simple_launch

This package provides a Python class to help writing ROS 2 launch files.

The motivation behind this package is that the ROS 2 launch Python syntax may not be suited for many simple cases such as running basic nodes, spawning a robot_state_publisher, and grouping nodes in namespaces or components.

Thanks: this package is inspired by ros2_launch_util.

The entry point is the SimpleLauncher class, which has several capabilities.

Basic syntax

Namespace and argument parser initialization

sl = SimpleLauncher(namespace = '', use_sim_time = None, scope_included_files = False)

• will initialize all nodes relative to the given namespace
• if use_sim_time is a Boolean, creates a use_sim_time launch argument with this value as the default and forwards it to all nodes, unless explicitely specified when running the node
• if use_sim_time is 'auto', then SimpleLauncher will set it to True if the /clock topic is advertized (case of an already running simulation). This may have side effects if the /clock topic is advertized but you want to use this launch file with system clock.
• if use_sim_time is None (default) then no particular value is forwarded to the nodes
• if scope_included_files is False (default) then including another launch file that shares the same argument but while passing another value for this argument, will also modify the value of this argument after the inclusion. This is the default behavior of launch but can be undesired. Inside an OpaqueFunction the behavior is to be scoped anyway (the argument value is resolved) and scope_included_files has no effect.

Node registration

sl.node(package, executable, **node_args) where

• package is the node package
• executable is the name of the executable
• node_args are any additional Node arguments

Launch file include

sl.include(package, launch_file, launch_dir = None, launch_arguments=None) where

• package is the package of the included launch file
• launch_file is the name of the launch file
• launch_dir is its directory inside the package share (None to have it found)
• launch_arguments is a dictionary of arguments to pass to the included launch file

Call a service at launch

This line runs a temporary client that waits for a service and calls it when available:

sl.call_service(server, request = None, verbosity = '') where

• server is the path to some service (possibly namespaced). The service type is deduced when it becomes available.
• request is a dictionary representing the service request. If None or incomplete, will use the service request default values.
• verbosity let the underlying node describe what it is doing: 'req' for request info, 'res' for response info or both with 'reqres'

If any request parameter is __ns it will be changed to the current namespace.

Setting parameters

This line runs a temporary client that waits for a node and changes its parameters when available:

sl.set_parameters(node_name, parameters: dict = {}, verbosity = '') where

• node_name is the name of the node (possibly namespaced)
• parameters is a dictionary of (name, value) parameters to be set
• verbosity let the underlying node describe what it is doing: 'req' for request info, 'res' for response info or both with 'reqres'

This calls the set_parameters service of the node with the passed types. Possible errors may happen if the parameters do not exist or are of a different type.

Robust types for parameters

In the launch API, differents types are expected for:

• node parameters: a list of dictionaries
• node remappings: a list of (key, value) pairs
• included launch arguments: a list of (key, value) pairs
• xacro arguments: a concatenation of key:=value strings

The sl.include, sl.node and xacro_args calls allow using any type (the simplest being a single dictionary) and will convert to the one expected by the API.

Launch arguments

simple_launch allows declaring launch arguments and getting them in return.

Declare a launch argument

sl.declare_arg(name, default_value, description = None): declare and returns the argument

Contrary to the base API, the default value is a raw Python type.

Retrieve a launch argument

sl.arg(name): returns the argument name as a SimpleSubstitution

Retrieve several arguments as a dictionary

sl.arg_map('robot', 'x', 'y'): returns {'robot': <robot arg value>, 'x': <x arg value>, 'y': <y arg value>}

Typical when forwarding some launch arguments to a node or an included launch file.

Groups or scopes

Groups are created through the with sl.group(): syntax and accept, a namespace an if/unless condition and an event.

Actions that are added in a scope inherit from all previous defined groups.

By namespace

  with sl.group(ns=sub_namespace):
    sl.node(package, executable)

From a condition

  with sl.group(if_condition=True):
    sl.node(package, executable)

  with sl.group(unless_condition=<some expression>):
    sl.node(package, executable)

• Only one condition can be set in a group, nested condition must be combined first, or used in nested groups.
• Combining conditions coming from launch arguments can be done with sl.py_eval as shown below.

