def guarantee_trailing_slash(directory_name: str) -> str:
    """Adds a trailling slash when missing

    Params:
        :directory_name: str, required
            A directory name to add trailling slash if missing

    Returns:
        A post processed directory name with trailling slash
    """
    if not directory_name.endswith('/'):
        return directory_name + '/'
    return directory_name