Server Methods Example

Source example: examples/highlevel/server_methods.py

This example concentrates on server-side methods. It shows how to group several callbacks under one object and how different OPC UA method signatures are declared in Python.

Explanation

Organizing methods below an object

calculator = server.add_object(
    "Calculator", server.objects_node, nodeid="ns=1;i=100"
)

A method without arguments

Reset is the smallest possible method example. The callback takes no parameters and returns an empty list.

def reset():
    return []

server.add_method("Reset", calculator, reset, nodeid="ns=1;i=2001")

Methods with typed inputs and outputs

Add pairs the Python callback with explicit input and output argument metadata.

def add(a, b):
    return [a + b]

server.add_method(
    "Add",
    calculator,
    add,
    input_args=[
        make_argument("A", "i=11", description="First operand"),
        make_argument("B", "i=11", description="Second operand"),
    ],
    output_args=[
        make_argument("Sum", "i=11", description="A + B"),
    ],
    nodeid="ns=1;i=2002",
)

Returning multiple values

Divide returns two outputs. The callback returns a list whose order matches the declared output arguments.

def divide(a, b):
    if b == 0:
        return [0, 0]
    quotient = int(a // b)
    remainder = a - quotient * b
    return [quotient, remainder]

Full source

#!/usr/bin/env python3

import time
from o6 import Server
from o6.server import make_argument


def main():
    server = Server(port=4840)

    calculator = server.add_object(
        "Calculator", server.objects_node, nodeid="ns=1;i=100"
    )

    def reset():
        print("  [Method] Reset called")
        return []

    server.add_method(
        "Reset",
        calculator,
        reset,
        nodeid="ns=1;i=2001",
    )

    def add(a, b):
        result = a + b
        print(f"  [Method] Add({a}, {b}) = {result}")
        return [result]

    server.add_method(
        "Add",
        calculator,
        add,
        input_args=[
            make_argument("A", "i=11", description="First operand"),
            make_argument("B", "i=11", description="Second operand"),
        ],
        output_args=[
            make_argument("Sum", "i=11", description="A + B"),
        ],
        nodeid="ns=1;i=2002",
    )

    def greet(name):
        message = f"Hello, {name}!"
        print(f"  [Method] Greet('{name}') -> '{message}'")
        return [message]

    server.add_method(
        "Greet",
        calculator,
        greet,
        input_args=[
            make_argument("Name", "i=12", description="Name to greet"),
        ],
        output_args=[
            make_argument("Greeting", "i=12", description="Greeting message"),
        ],
        nodeid="ns=1;i=2003",
    )

    def divide(a, b):
        if b == 0:
            print("  [Method] Divide: division by zero!")
            return [0, 0]
        quotient = int(a // b)
        remainder = a - quotient * b
        print(f"  [Method] Divide({a}, {b}) -> q={quotient}, r={remainder}")
        return [quotient, remainder]

    server.add_method(
        "Divide",
        calculator,
        divide,
        input_args=[
            make_argument("Dividend", "i=11", description="Dividend"),
            make_argument("Divisor", "i=11", description="Divisor"),
        ],
        output_args=[
            make_argument("Quotient", "i=11", description="Integer quotient"),
            make_argument("Remainder", "i=11", description="Remainder"),
        ],
        nodeid="ns=1;i=2004",
    )

    server.start()
    print("Server running at opc.tcp://localhost:4840")
    print("Methods available under Calculator (ns=1;i=100):")
    print("  - Reset      (ns=1;i=2001)  no args")
    print("  - Add        (ns=1;i=2002)  Double + Double -> Double")
    print("  - Greet      (ns=1;i=2003)  String -> String")
    print("  - Divide     (ns=1;i=2004)  Double, Double -> Double, Double")
    print("\nPress Ctrl+C to stop.\n")

    try:
        while True:
            time.sleep(1.0)
    except KeyboardInterrupt:
        print("\nShutting down…")
    finally:
        server.stop()
        print("Server stopped.")


if __name__ == "__main__":
    main()