Write a value to an OPC UA server in Python

The code

import asyncio
from asyncua import Client, ua

# umati is read-only. Start a local server first:
#   uaserver --populate
URL = "opc.tcp://localhost:4840/freeopcua/server/"

async def main():
    async with Client(url=URL) as client:
        node = client.get_node("ns=2;i=2")
        dtype = await node.read_data_type_as_variant_type()
        print("node expects:", dtype)
        await node.write_value(ua.DataValue(ua.Variant(42, dtype)))
        print("read back   :", await node.read_value())

asyncio.run(main())

What it prints

node expects: VariantType.Int64
read back   : 42

Notes

• `read_data_type_as_variant_type()` removes the guesswork. Int16 is not Int32, and a scalar is not a one-element array.
• Check AccessLevel before writing. Some servers accept a write, return Good, and discard it: the umati sample server does this deliberately, so always read back in a test.
• For an array node, write the whole array or use an IndexRange. Writing a scalar to an array node is a type mismatch, not a partial write.
• CLI equivalent: `uawrite -u <url> -n "ns=2;i=2" 42`.

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.