def get_library_isotopes(acelib_path):
    """
    Returns the isotopes in the cross section library

    Parameters
    ----------
    acelib_path : str
        Path to the cross section library
        (i.e. '/home/luke/xsdata/endfb7/sss_endfb7u.xsdata')

    Returns
    -------
    iso_array: array
        array of isotopes in cross section library:
    """
    lib_isos_list = []
    with open(acelib_path, 'r') as f:
        lines = f.readlines()
        for line in lines:
            iso = line.split()[0]
            lib_isos_list.append(iso)
    return lib_isos_list