def init_group(self, group, chunk_size,
                   compression=None, compression_opts=None):
        """Initializes a HDF5 group compliant with the stored data.

        This method creates the datasets 'items', 'labels', 'features'
        and 'index' and leaves them empty.

        :param h5py.Group group: The group to initializes.
        :param float chunk_size: The size of a chunk in the file (in MB).
        :param str compression: Optional compression, see
            :class:`h5features.writer` for details
        :param str compression: Optional compression options, see
            :class:`h5features.writer` for details

        """
        create_index(group, chunk_size)

        self._entries['items'].create_dataset(
            group, chunk_size, compression=compression,
            compression_opts=compression_opts)

        self._entries['features'].create_dataset(
            group, chunk_size, compression=compression,
            compression_opts=compression_opts)

        # chunking the labels depends on features chunks
        self._entries['labels'].create_dataset(
            group, self._entries['features'].nb_per_chunk,
            compression=compression,
            compression_opts=compression_opts)

        if self.has_properties():
            self._entries['properties'].create_dataset(
                group, compression=compression,
                compression_opts=compression_opts)
Initializes a HDF5 group compliant with the stored data.

        This method creates the datasets 'items', 'labels', 'features'
        and 'index' and leaves them empty.

        :param h5py.Group group: The group to initializes.
        :param float chunk_size: The size of a chunk in the file (in MB).
        :param str compression: Optional compression, see
            :class:`h5features.writer` for details
        :param str compression: Optional compression options, see
            :class:`h5features.writer` for details