def _walk_directory(root_directory):
        """
        Generates the paths of all files that are ancestors
        of `root_directory`.
        """

        paths = [os.path.join(root, name)
                 for root, dirs, files in os.walk(root_directory)  # noqa
                 for name in files]
        paths.sort()
        return paths
Generates the paths of all files that are ancestors
        of `root_directory`.