A few days ago I started writing a series of tutorials that ease the work of beginners in ROS. The first tutorial was about a template for a publisher node, and the second tutorial was about a template for a subscriber node in ROS.
Today, I continue the series of tutorials with a template for a publisher and a subscriber node in Python. In addition, I’ll test it by writing and running a publisher and subscriber node.
Template for a Simple ROS Subscriber and Publisher in Python (image source)
Copy the template into a ‘. py’ file, delete the information that you don’t need and replace the text in capital letters.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 | #ROS Node Subscriber & Publisher template #!/usr/bin/env python #remove or add the library/libraries for ROS import rospy, time, math, cv2, sys #remove or add the message type from std_msgs.msg import String, Float32, Image, LaserScan, Int32 varS=None #define function/functions to provide the required functionality def fnc_callback(msg): global varS varS==do_something(msg.data) if __name__=='__main__': #Add here the name of the ROS. In ROS, names are unique named. rospy.init_node('NODE_NAME') #subscribe to a topic using rospy.Subscriber class sub=rospy.Subscriber('TOPIC_NAME', THE_TYPE_OF_THE_MESSAGE, fnc_callback) #publish messages to a topic using rospy.Publisher class pub=rospy.Publisher('TOPIC_NAME', THE_TYPE_OF_THE_MESSAGE, queue_size=1) rate=rospy.Rate(10) while not rospy.is_shutdown(): if varS<= var2: varP=something() else: varP=something() pub.publish(varP) rate.sleep() |
I used the above template to write a ROS node that will display the random numbers received from another node, then I add some conditional statements, and finally I publish the results.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 | #!/usr/bin/env python import rospy from std_msgs.msg import Int32 varS=None def fnc_callback(msg): global varS varS=msg.data if __name__=='__main__': rospy.init_node('NODE_SUB_AND_PUB') sub=rospy.Subscriber('rand_no', Int32, fnc_callback) pub=rospy.Publisher('sub_pub', Int32, queue_size=1) rate=rospy.Rate(10) while not rospy.is_shutdown(): if varS<= 2500: varP=0 else: varP=1 pub.publish(varP) rate.sleep() |
To run the above node, navigate to the .py file and make it executable. The command is:
1 | chmod u+x my_python_file.py |
After the file is executable, you can run the node.
Step 1: open a new Terminal and run the command:
1 | roscore |
Step 2: open a new Terminal and run the publisher node with the following command:
1 | rosrun your_package your_ros_node_that_generates_random_number.py |
Step 3: open a new Terminal and run the publisher and subscriber node with the following command:
1 | rosrun your_package your_ros_node_for_publisher_and_subscriber.py |
aren’t you missing ros::spin()