From conditional arguments

  with sl.group(if_arg='use_gui'):
    sl.node(package, executable)

  with sl.group(unless_arg='dont_do'):
    sl.node(package, executable)

if_arg / unless_arg is expected to be the name of a launch argument. These two lines are equivalent:

  with sl.group(if_arg='use_gui'):
  with sl.group(if_condition=sl.arg('use_gui')):

If if_arg / unless_arg is not a string then it is considered as a if_condition / unless_condition.

From events

The when argument wraps events from the launch.event_handlers module. It combines an event and a delay (0 by default)

from simple_launch.events import When, OnProcessStart, OnProcessExit, OnProcessIO

  my_node = sl.node(...)   # reference node

  with sl.group(when = When(my_node, OnProcessStart, 1.)):
      sl.node(...)  # will run 1 s after main node starts

  with sl.group(when = When(my_node, OnProcessExit)):
      sl.node(...)  # will run as soon as the main node exists

  with sl.group(when = When(my_node, OnProcessIO, io = 'stdout'):
      # OnProcessIO events need a function changing the event into an action
      sl.add_action(lambda event: LogInfo(msg = 'Node says "{}"'.format(
                      event.text.decode().strip())))
      # several functions can be used if needed, they will be combined in a single one
      sl.add_action(lambda event: LogInfo(msg = 'Once again, node says "{}"'.format(
                      event.text.decode().strip())))

  with sl.group(when = When(delay = 2.)):
      sl.node(...)  # will run after 2 sec

Creating containers

This syntax adds the composition/composition::Talker as a ComposableNode

  with sl.container(name='my_container', output='screen'):
    sl.node(package='composition', plugin='Talker', name='talker')

Use the executable and package parameters if you want to use executors other than rclcpp_components’s component_container:

  with sl.container(name='my_container', output='screen', executable='component_container_isolated'):

It is currently impossible to have group blocks within a container block, as containers can only accept ComposableNodes. A GroupAction containing e.g. PushRosNamespace and a ComposableNode is not itself a ComposableNode.

use_sim_time

The current use_sim_time setting can be retrieved through sl.sim_time that may be:

• None, if use_sim_time was not set in the SimpleLauncher constructor
• a raw Boolean, if use_sim_time was set to 'auto', depending on the /clock topic being advertized
• a Boolean launch argument, if use_sim_time was set to True or False

In all cases, if the use_sim_time parameter is explicitely given to a node, it will be used instead of the SimpleLauncher instance one.

OpaqueFunction with implicit .perform(context)

Most of the use cases can be dealt with substitutions and with sl.group blocks. In order to design more imperative launch files, the OpaqueFunction approach can be used. The main drawback is that potential errors are harder to track.

To do this with simple_launch:

• the SimpleLauncher instance and the argument declaration should be done in the main body of your launch file.
• then, define a function (e.g. launch_setup) that takes no argument, where the logic of the launch file resides.
• all arguments obtained through sl.arg will be basic Python types, obtained from performing the substitutions.
• finally just export generate_launch_description = sl.launch_description(opaque_function = launch_setup).

Compare example_launch.py and example_opaque_launch.py to see the two approaches on the same logic.

Note that inside an OpaqueFunction the if/unless idiom reduces to a basic if/else:

# with substitutions
with sl.group(if_arg='some_condition'):
  # do stuff
with sl.group(unless_arg='some_condition'):
  # do other stuff

# with opaque function
if sl.arg('some_condition'):
  # do stuff
else:
  # do other stuff

Other one-liners

Methods listed below return instances of SimpleSubstitution that represent any Substitution, but that provides concatenation (+) and path concatenation (/) operators. It is still a Substitution, not a raw Python type. If run from an OpaqueFunction the underlying Python variable is returned.

