code: def url_for(self, operation, _external=True, **kwargs):
          
          return url_for(self.endpoint_for(operation), _external=_external, **kwargs)
docs: Construct a URL for an operation against a resource.
  
          :param kwargs: additional arguments for URL path expansion,
              which are passed to flask.url_for.
              In particular, _external=True produces absolute url.