People interested in frameworks should read bottle's code. It has a neat feature where the request handling is built by sub classing exceptions. This allows request processing to terminate at any point by simply raising the handler. There is a global try/except that then renders the result.

This mechanism is quite good for ensuring every request proper gets a response, though it also allows dodgy exception handling to interfere with processing. A discussion on the merits of exceptions themselves is beyond the scope of this comment. :)