Check out the Wing IDE - www.wingware.com . As part of it's general greatness it has a "debug probe" which lets you execute code snippets on active data in mid-debug execution.
It doesn't have precisely what you are after... you can't (yet) highlight code segments and say "run this, please", but I think it might almost have what you want for general workflow improvement. The main drawback is that it is a commercial product, albeit "cheap". The extra drawback is that the debug probe feature requires the professional version which is "less cheap", but still < $200. Well worth it for professional development IMO. They have a great demo policy... you should check it out. I tried several different IDEs (I've become accustomed to using IDEs over supe'd up text editors) and Wing was/is my favorite. -- http://mail.python.org/mailman/listinfo/python-list