ROS: saving object in a file when a ros node is killed

Question

I am running a rosnode with a kalman filter running. The kalman filter is an object with states that get updated as time plays out. Conventionally, a ros node has a run(self) method that runs at a specified frequency using the while condition

while not rospy.is_shutdown():
    do this

Going through each loop my kalman filter object updates. I just want to be able to save the kalman filter object when the node is shutdown either some external condition or when the user presses ctrl+C. I am not able to do this. In the run(self) method, I tried

while not rospy.is_shutdown():
    do this

# save in file
output = pathlib.Path('path/to/location')
results_path = output.with_suffix('.npz')
with open(results_path, 'xb') as results_file:
    np.savez(results_file,kfObj=kf_list)

But it has not worked. Is it not executing the save command? If ctrl+C is pressed does it stop short of executing it? Whats the way to do it?

Possible duplicate of [Saving an Object (Data persistence)](https://stackoverflow.com/questions/4529815/saving-an-object-data-persistence) — Benyamin Jafari, Nov 27 '19 at 06:16

score 0 · Answer 1 · answered Nov 24 '19 at 23:30

0

Check out the atexit module:

http://docs.python.org/library/atexit.html

import atexit

def exit_handler():
    output = pathlib.Path('path/to/location')
    results_path = output.with_suffix('.npz')
    with open(results_path, 'xb') as results_file:
    np.savez(results_file,kfObj=kf_list

atexit.register(exit_handler)

Just be aware that this works great for normal termination of the script, but it won't get called in all cases (e.g. fatal internal errors).

answered Nov 24 '19 at 23:30

prhmma

843
10
18

prhmma...thanks for the reply. But where should it be included? in the function whose termination should trigger this. It did not do anything when i simply added it to the run function – Zero Nov 24 '19 at 23:40
it triggers when your main function ends or when you ctrl+c to terminate your program. you should include it in your main function and store data from your specific function – prhmma Nov 24 '19 at 23:43

angelos.p · Answer 2 · 2019-12-01T09:27:13.250

Why not try the following python example class structure engaging shutdown hooks :

import rospy

class Hardware_Interface:
    def __init__(self, selectedBoard):

        ...

        # Housekeeping, cleanup at the end
        rospy.on_shutdown(self.shutdown)

        # Get the connection settings from the parameter server
        self.port = rospy.get_param("~"+self.board+"-port", "/dev/ttyACM0")

        # Get the prefix
        self.prefix = rospy.get_param("~"+self.board+"-prefix", "travel")

        # Overall loop rate
        self.rate = int(rospy.get_param("~rate", 5))
        self.period = rospy.Duration(1/float(self.rate))

        ...

    def shutdown(self):
        rospy.loginfo("Shutting down Hardware Interface Node...")

        try:
            rospy.loginfo("Stopping the robot...")

            self.controller.send(0, 0, 0, 0)
            #self.cmd_vel_pub.publish(Twist())
            rospy.sleep(2)
        except:
            rospy.loginfo("Cannot stop!")

        try:
            self.controller.close()
        except:
            pass
        finally:
            rospy.loginfo("Serial port closed.")
            os._exit(0)

This just an extract from a personal script, please modify it to your needs. I imagine that the on_shutdown will do the trick. Another similar approach comes from my friends in the Robot Ignite Academy in The Construct and seems like that

#!/usr/bin/env python

import rospy
from geometry_msgs.msg import Twist

class my_class():

    def __init__(self):
        ...
        self.cmd = Twist()
        self.ctrl_c = False
        self.rate = rospy.Rate(10) # 10hz
        rospy.on_shutdown(self.shutdownhook)

    def publish_once_in_cmd_vel(self):

        while not self.ctrl_c:
            ...


    def shutdownhook(self):
        # works better than the rospy.is_shutdown()
        self.ctrl_c = True

    def move_something(self, linear_speed=0.2, angular_speed=0.2):
        self.cmd.linear.x = linear_speed
        self.cmd.angular.z = angular_speed
        self.publish_once_in_cmd_vel()

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

This is obviously a sample of their code (for more please join the academy)

ROS: saving object in a file when a ros node is killed

2 Answers2