[ROS2] using dynamic parameters in Python

edit

I want to use dynamic parameters in my ROS2 code. I defined a callback function, if a parameter is changed with following line:

self.add_on_set_parameters_callback(self.cb_params)

I wrote a little example to test it:

import rclpy
from rclpy.node import Node
from rclpy.exceptions import ParameterNotDeclaredException

class MinimalParam(Node):

def __init__(self):
    super().__init__('minimal_param_node')
    timer_period = 2  # seconds
    self.timer = self.create_timer(timer_period, self.timer_callback)
    self.declare_parameter("name", "world")

    self.add_on_set_parameters_callback(self.cb_params)
    self.name = self.get_parameter("name").get_parameter_value().string_value

def cb_params(self, data):

    self.get_logger().warn("parameter changed...")
    self.successful = True
    self.name = self.get_parameter("name").get_parameter_value().string_value
    return self


def timer_callback(self):

    self.get_logger().info('Hello %s!' % self.name)

def main():
    rclpy.init()
    node = MinimalParam()
    rclpy.spin(node)

if __name__ == '__main__':
    main()

I changed the parameter over the ros2 command in a terminal with:

ros2 param set /minimal_param_node name "earth"

With the log message I can see that the callback is working, but the parameter is not changed the first time. When I call the ros2 command again, the parameter changed to the value set before.

FYI: The line:

self.successful = True
return self

is needed, because without them the code is crashing with an Error:

AttributeError: 'NoneType' object has no attribute 'successful'

I'm working with Foxy on Ubuntu 20.04 . Unfortunately I could not find any documentation/tutorial on this. What did i wrong?