[the_real_bto]

This is a great idea, although repr() would probably be better than str().

I've spent a lot of time debugging Python over the years. This honestly never occured to me until you mentioned it.

Edit:

Everyone else jumped in before me. I think it would be useful as a command line switch to python to turn on the behavior (maybe call it extended exceptions).

Python would need to run the extended exception generation under a try/except that fell back to regular exceptions if an exception was raised while calling repr()