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