Class 2: "name 'launch_ros' is not defined"

Course Support ROS2 In 5 Days Python
1

Hi!
I am having issues when launching the exercise 2.1 on ROS2.

The launch file is called my_package_launch_file.launch.py and it contains:

from launch import LaunchDescription

from launch_ros.actions import Node, LoadComposableNodes

def generate_launch_description():

return LaunchDescription([

    launch_ros.actions.Node(

        package='my_package',

        executable='simple',

        output='screen')

])

Then I launch it using the following command line:
user:~/ros2_ws$ ros2 launch my_package my_package_launch_file.launch.py

I’ve already build the project using colcon build, and then source install/setup.bash

[INFO] [launch]: All log files can be found below /home/user/.ros/log/2021-10-16-22-52-21-926965-1_xterm-4197
[INFO] [launch]: Default logging verbosity is set to INFO
Task exception was never retrieved
future: <Task finished name=‘Task-2’ coro=<LaunchService._process_one_event() done, defined at /opt/ros/foxy/lib/python3.8/site-packages/launch/launch_service.py:271> exception=InvalidLaunchFileError(‘py’)>
Traceback (most recent call last):
File “/opt/ros/foxy/lib/python3.8/site-packages/launch/launch_description_sources/any_launch_file_utilities.py”, line 53, in get_launch_description_from_any_launch_file
return loader(launch_file_path)
File “/opt/ros/foxy/lib/python3.8/site-packages/launch/launch_description_sources/python_launch_file_utilities.py”, line 68, in get_launch_description_from_python_launch_file
return getattr(launch_file_module, ‘generate_launch_description’)()
File “/home/user/ros2_ws/install/my_package/share/my_package/my_package_launch_file.launch.py”, line 6, in generate_launch_description
launch_ros.actions.Node(
NameError: name ‘launch_ros’ is not defined

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
File “/opt/ros/foxy/lib/python3.8/site-packages/launch/launch_service.py”, line 273, in _process_one_event
await self.__process_event(next_event)
File “/opt/ros/foxy/lib/python3.8/site-packages/launch/launch_service.py”, line 293, in __process_event
visit_all_entities_and_collect_futures(entity, self.__context))
File “/opt/ros/foxy/lib/python3.8/site-packages/launch/utilities/visit_all_entities_and_collect_futures_impl.py”, line 45, in visit_all_entities_and_collect_futures
futures_to_return += visit_all_entities_and_collect_futures(sub_entity, context)
File “/opt/ros/foxy/lib/python3.8/site-packages/launch/utilities/visit_all_entities_and_collect_futures_impl.py”, line 45, in visit_all_entities_and_collect_futures
futures_to_return += visit_all_entities_and_collect_futures(sub_entity, context)
File “/opt/ros/foxy/lib/python3.8/site-packages/launch/utilities/visit_all_entities_and_collect_futures_impl.py”, line 38, in visit_all_entities_and_collect_futures
sub_entities = entity.visit(context)
File “/opt/ros/foxy/lib/python3.8/site-packages/launch/action.py”, line 108, in visit
return self.execute(context)
File “/opt/ros/foxy/lib/python3.8/site-packages/launch/actions/include_launch_description.py”, line 125, in execute
launch_description = self.__launch_description_source.get_launch_description(context)
File “/opt/ros/foxy/lib/python3.8/site-packages/launch/launch_description_source.py”, line 84, inget_launch_description
self._get_launch_description(self.__expanded_location)
File “/opt/ros/foxy/lib/python3.8/site-packages/launch/launch_description_sources/any_launch_description_source.py”, line 53, in _get_launch_description
return get_launch_description_from_any_launch_file(location)
File “/opt/ros/foxy/lib/python3.8/site-packages/launch/launch_description_sources/any_launch_file_utilities.py”, line 56, in get_launch_description_from_any_launch_file
raise InvalidLaunchFileError(extension, likely_errors=exceptions)
launch.invalid_launch_file_error.InvalidLaunchFileError: Caught exception when trying to load file of format [py]: name ‘launch_ros’ is not defined

Any idea what can I do with this issue?
Thank you!

2

Hi Tomas,

Is your ros2 launch command autocompleting with tab while you are typing it? If not, then it probably means that your package wasn’t configured properly.

I think you could try adding Node() without the preceding launch_ros.actions, and make sure the setup for your .launch.py is correct, following the instructions step by step.

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