String / substitution concatenation

The following syntax builds the SimpleSubstitution corresponding to <robot arg>.xacro:

file_name = sl.arg('robot') + '.xacro'

deprecated: sl.name_join(sl.arg('robot'), '.xacro')

Path concatenation

The following syntax builds the SimpleSubstitution corresponding to <package_path>/urdf/<robot arg>.xacro:

file_name = sl.arg('robot') + '.xacro'
urdf_file = os.path.join(get_package_share_directory(package),'urdf')/file_name

Obviously if all the path elements are raw strings, you should use os.path.join all along.

deprecated: sl.path_join(get_package_share_directory(package), sl.arg('robot'), '.xacro')

Find a share file

path = sl.find(package, file_name = None, file_dir = None) where:

• package is the name of the package or None if file_name is already an absolute path
• file_name is the name of the file to find
• file_dir is the path inside the package

If file_dir is None but package and file_name are raw strings then the find function will actually look for the file inside the package share, using os.walk.

If file_name is None then the function just returns the path to the package share directory (e.g. get_package_share_directory(package))

Robot state publisher

It is quite common to run a robot_state_publisher from a urdf or xacro file. The line below runs it at the current namespace / condition level:

sl.robot_state_publisher(package, description_file, description_dir=None, xacro_args=None, **node_args) where

• description_file is a URDF or xacro file
• description_dir is the sub-directory of the file. If omitted, let the script search for the file assuming it is a raw string
• xacro_args is a dictionary of arguments to forward to xacro
• node_args are any additional arguments for robot_state_publisher (remappings / parameters)

Python expressions

sl.py_eval will evaluate the given arguments as a Python expression, possibly performed if in an Opaque Function.

# RGB color as a list of [0-255] integers
sl.declare_arg('color', [255,0,0])
# same color as a string of [0-1] numbers (URDF format), note the padding commas to get a string
xacro_color = "'" + sl.py_eval("' '.join(str(c/255) for c in ", sl.arg('color'), ')') + "'"

Conditions

sl.py_eval can be used to combine conditions. It is robust to lower case true or false and will return a SimpleSubstitution.

sl.declare_arg('some_condition', True)
opposed = sl.py_eval('not ', sl.arg('some_condition'))

Note that IfCondition and UnlessCondition cannot be combined, only the underlying Substitutions can.

Joint state publisher

sl.joint_state_publisher(use_gui, **node_args): fires up a joint_state_publisher, with or without the gui, in the current namespace.

Rviz

sl.rviz(config_file = None, warnings = False): runs RViz on the given configuration file. If warnings is False (default) then runs with log-level FATAL in order to avoid many messages in the console.

Classical use case: sl.rviz(sl.find('my_package', 'some_rviz_config.rviz'))

Fallback to low-level syntax

If any unavailable functionality is needed, the sl.add_action(action) function adds any passed Action at the current namespace / conditional / event level.

Interaction with Gazebo / Ignition

Note: Ignition being renamed to Gazebo, all tools in this section use Gazebo / gz names

An effort was made to be robust to Ignition versus Gazebo uses, i.e. ign prefix is used for foxy and galactic while gz prefix is used from humble. GZ_VERSION and IGNITION_VERSION environment variables are also used to identify which version should be preferred.

Launch Gazebo

The Gazebo launch file corresponding to the current ROS 2 distribution is launched with

sl.gz_launch(world_file, gz_arguments)

Namely, it will redirect to either ros_ign_gazebo/ign_gazebo.launch.py (foxy, galactic) or ros_gz_sim/gz_sim.launch.py (humble+). The given gz_arguments, if any, will be forwarded either as the ign_args or gz_args, accordingly.

If the world file can be parsed then SimpleLaunch will detect its name and forward it to GazeboBridge functions.

