docs: def __get_lookup(in_fn, selected_type=None):
    """Determine which lookup func to use based on inpt files and type option."""
    lookup_func = None
    if selected_type is not None:
      lookup_func = get_lookup_by_filetype(selected_type)
    else:
      extension = os.path.splitext(in_fn)[1]
      lookup_func = get_lookup_by_file_extension(extension)
    assert(lookup_func is not None)
    return lookup_func