Is it possible to have a conditional in a launch file using DeclareLaunchArgument

edit

I have a setup of hierarchical launch files, and I would like to use a launch argument to determine whether or not certain launch actions in them run. I already have a launch argument working via DeclareLaunchArgument that determines the namespace the node launches in, and I was hoping I could use another launch argument to run a simple if statement before adding an action to my launch description, launch_me.

The code below shows my attempt to implement this, but it does not work.

def generate_launch_description():
bridge_topics = join(get_package_share_directory('piracer'),
                     'config', 'ackermann_bridge_topics.yaml')
launch_directory = get_package_share_directory('piracer')

launch_me = LaunchDescription()

car_name_arg = DeclareLaunchArgument('car_name', default_value='car1',
                                    description='Sets the namespace for this car.')

hardware_arg = DeclareLaunchArgument('should_launch_hardware', default_value='true',
                                    description='Determines if hardware_nodes.launch is called.')

hardware_launch = IncludeLaunchDescription(
    PythonLaunchDescriptionSource([launch_directory, '/hardware_nodes.launch.py']),
    launch_arguments={'car_name': LaunchConfiguration('car_name')}.items()
)
ackermann_node = Node(
    package='piracer',
    namespace=[LaunchConfiguration('car_name')],
    executable='ackermann_controller',
    name='ackermann_controller',
    parameters=[bridge_topics]
)
launch_me.add_action(car_name_arg)
launch_me.add_action(hardware_arg)
if LaunchConfiguration('should_launch_hardware') == 'true':
    launch_me.add_action(hardware_launch)
launch_me.add_action(ackermann_node)

return launch_me

Is it possible to use a launch argument as a conditional in a launch file?

EDIT: Solved thanks to djchopp! To properly add the conditional, I had to do four things:

1. Add an import statement for IfCondition
2. Add a launch configuration, which I called should_launch_hardware.
3. Add the condition to the hardware_launch launch action.
4. Remove the useless if-statement around adding the hardware_launch action to launch_me. It will be added every time, but using the condition should_launch_hardware, ROS will decide if it actually gets executed or not.

A summary of the additions I made to the code are shown below:

from launch.conditions import IfCondition #1
....
should_launch_hardware = LaunchConfiguration('should_launch_hardware') #2
....
hardware_launch = IncludeLaunchDescription(
        PythonLaunchDescriptionSource([launch_directory, '/hardware_nodes.launch.py']),
        launch_arguments={'car_name': LaunchConfiguration('car_name')}.items(),
        condition=IfCondition(should_launch_hardware)   #3
    )

The entire new launch file looks like this:

# Standard library imports
from os.path import join

# Third-party imports
from ament_index_python.packages import get_package_share_directory
from launch_ros.actions import Node
from launch import LaunchDescription
from launch.actions import DeclareLaunchArgument
from launch.actions import IncludeLaunchDescription
from launch.conditions import IfCondition #1
from launch.launch_description_sources import PythonLaunchDescriptionSource
from launch.substitutions import LaunchConfiguration


def generate_launch_description():
    bridge_topics = join(get_package_share_directory('piracer'),
                         'config', 'ackermann_bridge_topics.yaml')
    launch_directory = get_package_share_directory('piracer')

    should_launch_hardware = LaunchConfiguration('should_launch_hardware')  #2

    launch_me = LaunchDescription()

    car_name_arg = DeclareLaunchArgument('car_name', default_value='car1',
                                         description='Sets the namespace for this car.')

    hardware_arg = DeclareLaunchArgument('should_launch_hardware', default_value='true',
                                         description='Determines if hardware_nodes.launch is called.')

    hardware_launch = IncludeLaunchDescription(
        PythonLaunchDescriptionSource([launch_directory, '/hardware_nodes.launch.py']),
        launch_arguments={'car_name': LaunchConfiguration('car_name')}.items(),
        condition=IfCondition(should_launch_hardware) #3
    )
    ackermann_node = Node(
        package='piracer',
        namespace=[LaunchConfiguration('car_name')],
        executable='ackermann_controller',
        name='ackermann_controller',
        parameters=[bridge_topics]
    )
    launch_me.add_action(car_name_arg)
    launch_me.add_action(hardware_arg)
    launch_me.add_action(hardware_launch) #4
    launch_me.add_action(ackermann_node)

    return launch_me