code: def _determine_hpp_url(self, platform, action):
          
          base_uri = settings.BASE_HPP_URL.format(platform)
          service = action + '.shtml'
          result = '/'.join([base_uri, service])
          return result
docs: This returns the Adyen HPP endpoint based on the provided platform,
          and action.
  
          Args:
              platform (str): Adyen platform, ie 'live' or 'test'.
              action (str):   the HPP action to perform.
              possible actions: select, pay, skipDetails, directory