code: def init_group(self, group, chunk_size,
                     compression=None, compression_opts=None):
          
          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)
docs: 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