Explicit type declarations; validation; initialization; delegation; notification; visualization.
Traits documentation; more info on Traits
Interactive 2-D plotting for applications.
Get Traits, Chaco, and Mayavi, as well as all the other open source projects that Enthought has created for building scientific applications:
ETS documentation; more info on ETS
One big bundle of Python goodness, including standard Python, ETS, and a whole bunch of other useful open source packages. No-cost for academic, non-profit and hobbyist users; fee-licensed for commercial and government users.