def METHOD_NAME(hdf5_filename, key=None):
    """
    Function to reconstruct streamlines from hdf5, mainly to facilitate
    decomposition into thousand of connections and decrease I/O usage.
    ----------
    hdf5_filename : str
        Filepath to the hdf5 file.
    key : str
        Key of the connection of interest (LABEL1_LABEL2).
    Returns
    -------
    streamlines : list of np.ndarray
        List of streamlines.
    """
    hdf5_file = hdf5_filename
    if key is not None:
        if key not in hdf5_file:
            return []
        group = hdf5_file[key]
        if 'data' not in group:
            return []
    else:
        group = hdf5_file
    data = np.array(group['data']).flatten()
    offsets = np.array(group['offsets'])
    lengths = np.array(group['lengths'])
    return reconstruct_streamlines(data, offsets, lengths)