code: def download_items(self, items, file_path, file_format='zip'):
          
          download_url = '/catalog/download_items'
          download_url += '?' + urlencode((('format', file_format),))
          item_data = {'items': list(items)}
  
          data = self.api_request(download_url, method='POST', data=json.dumps(item_data), raw=True)
  
          with open(file_path, 'w') as f:
              f.write(data)
  
          return file_path
docs: Retrieve a file from the server containing the metadata
          and documents for the speficied items
  
          :type items: List or ItemGroup
          :param items: List of the the URLs of the items to download,
              or an ItemGroup object
          :type file_path: String
          :param file_path: the path to which to save the file
          :type file_format: String
          :param file_format: the file format to request from the server: specify
              either 'zip' or 'warc'
  
          :rtype: String
          :returns: the file path
  
          :raises: APIError if the API request is not successful