`
        :param keyset: keys / ranges identifying rows to be retrieved

        :type index: str
        :param index: (Optional) name of index to use, rather than the
                      table's primary key

        :type partition_size_bytes: int
        :param partition_size_bytes:
            (Optional) desired size for each partition generated.  The service
            uses this as a hint, the actual partition size may differ.

        :type max_partitions: int
        :param max_partitions:
            (Optional) desired maximum number of partitions generated. The
            service uses this as a hint, the actual number of partitions may
            differ.

        :rtype: iterable of bytes
        :returns: a sequence of partition tokens

        :raises ValueError:
            for single-use snapshots, or if a transaction ID is
            already associtated with the snapshot.
        """
        if not self._multi_use:
            raise ValueError("Cannot use single-use snapshot.")

        if self._transaction_id is None:
            raise ValueError("Transaction not started.")

        database = self._session._database
        api = database.spanner_api
        metadata = _metadata_with_prefix(database.name)
        transaction = self._make_txn_selector()
        partition_options = PartitionOptions(
            partition_size_bytes=partition_size_bytes, max_partitions=max_partitions
        )

        response = api.partition_read(
            session=self._session.name,
            table=table,
            columns=columns,
            key_set=keyset._to_pb(),
            transaction=transaction,
            index=index,
            partition_options=partition_options,
            metadata=metadata,
        )

        return [partition.partition_token for partition in response.partitions]