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Here is the program I have written. The function is to make the turtlesim complete one revolution, with a fixed angular velocity and speed.

#!/usr/bin/env python

import rospy
from geometry_msgs.msg import Twist
from turtlesim.msg import Pose
from math import *


def pose_callback(msg):
    print(msg.x,msg.y,msg.theta)
    return 


def main():

    speed = 10
    angular_vel= 5

    rospy.init_node('turtle_revolve', anonymous=True)
    rate= rospy.Rate(10)
    velocity_publisher = rospy.Publisher('/turtle1/cmd_vel', Twist, queue_size=10)
    vel_msg = Twist()

    rospy.Subscriber("/turtle1/pose", Pose, pose_callback)
    posi = Pose()
    
#Setting the time of the start of the node to be t1
    t1= rospy.Time.now().to_sec()
#Initial positions x and y    
    x = posi.x
    y=  posi.y
    vel_msg.linear.x = speed
    vel_msg.angular.z = angular_vel
    rospy.logwarn(posi.x)
    rospy.logwarn(posi.y)

    while not rospy.is_shutdown():
           
        velocity_publisher.publish(vel_msg)

#Another time measurement t2 is taken for every iteration
    
    t2 = rospy.Time.now().to_sec()

# difference in time measurements t1 and t2 
 
    t_diff = t2 - t1

#If the time delay between measurements is greater than 0.5 secs and theta has reached 0 ,
#it means that the circle has been completed 
    
    if (t_diff >0.5  and posi.theta == 0.0 ): 
        
        
        break
        
    rospy.logwarn("Completing circle")
    
    

if __name__ == '__main__':
    try:
        main()
    except rospy.ROSInterruptException:
        pass





Now my reasoning here is that theta will become zero again when it reaches the initial position and x, y should be equal to the initial values. I tried to find out x, y with posi = Pose () , after subscribing to turtle1/Pose . However all values in posi, i.e x,y, theta etc are zero. Ive confirmed this with multiple print/logwarn statements that i removed. It seems posi is returning zero values continuosly.

However I have confirmed using rostopic in the terminal that the Pose topic is printing values. Furthermore the pose values are correctly being displayed by the pose_callback function. So the zero values are only in the main function. Could anyone point out anything wrong in the code ? Why does posi have all values zero all the time ?

Here is the output showing zero in logwarn and proper values for the pose_callback function : Terminal output

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The position values look like they're getting to the callback okay, but then you're not doing anything with them in the callback. I can't see where posi.x or posi.y are defined, but what if instead of:

def pose_callback(msg):
    print(msg.x,msg.y,msg.theta)
    return 

you had:

def pose_callback(msg):
    posi.x = msg.x
    posi.y = msg.y
    print(posi.x,posi.y,msg.theta)
    return 
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