Declare, get and set ROS 2 (Lyrical Luth) node parameters from Python, the CLI, and a launch file

Documented steps

1. Source ROS 2, then create the package: `cd ~/ros2_ws/src && ros2 pkg create --build-type ament_python --license Apache-2.0 python_parameters --dependencies rclpy`.
2. Fill in description, maintainer and license in package.xml (the --dependencies flag already added the rclpy depend line).
3. In python_parameters/python_parameters_node.py define class MinimalParam(Node): super().__init__('minimal_param_node'), then self.declare_parameter('my_parameter', 'world'). The parameter type is inferred from the default value.
4. Add self.timer = self.create_timer(1, self.timer_callback). In timer_callback read the value with self.get_parameter('my_parameter').get_parameter_value().string_value and log it.
5. To set a parameter from code build rclpy.parameter.Parameter('my_parameter', rclpy.Parameter.Type.STRING, 'world') and call self.set_parameters([that_param]).
6. Optional descriptor: `from rcl_interfaces.msg import ParameterDescriptor`; desc = ParameterDescriptor(description='...'); self.declare_parameter('my_parameter', 'world', desc). Add <depend>rcl_interfaces</depend> to package.xml.
7. Add the console_scripts entry point in setup.py: 'minimal_param_node = python_parameters.python_parameters_node:main'.
8. From the workspace root: `rosdep install -i --from-path src --rosdistro lyrical -y`, then `colcon build --packages-select python_parameters` and `source install/setup.bash`, then `ros2 run python_parameters minimal_param_node`.
9. Change it from the console: in another sourced terminal `ros2 param list` to see my_parameter, then `ros2 param set /minimal_param_node my_parameter earth` - expect 'Set parameter successful'.
10. Set it from a launch file: create launch/python_parameters_launch.py returning a LaunchDescription with Node(package='python_parameters', executable='minimal_param_node', name='custom_minimal_param_node', output='screen', emulate_tty=True, parameters=[{'my_parameter': 'earth'}]).
11. In setup.py add `import os`, `from glob import glob` and append (os.path.join('share', package_name, 'launch'), glob('launch/*')) to data_files so the launch file is installed. Rebuild, re-source, then `ros2 launch python_parameters python_parameters_launch.py`.
12. For namespaced parameters use a dot ('.') as the separator between namespace and name in declare/get/set calls.
13. Official doc: https://docs.ros.org/en/lyrical/Tutorials/Beginner-Client-Libraries/Using-Parameters-In-A-Class-Python.html