Add pathlib.Path to SomeSubstitutionsType

[DLu]

I'm making this change relatively blindly as a way to start discussion.

In ament/ament_index#73 I added the ability to load the path to a package as a pathlib.Path. I'd like to be able to directly use that in my launch files without manually converting to str. Consider the following example:

from ament_index_python.packages import get_package_share_path

from launch import LaunchDescription
from launch.actions import IncludeLaunchDescription
from launch.launch_description_sources import PythonLaunchDescriptionSource

def generate_launch_description():
    pkg_path = get_package_share_path('super_launch_package')
    launch_py_1 = PythonLaunchDescriptionSource(str(pkg_path / 'launch/launch_1.launch.py'))
    launch_py_2 = PythonLaunchDescriptionSource(str(pkg_path / 'launch/launch_2.launch.py'))
    ld.add_action(IncludeLaunchDescription(launch_py_1))
    ld.add_action(IncludeLaunchDescription(launch_py_2))
    return ld

Questions I have:

• Where are there repercussions of this that I'm missing?
• Where should tests be added?

[hidmic]

@DLu I see the convenience and simplicity of supporting implicit str coercions, but I wonder if we should open the door to (even) more opaque launch descriptions to spare code in some actions' __init__. I.e. PythonLaunchDescriptionSource could take a substitution or a path, and that'd yield the same UX.

[DLu]

@hidmic The other place where I want to use it is in specifying parameter files to load, a la

parameters = [[pkg_path, 'config', 'ompl_planning.yaml']]

[hidmic]

@DLu hmm, would it work if a pathlib.Path instance like pkg_path / 'config' / 'ompl_planning.yaml' was accepted?

[DLu]

@hidmic Yes, but I was also thinking about the wacky inclusion stuff that MoveIt configs sometimes do and so a pathlib might be combined with a LaunchConfiguration.

[hidmic]

Hmm, good point. @wjwwood @ivanpauno WDYT?

[ivanpauno]

Hmm, good point. @wjwwood @ivanpauno WDYT?

SomeSubstitutionsType is also used for things that are not a path, so I don't think we should modify that.

We could have add a SomePathType definition, similar to what's proposed here.
We would also need a method to resolve that SomePathType "substitution" when a context is provided.

[DLu]

I'm closing this for now, as calling str() is not too onerous.