Decode custom structures in Python

The code

import asyncio
from asyncua import Client

async def main():
    async with Client(url="opc.tcp://opcua.umati.app:4840") as client:
        # pull the server's DataTypeDefinitions and build Python classes
        await client.load_data_type_definitions()

        node = client.get_node("ns=4;i=7001")   # a structured value
        value = await node.read_value()
        print(type(value).__name__)
        for field, val in vars(value).items():
            print(f"  {field} = {val}")

asyncio.run(main())

What it prints

ProductionJobType
  Identifier = JOB-2026-0912
  State = 2
  PartsPlanned = 500
  PartsProduced = 431

Notes

• `load_data_type_definitions()` reads the server's DataTypeDefinition attributes and generates classes at runtime. Call it once after connecting, before reading any structure.
• If it returns nothing useful, the server may only publish a legacy binary type dictionary. That mismatch is a real interoperability gap, not a client bug.
• Unions and optional fields are where decoding usually breaks. Test those specifically rather than assuming the happy path generalises.
• For a one-off inspection, UaExpert shows decoded structures without any code.

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.