Variables¶

Demonstrates how to add scalar variables of different data types under the Objects folder, mark one of them read-only, and update their values from a server-side simulation loop.

Start any OPC UA client (e.g. C02_firststeps.py or C12_browser.py) against this server to read and (for the writable variables) write the values.

Server.addVariable() adds a scalar Variable to the address space. The Python type of the initial value decides the OPC UA data type: float → Double, int → Int32, str → String, bool → Boolean. Pass an explicit nodeid to keep ids stable across runs so client examples can reference them.

1. Scalar variables¶

Four common scalar types side by side: Double, Int32, String and Boolean. Each is added directly under the Objects folder with a fixed NodeId in namespace 1.

print("--- 1. Scalar Variables ---")

temperature = server.addVariable(
    "Temperature",
    server.objectsNode,
    22.5,
    nodeId="ns=1;i=1001",
)

pressure = server.addVariable(
    "Pressure",
    server.objectsNode,
    1013,  # int → OPC UA Int32
    nodeId="ns=1;i=1002",
)

machine_name = server.addVariable(
    "MachineName",
    server.objectsNode,
    "CNC-Mill-01",
    nodeId="ns=1;i=1003",
)

is_running = server.addVariable(
    "IsRunning",
    server.objectsNode,
    False,
    nodeId="ns=1;i=1004",
)

2. Read-only variable¶

writable=False clears the write bit in the access level so clients can read the value but any write request is rejected by the server with Bad_NotWritable. It can be useful for values that come from firmware/hardware and must not be modified.

print()
print("--- 2. Read-Only Variable ---")

firmware_version = server.addVariable(
    "FirmwareVersion",
    server.objectsNode,
    "v2.1.0",
    nodeId="ns=1;i=1005",
    writable=False,
)

3. Run and update loop¶

server.start() starts networking in the background. The simulation loop assigns new values to the Variable handles once per second; any subscribed client sees the changes immediately.

    print()
    print("--- 3. Run and Update Loop ---")

    server.start()
    print(f"Server running at {endpoint_url}")
    print("Press Ctrl+C to stop.")

    try:
        cycle = 0
        while True:
            cycle += 1
            temperature(22.5 + (cycle % 50) * 0.1)
            pressure(1013 + cycle % 20)
            is_running(cycle % 30 != 0)

            if cycle % 10 == 0:
                print(
                    f"Cycle {cycle}: Temp = {temperature():.1f}°C, "
                    f"Pressure = {pressure():.1f}hPa, Running = {is_running()}"
                )

            time.sleep(1.0)

    except KeyboardInterrupt:
        print("Shutting down...")
    finally:
        server.stop()
        print("Server stopped.")


if __name__ == "__main__":
    main()

Source Code¶

Download S04_variables.py to run it locally.