报错信息如下；
ERROR:asyncua.common.subscription:DataChange subscription created but handler has no datachange_notification method

上述报错原因在于创建监控句柄SubscriptionHandler类时，节点数据变化的函数名称有问题，不是默认的datachange_notification，我写的是data_change_notification

完整的监控句柄SubscriptionHandler类代码如下：

class SubscriptionHandler:
    """
    The SubscriptionHandler is used to handle the data that is received for the subscription.
    """

    def datachange_notification(self, node: Node, val, data):
        """
        Callback for asyncua Subscription.
        This method will be called when the Client received a data change message from the Server.
        """
        print(node, val, data)

    def event_notification(self, event: Event):
        """
        called for every event notification from server
        """
        print(event)

    def status_change_notification(self, status):
        """
        called for every status change notification from server
        """
        print(status)

 # 建立数据改变监控事件
 handler = SubscriptionHandler()
 subscription = await client.create_subscription(500, handler)  # 500ms 发布间隔
 # 获取订阅节点数据
 node = await client.nodes.root.get_child('Objects/2:Message/3:start_message')
 await subscription.subscribe_data_change(node)

opcua的Client监控多个节点数据变化的使用方法参考文章：
opcua-asyncio：订阅多个地址

import sys
sys.path.insert(0, "..")
import time
import asyncio
from asyncua import Client, ua, Node

url = 'opc.tcp://192.168.200.204:8999'

class SubscriptionHandler:
    def datachange_notification(self, node: Node, val, data):
        print(str(val))
        
async def main():
    client = Client(url=url)
    async with client:
        handler = SubscriptionHandler()
        subscription = await client.create_subscription(500, handler)
        nodes = [
            client.get_node("ns=2;s=CncData/MachineStatus/Status"),
            client.get_node("ns=2;s=CncData/MachineStatus/Part/PieceToDo"),
        ]
        await subscription.subscribe_data_change(nodes)
        while True:
            await asyncio.sleep(1)

if __name__ == "__main__":
    asyncio.run(main())