OPC UA with Python

The quickest way to get OPC UA is to talk to a server in code. This tutorial uses asyncua (the free, open-source opcua-asyncio library) to connect, browse, read, write and subscribe: the four things you'll do 90% of the time.

1. Install

pip install asyncua

2. Connect and read a value

import asyncio
from asyncua import Client

URL = "opc.tcp://localhost:4840/freeopcua/server/"

async def main():
    async with Client(url=URL) as client:
        # Read a node by its NodeId (namespace index + identifier)
        node = client.get_node("ns=2;i=2")
        value = await node.read_value()
        print("Value:", value)

asyncio.run(main())

3. Browse the address space

async with Client(url=URL) as client:
    objects = client.nodes.objects
    for child in await objects.get_children():
        name = await child.read_browse_name()
        print(name)

4. Write a value

from asyncua import ua

async with Client(url=URL) as client:
    node = client.get_node("ns=2;i=3")
    await node.write_value(ua.DataValue(ua.Variant(42.0, ua.VariantType.Double)))

5. Subscribe to data changes

Subscriptions push updates to you instead of polling: the right way to watch a value.

class Handler:
    def datachange_notification(self, node, val, data):
        print("Change:", node, val)

async with Client(url=URL) as client:
    handler = Handler()
    sub = await client.create_subscription(500, handler)  # 500 ms
    node = client.get_node("ns=2;i=2")
    await sub.subscribe_data_change(node)
    await asyncio.sleep(30)   # keep running to receive changes

Connecting securely

For Sign & Encrypt, load your certificate and key before connecting:

await client.set_security_string(
    "Basic256Sha256,SignAndEncrypt,client-cert.der,client-key.pem")

The server must trust your certificate (move it from rejected to trusted). Details: OPC UA security & certificates.

FAQ

Last updated 4 September 2026 · Educational resource. Library names are trademarks of their owners.

Next: OPC UA & PLCs (Siemens TIA Portal) →