Acting on information in a custom message on ROS-CodePudding

I have done the tutorials which shows you how to publish and subscribe to messages in ROS.

I have now set up a message talker and listener in a file, but this is where I am running into some issues. And this may be more of a Python issue than a ROS issue.

My message at the minute publishes a '0'. When the one is published, I want my turtle to complete a set action, however, I am having trouble extracting that 1 from the published message.

What I am trying to do is use that 0 in an if-statement. So, if 0, do this, if not, do that.

It was suggested that I make the information in my listener callback function that I want to extract to be global. This doesn't seem to have worked.

I am hoping someone can point out where I am going wrong, or suggest a path forward.

My code is below:

#!/usr/bin/env python

import rospy
from com760_week4_ws_pkg.msg import CustomChat
from std_msgs.msg import Header
from time import gmtime
from time import strftime

def callback(data):
    global name
    chatter = data.data
    header = data.header
    name = data.name
    timestamp = header.stamp.to_sec()
    rospy.loginfo("The message passed by %s is %s, sent at %s" % (name, chatter, strftime("%H:%M:%S", gmtime(timestamp))))
    
def listener():
    rospy.Subscriber("message_chatter", CustomChat, callback)

def talker(count):
    pub = rospy.Publisher("message_chatter", CustomChat, queue_size=10)
    
    msg = CustomChat()
    msg.name = "The ROS User xxxx"
    

    msg.header.stamp = rospy.Time.now()
    msg.data = "-HELLO, ROS-"
    pub.publish(msg)
    rospy.sleep(2)
    listener()
        
if __name__ == '__main__':
    rospy.init_node("chatter", anonymous=True)
    count = 0
    while not rospy.is_shutdown():
        talker(count)
        print(name)

When I run this I get an error saying

line 42, in <module>
    print(name)
NameError: global name 'name' is not defined

I did try adding name = " " at the top of my code, but when I got to the print at the bottom, all it did was print the empty string.

CodePudding user response：

One major problem you have is recreating the subscriber and publisher every time time you call talker. These should only be created once; preferably when you init the node.

As per your comments a couple more changes are needed. First you're getting the error because name will only be defined as global when the callback is called first; this is unlikely to happen before your first print so you get the undefined error. To fix this you can simple declare name as a global variable outside of your functions. This will create the variable when your script is loaded and take care of the error. For example:

...
from time import strftime

name = ""    

def callback(data):
    global name
    ...