code: def _complete_url(self, url_part, registration_prefix):
          
          parts = {
              'b': registration_prefix,
              'a': self.prefix,
              'e': url_part
          }
          return ''.join(parts[key] for key in self.url_part_order if parts[key])
docs: This method is used to defer the construction of the final url in
          the case that the Api is created with a Blueprint.
  
          :param url_part: The part of the url the endpoint is registered with
          :param registration_prefix: The part of the url contributed by the
              blueprint.  Generally speaking, BlueprintSetupState.url_prefix