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.

Note as of ROS 2 Humble the XML launch syntax is almost feature-equal to Python. For simple cases it might be enough, and simple_launch can be used for more advanced usage (notably Gazebo interaction).

ROS Packages

Humble	</td> </tr>
Jazzy	</td> </tr>
Rolling	</td> </tr> </tbody> </table> ## 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](https://robotics.stackexchange.com/questions/98997/ros2-foxy-python-launch-argument-scope-when-nesting-launch-files). 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. File truncated at 100 lines [see the full file](https://github.com/oKermorgant/simple_launch/tree/devel/./README.md)

Changelog for package simple_launch

1.11.3 (2026-02-11)

• do not add commas when performing xacro output
• as_raw_type instead of as_str
• Contributors: Olivier Kermorgant

1.11.2 (2026-02-02)

• always try_perform when returning a Substitution
• event example was missing
• Contributors: Olivier Kermorgant

1.11.1 (2025-11-21)

• update Readme about XML launch files
• typo in gz_generate_world
• Contributors: Olivier Kermorgant

1.11.0 (2025-02-16)

• handle gz world export to SDF after some delay
• make gz_launch compatible with substitutions
• Contributors: Olivier Kermorgant

1.10.1 (2024-07-05)

• check for gz/ign executable being available
• gz_world_tf + better guess on Gz vs Ign
• Contributors: Olivier Kermorgant

1.10.0 (2024-06-16)

• scope_included_files to include other launch files in a Group and avoid changing my scope
• remove dead code about prefixing gz plugins
• forward sim_time even for nodes that load a parameter file
• lazy GazeboBridge
• Contributors: Olivier Kermorgant

1.9.2 (2024-03-25)

• Gazebo basic example: launch SDF world + spawn from xacro
• remove dead code related to Gazebo.
• sl.arg returns a SimpleSubstitution to allow concatenation
• auto-detect Gazebo world name, allows running the simulation + spawn models in the same launch file
• Type debug on String being Iterable
• better handling of non-string choices in argument declaration
• more robust to various gz/ros combinations
• Contributors: Olivier Kermorgant

1.9.1 (2024-02-20)

• simplify + debug logic of SimpleSubstitution divisions
• image instead of /image to be detected as an image topic
• remove auto_sim_time function
• more robust to various combinations of ROS and Gazebo
• default Gazebo is still ignition Fortress, better error message about GZ_VERSION
• add new bridges for Gazebo
• Contributors: Olivier Kermorgant

1.9.0 (2023-11-23)

• update documentation
• warnings/errors on misuse of GazeboBridge
• move example executable to share directory
• dict cannot be updated with | in Foxy, fallback to dict.update
• absolute container name when loading in existing container
• py_eval can now handle conditions
• Contributors: Olivier Kermorgant

1.8.0 (2023-11-20)

• sync documentation and examples
• when instead of after for events
• scoped events
• add event wrapper
• Contributors: Olivier Kermorgant

1.7.2 (2023-10-02)

• simplify logic of try_perform
• Merge pull request #5 from okvik/devel Fix string-valued launch argument substitution
• slight refactor to prepare wrapping Delays
• Contributors: Olivier Kermorgant, Viktor Pocedulic
• slight refactor to prepare wrapping Delays
• Contributors: Olivier Kermorgant

1.7.1 (2023-05-16)

• list[] -> List[] for type hints in 20.04
• detail on __ns for services
• add service call at launch
• Contributors: Olivier Kermorgant

File truncated at 100 lines see the full file