how to launch a launch file from python code

edit

I use the following line

...
os.system("roslaunch turtlebot_follower follower.launch")
...

to be able to use os.system, you need to import corresponding package

import os

Something like this would work

  import roslaunch

  uuid = roslaunch.rlutil.get_or_generate_uuid(None, False)
  roslaunch.configure_logging(uuid)
  launch = roslaunch.parent.ROSLaunchParent(uuid, [file_path])
  launch.start()

I am using the following frequently, for instance in ros tests.

import roslaunch
package = 'package_name'
executable = 'node.py'
node_name = 'nodename'
node = roslaunch.core.Node(package=package, node_type=executable, name=node_name, output='screen')
launch = roslaunch.scriptapi.ROSLaunch()
launch.start()
process = launch.launch(node)

The only problem I still couldn't solve is, how to launch nodes outside of the main thread.

[EDIT]

I have now also solved the problem about launching nodes outside the main thread within callbacks. The following code snippet illustrates the solution. It also shows, how you can start nodes with launch file programmatically. However, instructing the command line programmatically is not nice but the only way I was able to make it work.

import rospy
import os
import rospy
import rospkg
import subprocess
import roslaunch
from std_srvs.srv import Trigger, TriggerResponse

def start_node_direct():
    """
    Does work as well from service/topic callbacks directly using rosrun
    """    
    package = 'YOUR_PACKAGE'
    node_name = 'YOUR_NODE.py'

    command = "rosrun {0} {1}".format(package, node_name)

    p = subprocess.Popen(command, shell=True)

    state = p.poll()
    if state is None:
        rospy.loginfo("process is running fine")
    elif state < 0:
        rospy.loginfo("Process terminated with error")
    elif state > 0:
        rospy.loginfo("Process terminated without error")


def start_node2():
    """
    Does work as well from service/topic callbacks using launch files
    """    
    package = 'YOUR_PACKAGE'
    launch_file = 'YOUR_LAUNCHFILE.launch'

    command = "roslaunch  {0} {1}".format(package, launch_file)

    p = subprocess.Popen(command, shell=True)

    state = p.poll()
    if state is None:
        rospy.loginfo("process is running fine")
    elif state < 0:
        rospy.loginfo("Process terminated with error")
    elif state > 0:
        rospy.loginfo("Process terminated without error")

def start_node():
    """
    Does not work if called from service/topic callbacks due to main signaling issue
    """
    package = 'YOUR_PACKAGE'
    launch_file = 'YOUR_LAUNCHFILE.launch'
    uuid = roslaunch.rlutil.get_or_generate_uuid(None, False)
    roslaunch.configure_logging(uuid)
    launch_file = os.path.join(rospkg.RosPack().get_path(package), 'launch', launch_file)
    launch = roslaunch.parent.ROSLaunchParent(uuid, [launch_file])
    launch.start()

def service_callback(req):

    # edit below for other options
    #start_node()
    #start_node2()
    start_node_direct()

    return TriggerResponse(success=True)

if __name__ == '__main__':
    rospy.init_node('test1', anonymous=True)

    service = rospy.Service('launch', Trigger, service_callback)

    rospy.spin()

In ROS 2 you need to follow these steps:

1. Create a LaunchDescription object. If you have seen a regular python launch file, this is what generate_launch_description() returns. This object contains information about nodes to be launched, etc.

2. Create a LaunchService object from launch python module.

3. Pass the LaunchDescription to LaunchService using include_launch_description method of the LaunchService instance.

4. Use run method of the LaunchService. (Use more advanced functions if you need async, etc. See more info here.)

Here's an example:

import launch
from launch.substitutions import Command
import launch_ros


def generate_launch_description():
    model_path = '/path/to/my/robot.urdf'

    robot_state_publisher_node = launch_ros.actions.Node(
        package='robot_state_publisher',
        executable='robot_state_publisher',
        parameters=[{'robot_description': Command(['xacro ', model_path])}]
    )

    spawn_entity = launch_ros.actions.Node(
        package='gazebo_ros', 
        executable='spawn_entity.py',
        arguments=['-entity', 'sam_bot', '-topic', 'robot_description'],
        output='screen'
    )

    return launch.LaunchDescription([
        launch.actions.ExecuteProcess(cmd=['gazebo', '--verbose', '-s', 'libgazebo_ros_factory.so'], output='screen'),
        robot_state_publisher_node,
        spawn_entity,
    ])


def main():
    launch_description = generate_launch_description()
    launch_service = launch.LaunchService()
    launch_service.include_launch_description(launch_description)
    launch_service.run()


if __name__ == '__main__':
    main()