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if __name__ == "__main__":
    listener()
    while not rospy.is_shutdown():
        print('inside main')
        rospy.spin()
My problem is that the print statement 'inside main' is executed only once and I don't understand why, shouldn't it be looping for as long as the rospy is up and running?

No, it shouldn't.

rospy.spin() is a blocking call: you're program will not advance to the next line as long as rospy.spin() is executing -- which will be forever by design. That is, until you make rospy.is_shutdown() true (by using ctrl+c for instance).

So your while is entered, the print(..) statement is executed once and then rospy.spin() takes over and never returns.

if __name__ == "__main__":
    listener()
    while not rospy.is_shutdown():
        print('inside main')
        rospy.spin()
My problem is that the print statement 'inside main' is executed only once and I don't understand why, shouldn't it be looping for as long as the rospy is up and running?

No, it shouldn't.

So your while is entered, the print(..) statement is executed once and then rospy.spin() takes over and never returns.

Edit: some things to note:

rospy.spin() doesn't really do anything: it just sits there, waiting. It's not needed in rospy for handling events.
if you're going to use something else instead of rospy.spin(), make sure to properly throttle your while-loop, or it will start using 100% cpu even while doing nothing.
you seem to be implementing a synchronous sampling system (global value; value = data.data): you may want to consider handling incoming msgs directy in the callback and keep everything event-based, or take a look at message_filters/Cache.