docs: def report_unknown(bytes_so_far, total_size, speed, eta):
      '''
      This callback for the download function is used
      when the total size is unknown
      '''
      sys.stdout.write(
          "Downloading: {0} / Unknown - {1}/s      ".format(approximate_size(bytes_so_far),
                                                            approximate_size(speed)))
  
      sys.stdout.write("\r")
      sys.stdout.flush()