Using the ROSlaunch Python API

The ROSlaunch API allows you to launch nodes (and other launch files) from inside your own nodes! The official documentation for the roslaunch API is the best place to go to find out how to use this, but here's a quick example, if you're short on time.

Recall the SLAM Exercise from Week 3. Here we had SLAM running in the background, while we drove our robot around a simulated environment (using the turtlebot3_teleop node). Once we'd generated a full map we used a rosrun command (in the terminal) to save a copy of this map:

rosrun map_server map_saver -f {map name}

Suppose we wanted to do this programmatically instead...

The following is an example of a very simple ROS node that does just that. Once the code below is executed it will launch the map_saver node from the map_server package in exactly the same way as above (but using the ROSlaunch API instead). The key difference here is that instead of {map name}, we need to define the full file path for the map that we want to generate.

map_saver.py

#!/usr/bin/env python3

import roslaunch
import rospy

map_path = "path/to/my_map"

rospy.init_node("map_getter", anonymous=True)

launch = roslaunch.scriptapi.ROSLaunch()
launch.start()

print(f"Saving map at time: {rospy.get_time()}...")
node = roslaunch.core.Node(package="map_server",
                           node_type="map_saver",
                           args=f"-f {map_path}")
process = launch.launch(node)

You could wrap this into one of your own ROS nodes, or even build a standalone node inside your package to call the map_saver node when required. Perhaps you could even wrap the last three of the above commands inside a while loop and get the node to update a map file at regular intervals (say 5 seconds?) while your robot explores its environment...