Callback function not changing the value of Global variable

edit

i am using ar track alvar to recieve co ordinates from my camera... The data seems to be fine but i want to use this data.For this i made a subscriber as below

start = (0,0) 

def callback(msg):
    x=msg.pose.position.x
    y=msg.pose.position.y
    rospy.loginfo(rospy.get_caller_id() + "I heard x = %f", x)
    rospy.loginfo(rospy.get_caller_id() + "I heard  y= %f", y)XPos1 =x
    YPos1 =y  
    XPos1 += 0.11
    YPos1 += 0.1
    XPos1 = XPos1 * 22.69
    YPos1 = YPos1 * 24
    XPos1 = round(XPos1) -1
    YPos1 = round(YPos1) 
    XPos1 =int(XPos1)
    YPos1= int(YPos1)    
    start = (XPos1,YPos1)
    print start


print start

where start is a global variable which i initialize in the begining as start(0,0) but when my code is executed even though this call back function is invoked as i can see the value of x and y on terminal and the value of start ( which is a global variable) is also what i expected but as soon as the program exits callback function the the last print statement prints (0,0) as the value of start (which is what i initialized start variable to in the begining of program as global variable) This is the Output

[INFO] [1535398460.244633]: /listener_11195_1535398438793I heard x = 1.294724
[INFO] [1535398460.250098]: /listener_11195_1535398438793I heard  y= -0.089722

(31, 0)
^C(0, 0)