rospy.Subscriber

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It is not necessary to have a def callback in the rospy.Subscriber constructor(It's an optional argument). However, if no callback functions are set, the messages won't be processed. You can later add callbacks using the add_callback() (source)

If you look at the question that you are referring to, he is using message_filter to approximately synchronise messages and process them together in a single callback, which he registers by calling self.ts.registerCallback(self.callback).