Python call callback function on input change

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Some basic programming question:

My node is subscribed to the /joint_states topic, more exactly /joint_states/position[5]. I am trying to publish a message, each time the joint6's state has changed. The /joint_states topic has a high rate 0.1 sec/ published messages, so right now i publish the same value, 10x a sec. I would like to call the callback function only on input change.

Does python have some special function for that?

Any ideas?

btw: i would have a much easier task if i could publish fake_gripperarm_controller using demo.launch from my MoveIt! package