Subscribe to data changes in Python

The code

import asyncio
from asyncua import Client, ua

class Handler:
    def datachange_notification(self, node, val, data):
        dv = data.monitored_item.Value
        print(f"{node} = {val}  status={dv.StatusCode}  src={dv.SourceTimestamp}")

    def status_change_notification(self, status):
        print("subscription status:", status)

async def main():
    async with Client(url="opc.tcp://opcua.umati.app:4840") as client:
        handler = Handler()
        # publishing interval = what the consumer needs, not the fastest possible
        sub = await client.create_subscription(1000, handler)

        node = client.get_node("i=2258")
        mfilter = ua.DataChangeFilter(
            Trigger=ua.DataChangeTrigger.StatusValue,
            DeadbandType=ua.DeadbandType.Absolute.value,
            DeadbandValue=0.5,
        )
        await sub.subscribe_data_change(node, queuesize=10, monitorfilter=mfilter)

        await asyncio.sleep(5)
        await sub.delete()

asyncio.run(main())

What it prints

i=2258 = 2026-09-08 03:45:00.102000  status=StatusCode(Good)  src=2026-09-08 03:45:00.102000
i=2258 = 2026-09-08 03:45:01.104000  status=StatusCode(Good)  src=2026-09-08 03:45:01.104000
i=2258 = 2026-09-08 03:45:02.101000  status=StatusCode(Good)  src=2026-09-08 03:45:02.101000

Notes

• The 1000 in create_subscription is the publishing interval in milliseconds: how often the server delivers. The sampling interval, set per item, is how often it looks. Confusing the two is the most common subscription mistake in OPC UA.
• Percent deadband needs EURange on the node. Without it the deadband is silently ignored, so use Absolute unless you have checked.
• Size queuesize from the publishing-to-sampling ratio. If it overflows, the overflow bit is set and the status still reads Good: the loss is silent.
• Keep the handler non-blocking. It runs on the client's loop; heavy work there stalls the subscription.
• Add a heartbeat if you need to distinguish 'value unchanged' from 'connection dead'.

What breaks first

Last verified 7 September 2026 against OPC UA 1.05.07. OPC UA Connect is an educational resource and is not affiliated with the OPC Foundation.