code: def fetch_by_client_id(self, client_id):
          
          if client_id not in self.clients:
              raise ClientNotFoundError
  
          return self.clients[client_id]
docs: Retrieve a client by its identifier.
  
          :param client_id: Identifier of a client app.
          :return: An instance of :class:`oauth2.Client`.
          :raises: ClientNotFoundError