Connect to an OPC UA server in Python

The code

import asyncio
from asyncua import Client

URL = "opc.tcp://opcua.umati.app:4840"

async def main():
    async with Client(url=URL) as client:
        print("connected to", URL)
        print("namespaces:")
        for i, uri in enumerate(await client.get_namespace_array()):
            print(f"  ns={i}  {uri}")

asyncio.run(main())

What it prints

connected to opc.tcp://opcua.umati.app:4840
namespaces:
  ns=0  http://opcfoundation.org/UA/
  ns=1  urn:umati:sample-server
  ns=2  http://opcfoundation.org/UA/DI/
  ns=3  http://opcfoundation.org/UA/Machinery/
  ns=4  http://opcfoundation.org/UA/MachineTool/

Notes

• Print the namespace array on every first connection. It tells you which companion specifications the server implements before you browse anything, and you need the URI-to-index mapping anyway.
• Connect with security None first even on a server you intend to secure. If None works and SignAndEncrypt does not, the problem is certificates, and you have just halved your search space.
• For scripts and notebooks, `from asyncua.sync import Client` gives the same API without asyncio.

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.