Subscribe to alarms and events in Python

The code

import asyncio
from asyncua import Client, ua

class Handler:
    def event_notification(self, event):
        print(f"[{event.Severity}] {event.SourceName}: {event.Message.Text}")

async def main():
    async with Client(url="opc.tcp://milo.digitalpetri.com:62541/milo") as client:
        handler = Handler()
        sub = await client.create_subscription(500, handler)

        server_node = client.get_node("i=2253")   # the Server object is the event notifier
        await sub.subscribe_events(server_node)

        await asyncio.sleep(10)
        await sub.delete()

asyncio.run(main())

What it prints

[500] MachineTool_1: Tool life below threshold
[800] MachineTool_1: Tool breakage detected
[100] Server: Session activated

Notes

• The Server object (i=2253) is the standard event notifier. Some servers expose additional notifiers on individual machines: browse for the EventNotifier attribute.
• Pass an explicit list of event types or fields to subscribe_events to narrow what you receive. The default selection is broad.
• Call ConditionRefresh after a reconnect, or your client will believe the plant is healthy while alarms are still active.
• For OEE work, select ConditionClassId. MachineTool's condition classes give you a standardised downtime taxonomy that survives a change of machine vendor.

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.