How to stop a turtle moving in circular path after reaching its initial position

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#!/usr/bin/env python

import rospy

from geometry_msgs.msg import Twist

from turtlesim.msg import Pose

import sys

robot_x = 0 distance = 0

def pose_callback(pose): global robot_x

rospy.loginfo("Robot X = %f\n",pose.x)

robot_x = pose.x

def move_turtle(lin_vel,ang_vel):

global robot_x

rospy.init_node('move_turtle', anonymous=True)
pub = rospy.Publisher('/turtle1/cmd_vel', Twist, queue_size=10)

rospy.Subscriber('/turtle1/pose',Pose, pose_callback)

rate = rospy.Rate(10) # 10hz

vel = Twist()
while not rospy.is_shutdown():

vel.linear.x = lin_vel
vel.linear.y = 0
vel.linear.z = 0

vel.angular.x = 0
vel.angular.y = 0
vel.angular.z = ang_vel



    rospy.loginfo("Linear Vel = %f: Angular Vel = %f",lin_vel,ang_vel)
    global distance  
    distance = 2*(3.1415)*(lin_vel/ang_vel)

    if(robot_x == distance):

         rospy.loginfo("Robot Reached destination")
     rospy.logwarn("Stopping robot")

         break


    pub.publish(vel)

    rate.sleep()

if __name__ == '_main_': try: move_turtle(float(sys.argv[1]),float(sys.argv[2])) except rospy.ROSInterruptException: pass