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Using Python, XML, and YAML for ROS 2 Launch Files

ROS 2 launch files can be written in Python, XML, and YAML. This guide shows how to use these different formats to accomplish the same task, as well as has some discussion on when to use each format.

Launch file examples

Below is a launch file implemented in Python, XML, and YAML. Each launch file performs the following actions:

• Setup command line arguments with defaults

• Include another launch file

• Include another launch file in another namespace

• Start a node and setting its namespace

• Start a node, setting its namespace, and setting parameters in that node (using the args)

• Create a node to remap messages from one topic to another

PythonXMLYAML

# example.launch.py

import os

from ament_index_python import get_package_share_directory

from launch import LaunchDescription
from launch.actions import DeclareLaunchArgument
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


def generate_launch_description():

    # args that can be set from the command line or a default will be used
    background_r_launch_arg = DeclareLaunchArgument(
        "background_r", default_value=TextSubstitution(text="0")
    )
    background_g_launch_arg = DeclareLaunchArgument(
        "background_g", default_value=TextSubstitution(text="255")
    )
    background_b_launch_arg = DeclareLaunchArgument(
        "background_b", default_value=TextSubstitution(text="0")
    )
    chatter_ns_launch_arg = DeclareLaunchArgument(
        "chatter_ns", default_value=TextSubstitution(text="my/chatter/ns")
    )

    # include another launch file
    launch_include = IncludeLaunchDescription(
        PythonLaunchDescriptionSource(
            os.path.join(
                get_package_share_directory('demo_nodes_cpp'),
                'launch/topics/talker_listener.launch.py'))
    )
    # include another launch file in the chatter_ns namespace
    launch_include_with_namespace = GroupAction(
        actions=[
            # push-ros-namespace to set namespace of included nodes
            PushRosNamespace(LaunchConfiguration('chatter_ns')),
            IncludeLaunchDescription(
                PythonLaunchDescriptionSource(
                    os.path.join(
                        get_package_share_directory('demo_nodes_cpp'),
                        'launch/topics/talker_listener.launch.py'))
            ),
        ]
    )

    # start a turtlesim_node in the turtlesim1 namespace
    turtlesim_node = Node(
            package='turtlesim',
            namespace='turtlesim1',
            executable='turtlesim_node',
            name='sim'
        )

    # start another turtlesim_node in the turtlesim2 namespace
    # and use args to set parameters
    turtlesim_node_with_parameters = Node(
            package='turtlesim',
            namespace='turtlesim2',
            executable='turtlesim_node',
            name='sim',
            parameters=[{
                "background_r": LaunchConfiguration('background_r'),
                "background_g": LaunchConfiguration('background_g'),
                "background_b": LaunchConfiguration('background_b'),
            }]
        )

    # perform remap so both turtles listen to the same command topic
    forward_turtlesim_commands_to_second_turtlesim_node = Node(
            package='turtlesim',
            executable='mimic',
            name='mimic',
            remappings=[
                ('/input/pose', '/turtlesim1/turtle1/pose'),
                ('/output/cmd_vel', '/turtlesim2/turtle1/cmd_vel'),
            ]
        )

    return LaunchDescription([
        background_r_launch_arg,
        background_g_launch_arg,
        background_b_launch_arg,
        chatter_ns_launch_arg,
        launch_include,
        launch_include_with_namespace,
        turtlesim_node,
        turtlesim_node_with_parameters,
        forward_turtlesim_commands_to_second_turtlesim_node,
    ])

Using the Launch files from the command line

Launching

Any of the launch files above can be run with ros2 launch. To try them locally, you can either create a new package and use

ros2 launch <package_name> <launch_file_name>

or run the file directly by specifying the path to the launch file

ros2 launch <path_to_launch_file>

Setting arguments

To set the arguments that are passed to the launch file, you should use key:=value syntax. For example, you can set the value of background_r in the following way:

ros2 launch <package_name> <launch_file_name> background_r:=255

or

ros2 launch <path_to_launch_file> background_r:=255

Controlling the turtles

To test that the remapping is working, you can control the turtles by running the following command in another terminal:

ros2 run turtlesim turtle_teleop_key --ros-args --remap __ns:=/turtlesim1

Python, XML, or YAML: Which should I use?

Nota

Launch files in ROS 1 were written in XML, so XML may be the most familiar to people coming from ROS 1. To see what’s changed, you can visit Migrating launch files from ROS 1 to ROS 2.

For most applications the choice of which ROS 2 launch format comes down to developer preference. However, if your launch file requires flexibility that you cannot achieve with XML or YAML, you can use Python to write your launch file. Using Python for ROS 2 launch is more flexible because of following two reasons:

• Python is a scripting language, and thus you can leverage the language and its libraries in your launch files.

• ros2/launch (general launch features) and ros2/launch_ros (ROS 2 specific launch features) are written in Python and thus you have lower level access to launch features that may not be exposed by XML and YAML.

That being said, a launch file written in Python may be more complex and verbose than one in XML or YAML.