ROS2 Humble: How to pass arguments to python launch files from command-line?

Question

Ubuntu 22.04, ROS 2 Humble

Hi, I am following the official tutorial to pass arguments to a cpp node via a python launch file

https://docs.ros.org/en/humble/How-To-Guides/Launch-file-different-formats.html

My launch file is as follows

import os

# https://docs.ros.org/en/humble/How-To-Guides/Launch-file-different-formats.html
from launch import LaunchDescription
from launch.actions import DeclareLaunchArgument, LogInfo
from launch.actions import IncludeLaunchDescription
from launch.actions import GroupAction
from launch.launch_description_sources import PythonLaunchDescriptionSource
from launch.substitutions import LaunchConfiguration
from launch.substitutions import TextSubstitution
from launch_ros.actions import Node
from launch_ros.actions import PushRosNamespace

from ament_index_python.packages import get_package_share_directory #* Required to get YAML file


# Define path to package
home_directory = os.path.expanduser("~")
path_to_orbslam3_pck = home_directory + "/" + "ros2_ws/src/orb_slam3_ros2"

# Build global paths
voc_path = os.path.join(path_to_orbslam3_pck, 'orb_slam3/Vocabulary/ORBvoc.txt.bin')
settings_path = os.path.join(path_to_orbslam3_pck, 'orb_slam3/config/Monocular/')

# https://roboticscasual.com/tutorial-ros2-launch-files-all-you-need-to-know/#cmd-line-args-hand-over

def generate_launch_description():
    ld = LaunchDescription()

    # If you want to load some YAML files
    # config_robot1 = os.path.join(
    #     get_package_share_directory('my_launcher'), #* Nothing fancy here, this is how the syntax goes
    #     'config',
    #     'params.yaml'
    # )

    # voc_file_launch_arg = DeclareLaunchArgument(
    #         "voc_file",
    #         default_value=TextSubstitution(text=voc_path),
    #         # description='Path to the Vocabulary file for ORB-SLAM3',
    #     ),
    
    # settings_file_path_launch_arg = DeclareLaunchArgument(
    #         "settings_file_path",
    #         default_value=TextSubstitution(text=settings_path),
    #         #description='Path to the Vocabulary file for ORB-SLAM3',
    #     ),
    
    settings_file_name_launch_arg = DeclareLaunchArgument(
            "settings_file_name",
            default_value=TextSubstitution(text='file_not_set'), # EuRoC, TUM2
            # description='Settings file name i.e. EuoRoC, TUM2 etc',
        ),


    robot0_vslam_node = Node(
        package="orb_slam3_ros2", # Remember, the (,) are very important
        executable="agent0",
        name = "robot0_vslam",
        # # if you have a namespace, use this tutorial to add it https://www.youtube.com/watch?v=wY8MrBGVxYA
        #parameters=[config_robot1] # when using YAML configuration files
        
        #* List with tuples
        parameters=[
                    {"agent_name_arg": 'robot0'}, 
                    #{"voc_file_arg": LaunchConfiguration('voc_file')},
                    #{"settings_file_path_arg": LaunchConfiguration('settings_file_path')},
                    {"settings_file_name_arg": LaunchConfiguration('settings_file_name')}
        ] # When setting individual parameters, set it as a tuple
    )

    # https://answers.ros.org/question/382167/is-it-possible-to-have-a-conditional-in-a-launch-file-using-declarelaunchargument/

    #ld.add_action(voc_file_launch_arg)
    #ld.add_action(settings_file_path_launch_arg)
    ld.add_action(settings_file_name_launch_arg)
    ld.add_action(robot0_vslam_node) # This adds the node to the description

    return ld 

This code builds without any issue. To make the testing simpler, I am only trying to pass one argument to my CPP node to see if it receives it.

However, when I try running this command

ros2 launch my_launcher my_app_launch.py settings_file_name:="TUM2"

I am getting this following error

[ERROR] [launch]: Caught exception in launch (see debug for traceback): 'tuple' object has no attribute 'describe_sub_entities'

Note that, I had tried returning LaunchDescription in the following way

return LaunchDescription([
    launch_args,
    node
])

Both gives the error shown above.

My questions are the following

1. What is `describe_sub_entities' in context of the above problem?
2. What changes are required to get the desired behavior?

Thanks @Mechazo11

Answer 1

I think the problem is the trailing comma on your launch arg declaration:

settings_file_name_launch_arg = DeclareLaunchArgument(
            "settings_file_name",
            default_value=TextSubstitution(text='file_not_set'), # EuRoC, TUM2
            # description='Settings file name i.e. EuoRoC, TUM2 etc',
        ), #<<< trailing comma here 

Instead of the expected launch object, which would have a .describe_sub_entities() method, you're then passing a one-element tuple to the launch system:

(settings_file_name_launch_arg, )