Those are intesting options, I will think about this. I am still afraid of adding more APIs in this area as having arbitrary Python getting call in the middle of handling the exceptions it can be quite tricky: think that we are handling an exception already so other exceptions can mess up stuff in very subtle ways or we need to ignore exceptions and that can also be dangerous.

Another thing we could do is reproduce the feature in idle itself with a custom display hook, which would be easier as we can just use difflib. It would be slightly different than the built-in one but honestly I don't think that's a problem at all. I understand that other people may think differently so I am open minded as well. But I wanted to clarify what are my worries :)