Spawn a model

The following function allows easily spawing a model from its robot_description:

sl.spawn_gz_model(name, topic, model_file = None, spawn_args = [], only_new = True)

• name is the name this model will get in Gazebo
• topic is the topic to obtain the model from, default is robot_description (relative to the current namespace)
• model_file is the path to the (urdf or sdf) file. If defined then this will spawn this model and ignore the topic
• only_new if True, will not spawn the model if it already exists in a running Gazebo instance
• spawn_args are any additional spawn arguments, e.g. the initial pose

example: sl.spawn_gz_model('my_robot', model_file = sl.find('my_pkg', 'my_model.urdf'))

Declare initial pose

Calling sl.declare_gazebo_axes() will declare all 6 parameters (x,y,z,roll,pitch,yaw) with 0 as default values. If any axis is given (e.g. sl.declare_gazebo_axes(yaw = 3.14) then only this parameter will be declared.

Such parameters can be retrieved through sl.gazebo_axes_args(). As a consequence, it is easy to spawn a model with:

sl.declare_gazebo_axes()
sl.robot_description(...)
sl.spawn_gz_model(name, spawn_args = sl.gazebo_axes_args())

Gazebo bridge

The GazeboBridge class allows easily creating bridges when using Gazebo.

An instance is created with: bridge = GazeboBridge(<gazebo_topic>, <ros_topic>, <ros_message>, direction, <gz_message> = None) where direction is either:

• GazeboBridge.gz2ros for Gazebo -> ROS
• GazeboBridge.ros2gz for ROS -> Gazebo
• GazeboBridge.bidirectional for both

The Gazebo message type is deduced from the ROS message type if not set. Remapping will be set to the given ros_topic.

The SimpleLauncher instance can then run a node dealing with declared bridges, as illustrated in the examples at this end of this document.

• sl.create_gz_bridge([bridges], <node_name>)

If some bridges involve sensor_msgs/Image then a dedicated ros_gz_image bridge will be used. The corresponding camera_info topic will be also bridged.

Interaction with Gazebo sim

Some bridges (e.g. joint_states) need to have information on the world name. This name can only be obtained while Gazebo is running, or by having a priori knowledge of the world file.

The GazeboBridge class has a few static methods to get information on the simulated world, namely:

• GazeboBridge.world() returns the current world name
• GazeboBridge.set_world_name(world) imposes the world name
• GazeboBridge.model_prefix(model) builds the Gazebo topic relative to the given model /world/<world>/model/<model>

They can be used under these conditions:

• sl.gz_launch was called first (in the same launch setup) and the world file could be parsed, in this case the world name from the file is used
• or GazeboBridge.set_world_name(world) was called first (in the same launch setup), in this case this world name is used
• if none of the above and a running Gazebo instance exists, in this case GazeboBridge will request information on the world

If none of these conditions hold, the launch file will not be able to get information on the world, and launch fill probably fail.

Built-in bridges

A common instance of the bridge is the clock. This one can be:

• created with GazeboBridge.clock(): returns a GazeboBridge instance, not added to any node yet
• or run directly with sl.create_gz_clock_bridge() (actually runs sl.create_gz_bridge(GazeboBridge.clock()))

Another common but tedious bridge instance is the joint state topic of a given model. This topic includes, in Gazebo, the name of the world and of the model. Assuming the world name is known, the corresponding bridge can be created with:

• GazeboBridge.joint_states_bridge(model)
• actually runs GazeboBridge('/world/<world>/model/<model>/joint_state', 'joint_states', 'sensor_msgs/JointState', GazeboBridge.gz2ros)

World TF publisher

If /tf is used with Gazebo, then the root frame used by Gazebo depends on the world name. This frame is usually world on the ROS side. A trivial static_transform_publisher is run to connect these two frames if they have different names:

• sl.gz_world_tf(world_frame = None): run a static_transform_publisher from world to world_frame. If None then detect the one currently used by Gazebo.

Examples

Examples can be found in the corresponding folder.

