Question

I have a node in a package that I set up using ros2 pkg create --build-type ament_python --node-name my_node my_package

my_node.py looks like this:

import rclpy
from rclpy.node import Node

class MyNode(Node):
    def __init__(self, arg):
        super().__init__("my_node")
        self.arg = arg

def main(args = None):

    rclpy.init(args = args)

    node = MyNode(9)
    node.get_logger().info("Hello from my_node")

    try:
        rclpy.spin(node)
    except:
        pass

    node.destroy_node()
    rclpy.shutdown()

if __name__ == '__main__':
    main()

And my launch file looks like this:

from launch import LaunchDescription from launch_ros.actions import Node import launch

def generate_launch_description():
    return LaunchDescription([

        Node(
            package='my_package',
            node_namespace='test',
            node_executable='my_node',
            emulate_tty=True,
            node_name='my_node',
        ),
        launch.actions.DeclareLaunchArgument('arg1')
])

I am launching the node using ros2 launch -a ./launch/my_launch.py arg1:=9

How do I access arg1 from inside the main function? I want to use it to pass into the constructor of the Node.

Alternatively, is there another way to pass parameters into the main function? I just want to populate the args in the def main(args = None) function, but it needs to work when launched from a launch file.

Thanks

Asked by CraigH92 on 2020-05-20 03:58:32 UTC

Answer 1

same i cant find it anywhere... their is no way to put it to your node? you can only put whole launch files with

parameters= ["path/params.yaml"]

and if you want to just add a normal argument with:

arguments= ["test"]

you can but if you want to parse it from your launch file its just yikes since you cant just give a normal argument to the launch command it has to be:

arg1:= test

you have to do some big yikes stuff in python like getting argv[position] and than split on := get the second part and than use that in the arguments array and than in your main you have to do argv[position'] again.....

Asked by teunes1 on 2020-05-20 07:15:25 UTC

Comments

I've found a workaround. I'll post it as an answer here in a moment.

Asked by CraigH92 on 2020-05-20 08:44:08 UTC

Answer 2

I've found a workaround, but I don't think this is the idiomatic way to do it.

Use the launch file to accept an argument:

def generate_launch_description():
    return LaunchDescription([

        DeclareLaunchArgument("simulation_mode"),

        Node(
            package='my_package',
            node_executable='my_node',
            emulate_tty=True,
            arguments = [launch.substitutions.LaunchConfiguration('simulation_mode')]
        )
])

Then launch the launch file with:

ros2 launch -a launch/my_launch.py simulation_mode:=False

And then this argument can be used in the node like this:

import rclpy
from rclpy.node import Node
import sys
import typing

class MyNode(Node):
    def __init__(self, simMode : bool):
        super().__init__("my_node")
        self.simMode = simMode

def main():

    rclpy.init()

    print(str(sys.argv[1]))

    node = MyNode(sys.argv[1])
    node.get_logger().info("Hello from my_node")

    try:
        rclpy.spin(node) 
    except:
        pass

    node.destroy_node()
    rclpy.shutdown()

if __name__ == '__main__':
    main()

argv[1] becomes the right-hand side of the assignment simulation_mode:=False. It's a positional argument instead of a named argument, so it could be more prone to errors, but it will do until I find a better solution.

Asked by CraigH92 on 2020-05-20 08:50:05 UTC

Comments

@CraigH92 Did you find a better solution?

Asked by ljaniec on 2022-03-25 12:23:24 UTC

rclpy: Accessing ros2 launch args from main function

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