def METHOD_NAME(self, params):
    # Requested params are ignored, result = params set during create_subscription.
    self.logger.info("modify subscription")
    result = ua.ModifySubscriptionResult()
    try:
        sub = self.subscriptions[params.SubscriptionId]
        result.RevisedPublishingInterval = sub.data.RevisedPublishingInterval
        result.RevisedLifetimeCount = sub.data.RevisedLifetimeCount
        result.RevisedMaxKeepAliveCount = sub.data.RevisedMaxKeepAliveCount
        return result
    except KeyError:
        raise utils.ServiceError(ua.StatusCodes.BadSubscriptionIdInvalid)