Launch with arguments

Here is a file spawning a robot_state_publisher and its joint_state_publisher, where:

• prefix, x and y are launch arguments that are passed to xacro
• use_gui tells whether joint_state_publisher should run the gui

from simple_launch import SimpleLauncher

def generate_launch_description():
    '''
    Launch description for a single robot - runs the two nodes in their own namespace
    '''
    sl = SimpleLauncher()

    sl.declare_arg('prefix', default_value = '', description='name of the robot (+ tf prefix)')
    sl.declare_arg('x', default_value = 0, description='x-offset of the robot')
    sl.declare_arg('y', default_value = 0, description='y-offset of the robot')
    sl.declare_arg('use_gui', default_value = True, description='Use JSP gui')

    xacro_args = sl.arg_map('prefix', 'x', 'y')
    xacro_args['prefix'] += '/'  # can sum substitutions and strings

    with sl.group(ns=sl.arg('prefix')):
        sl.robot_state_publisher('simple_launch', 'turret.xacro', xacro_args = xacro_args)
        sl.joint_state_publisher(use_gui = sl.arg('use_gui'))

    return sl.launch_description()

Conditional blocks and includes

The file below fires up either robot1 or robot2 (or both) and also has a boolean argument to spawn RViz2:

from simple_launch import SimpleLauncher

def generate_launch_description():

    sl = SimpleLauncher()

    # conditional args
    sl.declare_arg('robot1', default_value=True, description='use robot 1')
    sl.declare_arg('robot2', default_value=True, description='use robot 2')
    sl.declare_arg('no_robot2', default_value=False, description='cancel use of robot 2')
    sl.declare_arg('rviz', default_value=True, description='Bringup RViz2')

    # numeric args
    sl.declare_arg('robot2_x', default_value=1, description='x-offset of robot 2')
    sl.declare_arg('robot2_y', default_value=1, description='y-offset of robot 2')

    with sl.group(if_arg='robot1'):
        sl.include('simple_launch', 'included_launch.py',
                   launch_arguments = {'prefix': 'robot1'})

    with sl.group(if_arg='robot2'):

        with sl.group(unless_arg='no_robot2'):

            args = {'prefix': 'robot2', 'x':sl.arg('robot2_x'), 'y': sl.arg('robot2_y')}
            sl.include('simple_launch', 'included_launch.py', launch_arguments=args)

    with sl.group(if_arg='rviz'):
        rviz_config = sl.find('simple_launch', 'turret.rviz')
        sl.node('rviz2', 'rviz2', arguments = ['-d', rviz_config])

    return sl.launch_description()

Conditions with OpaqueFunction

The file below does the same as the previous one, but using an OpaqueFunction:

from simple_launch import SimpleLauncher

# declare simple launcher and the launch arguments in the main body
sl = SimpleLauncher()

# conditional args
sl.declare_arg('robot1', default_value=True, description='use robot 1')
sl.declare_arg('robot2', default_value=True, description='use robot 2')
sl.declare_arg('no_robot2', default_value=False, description='cancel use of robot 2')
sl.declare_arg('rviz', default_value=True, description='Bringup RViz2')

# numeric args
sl.declare_arg('robot2_x', default_value=1, description='x-offset of robot 2')
sl.declare_arg('robot2_y', default_value=1, description='y-offset of robot 2')

# string args
sl.declare_arg('included', default_value = 'included_launch')


# define the opaque function, context will be wrapped in the SimpleLauncher instance
def launch_setup():

    # we can use raw if as `robot1` argument is performed to a Boolean
    if sl.arg('robot1'):
        sl.include('simple_launch', 'included_launch.py', launch_arguments = {'prefix': 'robot1'})

    # and even combine conditions
    if sl.arg('robot2') and not sl.arg('no_robot2'):

            args = {'prefix': 'robot2', 'x':sl.arg('robot2_x'), 'y': sl.arg('robot2_y')}
            # summing up args and strings
            sl.include('simple_launch', sl.arg('included') + '.py', launch_arguments=args)

    if sl.arg('rviz'):
        rviz_config = sl.find('simple_launch', 'turret.rviz')
        sl.node('rviz2', 'rviz2', arguments = ['-d', rviz_config])

    return sl.launch_description()


# tell SimpleLauncher to rely on the opaque_function in the launch description
# /!\ no `def generate_launch_description():`

generate_launch_description = sl.launch_description(opaque_function = launch_setup)

Combining conditions

The file below shows how to use sl.py_eval to combine conditions. We have to build a valid Python expression, not forgetting the spaces around and, or, not, etc.

