party developed node that uses an argument on startup (and no parameters). I have a launch file that loads an YAML and uses it to feed the parameters to several nodes being launched. One of the configurations in that YAML file is the argument I need to pass to that 3rd-party node.

Currently, what I do, is to use an opaque function to process the YAML file and retrieve that configuration (it is not a ros__parameters) so as to pass it as an argument to the node in the launch file. Does this make sense, or is there a better way to deal with this need?

I am using a Python-based launch file with SimpleLauncher, but using normal LaunchDescription would be an option to.

The yaml file has this contents:

  world_package: "worlds"
  world_file: "stage_worlds/map_laser_with_landmarks2.world"
    use_sim_time: True

The launch file has the following:

from simple_launch import SimpleLauncher
import yaml
import os

sl = SimpleLauncher(use_sim_time=True)

# Declare arguments
               sl.find('tw07', 'params.yaml', 'config'),
               description='(Optional) Complete path to the parameters file')
sl.declare_arg('start_stage', True,
               description='If True, the Stage simulator is started')

def launch_opaque_setup():

    with sl.group(if_arg='start_stage'):
        # Get the YAML configuration file
        params = sl.arg('params_file')
        with open(params, 'r') as file:
            params_raw = yaml.safe_load(file)

        # Get the world file from the parameters file
        world_pkg = params_raw['stage']['world_package']
        world_path = params_raw['stage']['world_file']
        world_dir, world_filename = os.path.split(world_path)

        # Stage simulator
        sl.node('stage_ros', 'stageros',
                arguments=sl.find(world_pkg, world_filename, world_dir))

    return sl.launch_description()

generate_launch_description = \
  • $\begingroup$ Hi! Please add the relevant part of your launch file to the post. Moreover, could you clarify what is the problem with the method you are using? If I understood correctly, it works just as you want, but you find the implementation too hackish. Am I right? $\endgroup$
    – Luca
    Mar 18 at 10:52
  • $\begingroup$ Hi @Luca , thanks for your comment. I added the yam and launch files. Indeed, I was hoping that using "Substitutions" there was some way to do this without processing the YAML myself. $\endgroup$
    – hugo
    Mar 18 at 15:26


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