subscriber callback in a python class is overwriting the class variables with the most recent data before the first callback processing finishes.

edit

I am on ROS Melodic. When a subscriber is receiving data at a faster rate than it could process, the class variables are being updated before the callback process is finished. For example, when I run below subscriber. When the first callback is triggered tracker is set to 1 and after 5 seconds it was supposed to print 1, now before the completion of these 5 seconds, the subscriber receives another data, the call back will update tracker to 2. Now, this value 2 is printed twice instead of 1 and then 2. I am trying to say that the tracker is being set to most recent callback value before even the print process has been finished.

class listener:
    def printer(self):
        rospy.sleep(5)
        print (self.tracker)
        print("Look Above for value")

    def callback(self, data):
        rospy.loginfo("I heard %s", data)
        self.tracker = self.tracker + 1
        self.printer()

    def __init__(self):

        rospy.init_node('listener', anonymous=True)
        self.tracker = 0
        rospy.Subscriber('/path_query', PathQuery, self.callback,
                         queue_size=1)
        rospy.spin()

if __name__ == '__main__':
    listener()