Callback function not changing the value of Global variable

Question

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)

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Originally posted by Sufyan on ROS Answers with karma: 17 on 2018-08-27

Post score: 0

Answer 1

This is most likely a Python problem, not a ROS problem.

The start in your callback(msg) method is a different variable. The name is the same, but that variable shadows the start in the global scope. To refer to the global start, add global before it.

See Python Global, Local and Nonlocal variables (random Google search result) for some more explanation.

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Originally posted by gvdhoorn with karma: 86574 on 2018-08-27

This answer was ACCEPTED on the original site

Post score: 4

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Original comments

Comment by Sufyan on 2018-08-27:
Yes you are right it solved the problem. Thanks a lot!!!