def _gen_subname(videofile, language, ext):
        """ generate filename of subtitles
        :TODO: fix the conflict of subname
        """
        root, basename = os.path.split(videofile)
        name, _ = os.path.splitext(basename)
        subname = '{basename}.{language}.{ext}'.format(
            basename=name,
            language=language,
            ext=ext)
        return subname
generate filename of subtitles
        :TODO: fix the conflict of subname