code: def register_event(cls, event_name, event, method):
          
          log.info('@Registry.register_event `{}` with subscriber `{}`'
                   .format(event_name, method.__name__))
  
          if event_name not in cls._events:
              cls._events[event_name] = {}
  
          if event not in cls._events[event_name]:
              cls._events[event_name][event] = []
  
          cls._events[event_name][event].append(method)
docs: Register an event class on it's name with a method to process it.
          :param event_name: name of the event.
          :param event: class of the event.
          :param method: a method used to process this event.