Returns the lastfm360k dataset, downloading locally if necessary.
    Returns a tuple of (artistids, userids, plays) where plays is a CSR matrix

def get_lastfm():
    """ Returns the lastfm360k dataset, downloading locally if necessary.
    Returns a tuple of (artistids, userids, plays) where plays is a CSR matrix """

    filename = os.path.join(_download.LOCAL_CACHE_DIR, "lastfm_360k.hdf5")
    if not os.path.isfile(filename):
        log.info("Downloading dataset to '%s'", filename)
        _download.download_file(URL, filename)
    else:
        log.info("Using cached dataset at '%s'", filename)

    with h5py.File(filename, 'r') as f:
        m = f.get('artist_user_plays')
        plays = csr_matrix((m.get('data'), m.get('indices'), m.get('indptr')))
        return np.array(f['artist']), np.array(f['user']), plays