code: def _DownloadAuthUrl(self, url, dest_dir):
      
      dest_file = tempfile.NamedTemporaryFile(dir=dest_dir, delete=False)
      dest_file.close()
      dest = dest_file.name
  
      self.logger.info(
          'Downloading url from %s to %s using authentication token.', url, dest)
  
      if not self.token:
        response = self.watcher.GetMetadata(
            self.token_metadata_key, recursive=False, retry=False)
  
        if not response:
          self.logger.info(
              'Authentication token not found. Attempting unauthenticated '
              'download.')
          return self._DownloadUrl(url, dest_dir)
  
        self.token = '%s %s' % (
            response.get('token_type', ''), response.get('access_token', ''))
  
      try:
        request = urlrequest.Request(url)
        request.add_unredirected_header('Metadata-Flavor', 'Google')
        request.add_unredirected_header('Authorization', self.token)
        content = urlrequest.urlopen(request).read().decode('utf-8')
      except (httpclient.HTTPException, socket.error, urlerror.URLError) as e:
        self.logger.warning('Could not download %s. %s.', url, str(e))
        return None
  
      with open(dest, 'wb') as f:
        f.write(content)
  
      return dest
docs: Download a Google Storage URL using an authentication token.
  
      If the token cannot be fetched, fallback to unauthenticated download.
  
      Args:
        url: string, the URL to download.
        dest_dir: string, the path to a directory for storing metadata scripts.
  
      Returns:
        string, the path to the file storing the metadata script.