docs: def get_downloader(session, class_name, args):
      """
      Decides which downloader to use.
      """
  
      external = {
          'wget': WgetDownloader,
          'curl': CurlDownloader,
          'aria2': Aria2Downloader,
          'axel': AxelDownloader,
      }
  
      for bin, class_ in iteritems(external):
          if getattr(args, bin):
              return class_(session, bin=getattr(args, bin),
                            downloader_arguments=args.downloader_arguments)
  
      return NativeDownloader(session)