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Inconsistent syntax for arg tag in XML launch #720

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mjforan opened this issue Jul 13, 2023 · 3 comments · May be fixed by #848
Open

Inconsistent syntax for arg tag in XML launch #720

mjforan opened this issue Jul 13, 2023 · 3 comments · May be fixed by #848
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@christophebedard
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enhancement New feature or request

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@mjforan
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mjforan commented Jul 13, 2023

Bug report

Required Info:

  • Operating System:
    • Ubuntu 22.04
  • Installation type:
    • Humble
    • Binaries
    • ros-humble-desktop/jammy,now 0.10.0-1jammy.20230624.051647 amd64

Steps to reproduce issue

Example section of XML launch file for starting Gazebo simulator

<arg name="world_name"      default="warehouse"/>  <!-- world name -->
<!-- Launch Ignition Gazebo --> 
<include file="/launch/ign_gazebo.launch.py">
    <arg name="ign_args" value="/config/worlds/$(var world_name).sdf $(var use_levels)"/>
</include>

Expected behavior

According to the launch migration guide, <arg> should be used with default and <let> should be used with value. However, When arg is nested under an include tag it is necessary to use the value instead. You can see in the example how arg has two different syntaxes, depending on the context.

Actual behavior

Attempting to use the suggested syntax triggers the following error:

Caught exception when trying to load file of format [xml]: Attribute value of type <class 'str'> not found in Entity arg
@denseishin
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I'm experiencing the same under Ubuntu 24.04 & ROS2 Jazzy

@christophebedard
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This is still the case today in all distros. I was also surprised by this when I worked on ros2/ros2_documentation#5104.

The simple answer is that the code indeed expects <arg> entities with name and value attributes, but it doesn't actually parse them (using parser.parse_action(e) for e in entity.children, otherwise they would be actual <arg>/DeclareLaunchArgument actions):

launch/launch/launch/actions/include_launch_description.py

Lines 91 to 101 in 6fcc79e

args = entity.get_attr('arg', data_type=List[Entity], optional=True)
if args is not None:
kwargs['launch_arguments'] = [
(
parser.parse_substitution(e.get_attr('name')),
parser.parse_substitution(e.get_attr('value'))
)
for e in args
]
for e in args:
e.assert_entity_completely_parsed()
. It then creates SetLaunchConfiguration actions from those, which are equivalent to the frontend's let action, not arg (DeclareLaunchArgument):

launch/launch/launch/actions/include_launch_description.py

Lines 187 to 188 in 6fcc79e

for name, value in self.launch_arguments:
set_launch_configuration_actions.append(SetLaunchConfiguration(name, value))
.

Examples: https://docs.ros.org/en/rolling/Tutorials/Intermediate/Launch/Using-Substitutions.html#parent-launch-file

XML 
<launch>
  <let name="background_r" value="200"/>
  <include file="$(find-pkg-share launch_tutorial)/launch/example_substitutions_launch.xml">
    <arg name="turtlesim_ns" value="turtlesim2" />
    <arg name="use_provided_red" value="True" />
    <arg name="new_background_r" value="$(var background_r)" />
  </include>
</launch>
YAML 
launch:
  - let:
      name: 'background_r'
      value: '200'
  - include:
      file: '$(find-pkg-share launch_tutorial)/launch/example_substitutions_launch.yaml'
      arg:
        - name: 'turtlesim_ns'
          value: 'turtlesim2'
        - name: 'use_provided_red'
          value: 'True'
        - name: 'new_background_r'
          value: '$(var background_r)'

Now as to why it does this, I believe it's because ROS 1's launch XML <include> used <arg>: https://wiki.ros.org/roslaunch/XML/include#Elements. ROS 1 also didn't have <let>.

I think it could probably simply also accept <let>.

@christophebedard christophebedard linked a pull request Mar 24, 2025 that will close this issue
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@christophebedard
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I think it could probably simply also accept <let>.

I've implemented that in #848

@christophebedard christophebedard added the enhancement New feature or request label Apr 3, 2025
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@christophebedard christophebedard

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Allow providing launch args to include using let in frontends ros2/launch
4 participants
@mjcarroll @christophebedard @mjforan @denseishin