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ROS python launch system

ROS / ros eco / launch

Actions

Substitute

Events and condition

Demos

Gazebo

Rviz

Process

A Python launch file is not a script. It is a description factory that build declarative event graph

  1. imports your launch file
  2. calls generate_launch_description()
  3. builds a graph of actions
  4. executes that graph asynchronously

Anything that happens before step 3 is configuration time. Anything that happens after is launch runtime.

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Python import
   ↓
generate_launch_description()
   ↓
LaunchDescription created
   ↓
--------------------------------
Runtime starts
   ↓
Actions executed
   ↓
Nodes/processes run
   ↓
Events handled
   ↓
Shutdown

Demo: Example of launch life cycle

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from launch import LaunchDescription
from launch.actions import DeclareLaunchArgument, LogInfo, RegisterEventHandler
from launch.substitutions import LaunchConfiguration
from launch_ros.actions import Node
from launch.event_handlers import OnProcessExit

def generate_launch_description():
    ld = LaunchDescription()

    # ---- Substitution ----
    node_name_arg = DeclareLaunchArgument(
        'node_name',
        default_value='my_talker'
    )

    node_name = LaunchConfiguration('node_name')

    node = Node(
        package='demo_nodes_cpp',
        executable='talker',
        name=node_name,
        output='screen'
    )

    # ---- Runtime logging ----
    ld.add_action(node_name_arg)
    ld.add_action(LogInfo(msg=['Launching node: ', node_name]))

    # ---- Event handler ----
    ld.add_action(
        RegisterEventHandler(
            OnProcessExit(
                target_action=node,
                on_exit=[
                    LogInfo(msg=['Node exited: ', node_name])
                ]
            )
        )
    )

    ld.add_action(node)
    return ld
  • Python code → describe
  • Actions → do
  • Substitutions → resolve at runtime
  • Event handlers → react
Concept What it is
Node An action that starts a ROS process
LaunchConfiguration A runtime placeholder
DeclareLaunchArgument Makes a substitution configurable
LogInfo Runtime logging action
OnProcessExit Event emitted by launch
RegisterEventHandler Reaction to an event