input: Write a function to provide a command-line interface for downloading subtitles for movies and TV shows. The function should accept one or more release paths as arguments and optionally specify the language and subtitle site to use. The available commands are "download", "exist", "test", "list", "help", and "version". If the "download" command is used, the function should attempt to download subtitles for the specified release paths using the specified or default engine and language. If the "exist" command is used, the function should check whether subtitles exist for the specified release paths. If the "test" command is used, the function should test the specified or all available engines. If the "list" command is used, the function should list all available subtitle sites. If the "help" command is used, the function should display a help message. If the "version" command is used, the function should display the version of the script.
  
  def subtitles_client(command, **kwargs):
      if command == 'download':
          download_subtitles(kwargs['<releases_path>'], kwargs['--engine'], kwargs['--lang'])
      elif command == 'exist':
          is_subtitles_exist(kwargs['<releases_path>'], kwargs['--engine'], kwargs['--lang'])
      elif command == 'test':
          test_engines(kwargs['<engines>'])
      elif command == 'list':
          for sub_site in SUBTITLE_SITE_LIST.keys():
              sub_dict = SUBTITLE_SITE_LIST.get(sub_site)
              print(sub_dict.get('class_name'))
      elif command == 'help':
          print(__doc__)
      elif command == 'version':
          print('Subtitles Client %s' % __version__)
  
  assert subtitles_client('version') == None
  assert subtitles_client('download', **{'<releases_path>': ['example_release'], '--engine': ['default_engine'], '--lang': 'en'}) == None
  assert subtitles_client('exist', **{'<releases_path>': ['example_release'], '--engine': ['default_engine'], '--lang': 'en'}) == None
  assert subtitles_client('test', **{'<engines>': []}) == None
  assert subtitles_client('list') == None
  assert subtitles_client('help') == None
output: 
  def subtitles_client(command, **kwargs):
      if command == 'download':
          download_subtitles(kwargs['<releases_path>'], kwargs['--engine'], kwargs['--lang'])
      elif command == 'exist':
          is_subtitles_exist(kwargs['<releases_path>'], kwargs['--engine'], kwargs['--lang'])
      elif command == 'test':
          test_engines(kwargs['<engines>'])
      elif command == 'list':
          for sub_site in SUBTITLE_SITE_LIST.keys():
              sub_dict = SUBTITLE_SITE_LIST.get(sub_site)
              print(sub_dict.get('class_name'))
      elif command == 'help':
          print(__doc__)
      elif command == 'version':
          print('Subtitles Client %s' % __version__)
  