Declaring parameter fails unless value and descriptor are provided

Bug report

Required Info:

• Operating System:
  • Ubuntu 18.04
• Installation type:
  • From source
• Version or commit hash:
  • 0be24021af4269ce7c83fe9db550e326a8428a9b
• DDS implementation:
  • Fast-RTPS
• Client library (if applicable):
  • rclpy

Steps to reproduce issue

Write the following node:

import rclpy
from rclpy.node import Node


class MyNode(Node):
    def __init__(self):
        super().__init__("foo")

        self.declare_parameter("my_parameter")


def main(args=None):
    rclpy.init(args=args)
    node = MyNode()
    rclpy.spin(node)
    node.destroy_node()
    rclpy.shutdown()


if __name__ == "__main__":
    main()

Expected behavior

The node should be completely useless, but run. And have a parameter available.

Actual behavior

Traceback with “rclpy.exceptions.ParameterNotDeclaredException: (‘Invalid access to undeclared parameter(s)’, ‘my_parameter’)”

Additional information

The only way this works is if one supplies the optional fields value and descriptor in the declare_parameter() function. I’d say either these need to actually be optional and work by default, or actually be required in the API.

[wjwwood]

Calling declare with just a name and no value is fine. It just defaults to the not set value. You can do this in rclcpp already. I think the rclpy signature should be updated to allow it. In the meantime you can just explicitly pass a default initialized parameter value there, which will have the same behavior.

[wjwwood]

Also setting a value to not set still undeclares it in dashing. See: https://github.com/ros2/rclcpp/blob/0ae3da49e705eb2a019704c85fd0bb15ec4e968b/rclcpp/include/rclcpp/node.hpp#L527