How to use variable's value (subscriber function) in another function

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Hello all !

I want to pass the variable which is set to its value when called the callback function. and then want to use the value of the variable in another function.

def where1(msg):
    global x1
    x1 = msg.data

def where2(gsm):
    global y1
    y1 = gsm.data
def ground():
    global x1
    global y1
    rospy.init_node("ground")
    rospy.Subscriber("/box_positiona", Float64, where1)
    rospy.Subscriber("/box_positionb", Float64, where2)

    print x1
    print y1
    rospy.spin() # this will block untill you hit Ctrl+C

if __name__ == '__main__':
    try:
            ground()
    except rospy.ROSInterruptException:
            rospy.loginfo("Ground  motion node is shut down.")

But i got the following error print x1 NameError: global name 'x1' is not defined