ROS: saving object in a file when a ros node is killed - python-2.7

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?

Answer 1

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).

Answer 2

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)