code: def register(self, request_class: Request, mapper_func: Callable[[Request], BrightsideMessage]) -> None:
          
  
          key = request_class.__name__
          if key not in self._registry:
              self._registry[key] = mapper_func
          else:
              raise ConfigurationException("There is already a message mapper defined for this key; there can be only one")
docs: Adds a message mapper to a factory, using the requests key
          :param mapper_func: A callback that creates a BrightsideMessage from a Request
          :param request_class: A request type