from simple_launch import SimpleLauncher

def generate_launch_description():

    sl = SimpleLauncher()

    cond1 = sl.declare_arg('cond1', True)
    cond2 = sl.declare_arg('cond2', False)

    for logic in ('and', 'or'):

        # builds <cond1> <logic> <cond2> Python expression
        combined = sl.py_eval(cond1, f' {logic} ', cond2)

        sl.log_info([f'{logic} condition is ', combined])

        with sl.group(if_condition = combined):
            sl.node('demo_nodes_cpp', 'talker', name = f'talker_{logic}')

        # also works with sl.node(..., condition = IfCondition(combined))

    return sl.launch_description()

Composition

The file below is another way to write the composition launch example:

from simple_launch import SimpleLauncher

def generate_launch_description():

    sl = SimpleLauncher()

    with sl.container(name='my_container', output='screen'):
        sl.node(package='composition', plugin='Talker', name='talker')
        sl.node(package='composition', plugin='Listener', name='listener')

    return sl.launch_description()

Events and parameters

This example file is another way to write the event launch example. If does a little more work and is a little smarter, as the passed new_background_r is applied as soon as it has a valid value.

auto sim time

Here we run Gazebo and force all other nodes to use_sim_time:=True, unless this file is included from another one with use_sim_time:=False. This is unlikely as this launch file spawns a simulator.

from simple_launch import SimpleLauncher, GazeboBridge

def generate_launch_description():

    # all nodes in this launch file will use_sim_time:=True
    sl = SimpleLauncher(use_sim_time=True)

    # run Gazebo + clock bridge
    sl.gz_launch(<path/to/some/sdf/world>, '-r')
    sl.create_gz_clock_bridge()

    # run other nodes with sim time

    return sl.launch_description()

Robot description and conditionnal Gazebo bridge

The file below only runs by default a robot_state_publisher with use_sim_time:=False. However, if it is included from another file with use_sim_time:=True then it also spawns the robot into Gazebo and runs two bridges for joint states and pose.

from simple_launch import SimpleLauncher, GazeboBridge

def generate_launch_description():

    sl = SimpleLauncher(use_sim_time=False)

    # namespace is a launch argument, not a Python string
    sl.declare_arg('robot', default_value = 'robot1')
    robot = sl.arg('robot')

    with sl.group(ns = robot):
        # robot_state_publisher is always run
        sl.robot_state_publisher('my_description', 'my_robot.xacro')

        with sl.group(if_condition = sl.sim_time):
            # only execute this group if use_sim_time was set to True

            # spawn in Gazebo at default pose if not already here
            sl.spawn_gz_model(robot)

            # create a bridge for joint states @ /world/<world>/model/<robot>/joint_state
            # note the relative ROS topic 'joint_states' that is actually namespaced
            gz_js_topic = GazeboBridge.model_prefix(robot)/'joint_state'
            js_bridge = GazeboBridge(gz_topic = gz_js_topic,
                                     ros_topic = 'joint_states',
                                     msg = 'sensor_msgs/JointState',
                                     direction = GazeboBridge.gz2ros)

            # pose publisher bridge @ /model/<robot>
            pose_bridge = GazeboBridge('/model'/robot/'/pose',
                                       'pose_gt', 'geometry_msgs/Pose', GazeboBridge.gz2ros)

            # create bridge node with these two topics with default name gz_bridge
            sl.create_gz_bridge([js_bridge, pose_bridge])

    return sl.launch_description()

Full Gazebo simulation

The file example/gazebo/gazebo_launch.py runs a basic simulation of a turret robot with a camera, also displayed in RViz.

ros2 launch simple_launch gazebo_launch.py

You can move the robot around with a joint velocity setpoint (std_msgs/Float64) on /turret/joint# and see the simulated camera on /turret/image.

I want more examples

Other self-contained examples (and slides) have been used to teach the ROS 2 launch file system and the nav stack.

Various aspects of the launch systen are shown in the anf_launch package.

A simple tutorial on Nav2 using simple_launch is available in the anf_nav package.