code: def subscription_callback(self, subscription):
          
          subscription_record = self.__subscriptions.get(subscription)
          if not subscription_record:
              raise workflows.Error("Attempting to callback on unknown subscription")
          callback = subscription_record["callback"]
          if self.__callback_interceptor:
              return self.__callback_interceptor(callback)
          return callback
docs: Retrieve the callback function for a subscription. Raise a
          workflows.Error if the subscription does not exist.
          All transport callbacks can be intercepted by setting an
          interceptor function with subscription_callback_intercept().
          :param subscription: Subscription ID to look up
          :return: Callback function