docs: def default_classify_function(data):
      """
      A default classify_function which recieve `data` and return filename without
      characters just after the last underscore
  
      >>> # [<filename>] is mimicking `data`
      >>> default_classify_function(['./foo/foo_bar_hoge.piyo'])
      './foo/foo_bar.piyo'
      >>> default_classify_function(['./foo/foo_bar.piyo'])
      './foo/foo.piyo'
      >>> default_classify_function(['./foo/foo.piyo'])
      './foo/foo.piyo'
      >>> default_classify_function(['./foo/foo'])
      './foo/foo'
      """
      # data[0] indicate the filename of the data
      rootname, basename = os.path.split(data[0])
      filename, ext = os.path.splitext(basename)
      if '_' in filename:
          filename = filename.rsplit('_', 1)[0]
      filename = os.path.join(rootname, filename + ext)
      return filename