You can presumably run code that calls `sys.settrace` for that. Which makes it somewhat underwhelming to realize that you pretty much could also do that before, but perhaps convenient that now you don't have to have the foresight to have set that up beforehand.
Hmm, that's what pdb.set_trace() uses, right? That means monkey-patching live code will have an unavoidable performance hit, with the exception pointed out by sibling @skeledrew comment.
Which brings me back to my long-held belief that python functions are not quite first-class, not until you can rip them apart and put them back together as if nothing ever happened.
Not to say there haven't been improvements, as evinced by the decorator module rewrite from version 2->3 (or 3->4; can't remember) that completely changed how the resulting function was constructed.
