def set_config_filename(filename):
    """
    Change the "default" filename for the config file from the
    current value of CONFIG_FILE_NAME to the given filename string.

    The filename is assumed to be a simple filename (no directory
    components).  Partial directory paths might work, or might not.
    No sanity checking is done on the filename.

    This function has no useful effect if CONFIG_FILE is non-None;
    it is used only to compute the CONFIG_FILE, and if the CONFIG_FILE
    has already been set, then CONFIG_FILE will not change as a
    result of changing the CONFIG_FILE_NAME.
    """
    global _CONFIG_FILE_NAME
    _CONFIG_FILE_NAME = filename
    return filename
