ROS 2 YAML launch: "choices" is not supported in arg

[rafal-gorecki]

Description:

Using the "choices" key in an arg inside a ROS 2 YAML launch file causes a runtime error, but this limitation is not clearly documented.

Steps to reproduce:

1. Create YAML launch file:

launch:
  - arg:
      name: rviz
      default: "true"
      description: "Launch RViz2"
      choices: ["true", "false"]

2. Run:

ros2 launch <package> <file>.yaml

Observed behavior:

Launch fails with:

ValueError: Unexpected key(s) found in `arg`: {'choices'}

Expected behavior:

• Support "choices" in YAML launch files

Environment:

• ROS 2: Jazzy

[christophebedard]

This is totally not well documented (or even tested/used anywhere in the ROS 2 core), but looking at the implementation, the right syntax is:

launch:
  - arg:
      name: rviz
      default: "true"
      description: "Launch RViz2"
      choice:
        - value: "true"
        - value: "false"

and the XML syntax is:

<launch>
    <arg name="rviz" default="true" description="Launch RViz2">
        <choice value="true"/>
        <choice value="false"/>
    </arg>
</launch>

We should:

1. Include a full YAML example and improve the XML example in the underlying DeclareLaunchArgument Python class docstring:

   launch/launch/launch/actions/declare_launch_argument.py

   Lines 99 to 107 in 57c1aee

   	.. code-block:: xml
   	<launch>
   	<arg name="simple_argument"/>
   	<arg name="with_default_value" default_value="default"/>
   	<arg name="with_default_and_description" default_value="some_default"
   	description="this argument is used to configure ..."/>
   	</launch>
   	"""

2. Add a YAML test for it in launch_yaml similar to this XML test:

   launch/launch_xml/test/launch_xml/test_arg.py

   Lines 41 to 58 in 57c1aee

   	def test_arg_with_choices():
   	xml_file = \
   	"""\
   	<launch>
   	<arg name="my_arg" default="asd" description="something">
   	<choice value="asd"/>
   	<choice value="bsd"/>
   	</arg>
   	</launch>
   	"""
   	xml_file = textwrap.dedent(xml_file)
   	root_entity, parser = load_no_extensions(io.StringIO(xml_file))
   	ld = parser.parse_description(root_entity)
   	arg = ld.entities[0]
   	assert 'my_arg' == arg.name
   	assert 'asd' == ''.join([x.perform(None) for x in arg.default_value])
   	assert "something. Valid choices are: ['asd', 'bsd']" == arg.description
   	assert ['asd', 'bsd'] == arg.choices

3. Add examples somewhere in the ROS 2 docs

Any help would be appreciated!

[mjcarroll]

Oh, @christophebedard and I looked at this at the same time...

Well I added a test that demonstrates in #941

[christophebedard]

ahh you had assigned it to yourself, sorry!

[rafal-gorecki]

Thank you for the quick response and the link where it is implemented.