code: def lookup(self, request_class: Request) -> Callable[[Request], BrightsideMessage]:
          
          key = request_class.__class__.__name__
          if key not in self._registry:
              raise ConfigurationException("There is no message mapper associated with this key; we require a mapper")
          else:
              return self._registry[key]
docs: Looks up the message mapper function associated with this class. Function should take in a Request derived class
           and return a BrightsideMessage derived class, for sending on the wire
          :param request_class:
          :return: