code: def subscribe(self, handler):
      if self.__emitting:
          self.__deferred.append((self.__subscribeImpl, handler))
      elif handler not in self.__handlers:
          self.__subscribeImpl(handler)
  
imports: import abc
  import six
  from pyalgotrade import dispatchprio
label_desc: Use image node_id 5 for calling the Event obj's underlying member method code with example usage: obj.subscribe(handler) without return types