[SOLVED] ROS: Can't publish on Topic using python

Issue

I'm new in ROS and I'm developing a little project for an exam. I just finished the project, but I would change a little thing: instead of publish a message on a Topic using " rostopic pub /start std_msgs/String "start" " command in Ubuntu, I'd like to do it using a python code as shown:

#!/usr/bin/env python
from __future__ import print_function
from std_msgs.msg import String
import sys
import rospy

def start_stop(command):
    pub = rospy.Publisher('start', String, queue_size=10)
    command_str = String()
    command_str.data = command
    pub.publish(command_str)
    rospy.loginfo("OK")

if __name__ == "__main__":
    rospy.init_node('utility')

    executed = False
    if len(sys.argv) == 3:
        if sys.argv[1].lower() == "command":
            start_stop(sys.argv[2])
            executed = True
        elif ....
             ....
        else:
            executed = False
    else:
        executed = False

    if executed:
         print("Command sent")
    else:
         print("Command not sent.")

Now, if I use in ubuntu terminal " rosrun simulazione_traffico utility.py command start " command, I can see "OK" and "Sent" on the terminal. But, using " rostopic echo /start ", I can't see anything, so the String has not been sent to the Topic. Can you tell me why? I really don't know if I'm missing something important.

Thank you so much.

Solution

You are not "spinning" your ros node. You need to add something like this to your main function:

rate = rospy.Rate(10) # 10hz
while not rospy.is_shutdown():
    rate.sleep()

See here for details: https://wiki.ros.org/ROS/Tutorials/WritingPublisherSubscriber%28python%29

Answered By - Christian Fritz

Answer Checked By - Mary Flores (WPSolving Volunteer)