code: def is_message_handler(type_, from_, cb):
      
  
      try:
          handlers = aioxmpp.service.get_magic_attr(cb)
      except AttributeError:
          return False
  
      return aioxmpp.service.HandlerSpec(
          (_apply_message_handler, (type_, from_)),
          require_deps=(
              SimpleMessageDispatcher,
          )
      ) in handlers
docs: Return true if `cb` has been decorated with :func:`message_handler` for the
      given `type_` and `from_`.