Allow conditional substitutions

[ryanewel]

##Feature request

Feature description

Add a conditional to a substitution so that a substitution only takes place if a value was provided. See https://answers.ros.org/question/314903/ros2-optionally-launch-node-with-no-arguments/ for some discussion.

[chapulina]

We'd also like this for gazebo_ros_pkgs. We currently have a hacky workaround on this PR using PythonExpression.

[ivanpauno]

I don't like the idea of a conditional substitution returning None when the condition is False, as described in https://answers.ros.org/question/314903/ros2-optionally-launch-node-with-no-arguments/. IMO, it adds complexity, and that's not the source of the problem.

---

https://answers.ros.org/question/314903/ros2-optionally-launch-node-with-no-arguments/

Something similar as described in https://github.com/ros2/launch/issues/263 will solve the issue. Here the problem is not the need of conditional substitutions, but that the representation chosen for arguments is not good enough.

---

We'd also like this for gazebo_ros_pkgs. We currently have a hacky workaround on this PR using PythonExpression.

This can be solved by using SetLaunchConfiguration. e.g. this could be replaced with:

SetLaunchConfiguration('version-arg', '--version', if=LaunchConfiguration('version'))

and then instead of _command('version'), do:

LaunchConfiguration('version-arg', default=''),

We could have a "ternary-if" substitution too, like:

class ConditionalSubstitution(Substitution):
    def __init__(self, condition, true_value, else_value):
        ...

Which I agree, it would be nice to have it.

---

There are other cases where it's difficult to conditionally add an extra "entity". e.g.: parameters and environment variables.

IMO, the problem here is again the representation chosen for them: https://github.com/ros2/launch/blob/0007307e4f5ad26b0531ca7265965d09482c5d68/launch/launch/actions/execute_process.py#L92

If we would have a class for representing an environment variable:

class EnvironmentVariable: # Nothing to do with launch.substitutions.EnvironmentVariable
    def __init__(self, name, value, *, condition):
        ...

ExecuteProcess could take an Iterable[Union[EnvironmentVariable, THE_DICT_ABOVE]] instead of just a dict.

A similar solution is possible for parameters.

[AlexKaravaev]

Is there any update on this? Would find this pretty useful too.

[asasine]

Another similar idea I've had is to conditionally include substitutions in the parameters kwarg for a launch_ros.actions.Node. This will allow us to include YAML config files as parameters if they exist, otherwise skip them.

Our current solution employs a custom substitution to write empty YAML files to the temp directory. It would be excellent if we could provide a simple condition to a path such that the path is skipped when the node/process is later executed.

[nachovizzo]

Any update on this? Anyone who managed to setup an "optional" argument in ROS2?

[russkel]

Another similar idea I've had is to conditionally include substitutions in the parameters kwarg for a launch_ros.actions.Node. This will allow us to include YAML config files as parameters if they exist, otherwise skip them.

Our current solution employs a custom substitution to write empty YAML files to the temp directory. It would be excellent if we could provide a simple condition to a path such that the path is skipped when the node/process is later executed.

Have a look at SetParametersFromFile.

[slim71]

What about being able to call a launch file with no arguments from CLI? This is what I've thought of for now

spawn_x = LaunchConfiguration("spawn_x", default='0.0')
spawn_x_argument = DeclareLaunchArgument(
        name="spawn_x",
        default_value="0.0",
        description="Position along x-axis",
    )

but a conditional way to add an argument would be great. What I'm doing seems a bit redundant, since I have to specify a default in both the LaunchConfiguration and in the DeclareLaunchArgument...