Thanks to all of the developers behind wsgi.
When I first heard about it, wsgi struck me as an unnecessary complication. Then someone showed me how it could simplify testing of web applications. And now:
Google App Engine supports any framework written in pure Python that speaks CGI (and any WSGI-compliant framework using a CGI adaptor), including Django, CherryPy, Pylons, and web.py. You can bundle a framework of your choosing with your application code by copying its code into your application directory.