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You also need to add your declared argument to the final LaunchDescription that you return.

You also One thing you need to do to expose the argument to the user is to add your declared argument it to the final LaunchDescription that you return.return:

return LaunchDescription([arg_n_drones, *l_include_drown_spawn])

One thing you need to do to expose the argument to the user is to add it to the final LaunchDescription that you return:

return LaunchDescription([arg_n_drones, *l_include_drown_spawn])

To use it in your launch file as a normal Python variable, you need to use OpaqueFunction:

https://answers.ros.org/question/415825/can-i-programmatically-call-a-launch-file-with-arguments-that-can-be-used-as-a-variable-for-python-operations/

https://answers.ros.org/question/416438/adding-node-from-within-opaquefunction-in-ros2-python-launch-file/

That latter one includes a concrete example of using a declared launch argument to do N things. There are a couple of ways to access its string value. You can use a LaunchConfiguration object inside your OpaqueFunction and use LaunchConfiguration.perform(context):

https://github.com/danzimmerman/dz_launch_examples/blob/rolling/launch/opaque_multi_nodes.launch.py#L9

or you can access it by name from the context.launch_configurations dictionary:

https://github.com/danzimmerman/dz_launch_examples/blob/rolling/launch/arg_examples.launch.py#L24

(This is what (LaunchConfiguration.perform(context) uses accesses this internally, see here).

One thing First you need to do to expose the add your declared argument to the user is to add it to the final LaunchDescription that you return:

return LaunchDescription([arg_n_drones, *l_include_drown_spawn])

To use it in your launch file as a normal Python variable, I believe you need to must use OpaqueFunction:

https://answers.ros.org/question/415825/can-i-programmatically-call-a-launch-file-with-arguments-that-can-be-used-as-a-variable-for-python-operations/

https://answers.ros.org/question/416438/adding-node-from-within-opaquefunction-in-ros2-python-launch-file/

That latter one includes a concrete example of using a declared launch argument to do N things. There are a couple of ways to access its string value. You can use a LaunchConfiguration object inside your OpaqueFunction and use LaunchConfiguration.perform(context):

https://github.com/danzimmerman/dz_launch_examples/blob/rolling/launch/opaque_multi_nodes.launch.py#L9

or you can access it by name from the context.launch_configurations dictionary:

https://github.com/danzimmerman/dz_launch_examples/blob/rolling/launch/arg_examples.launch.py#L24

(LaunchConfiguration.perform(context) accesses this internally, see here).

First you need to add your declared argument to the final LaunchDescription that you return:

return LaunchDescription([arg_n_drones, *l_include_drown_spawn])

To use it in your launch file as a normal Python variable, I believe you must use OpaqueFunction:

https://answers.ros.org/question/415825/can-i-programmatically-call-a-launch-file-with-arguments-that-can-be-used-as-a-variable-for-python-operations/

https://answers.ros.org/question/416438/adding-node-from-within-opaquefunction-in-ros2-python-launch-file/

That latter one includes a concrete example of using a declared launch argument to do N things. There are a couple of ways to access its string value. You can use a LaunchConfiguration object inside your OpaqueFunction and use LaunchConfiguration.perform(context):

https://github.com/danzimmerman/dz_launch_examples/blob/rolling/launch/opaque_multi_nodes.launch.py#L9

or you can access it by name from the context.launch_configurations dictionary:

https://github.com/danzimmerman/dz_launch_examples/blob/rolling/launch/arg_examples.launch.py#L24

(LaunchConfiguration.perform(context) accesses this internally, see here).

First you need to add your declared argument to the final LaunchDescription that you return:

return LaunchDescription([arg_n_drones, *l_include_drown_spawn])

To use it in your launch file as a normal Python variable, I believe you must use OpaqueFunction:

https://answers.ros.org/question/415825/can-i-programmatically-call-a-launch-file-with-arguments-that-can-be-used-as-a-variable-for-python-operations/

https://answers.ros.org/question/416438/adding-node-from-within-opaquefunction-in-ros2-python-launch-file/

That latter one second answer includes a concrete example of using a declared launch argument to do N things. There are a couple of ways to access its string value. You can use a LaunchConfiguration object inside your OpaqueFunction and use LaunchConfiguration.perform(context):

https://github.com/danzimmerman/dz_launch_examples/blob/rolling/launch/opaque_multi_nodes.launch.py#L9

or you can access it by name from the context.launch_configurations dictionary:

https://github.com/danzimmerman/dz_launch_examples/blob/rolling/launch/arg_examples.launch.py#L24

(LaunchConfiguration.perform(context) accesses this internally, see here).