Well, some parts of Twisted. All code added and edited these days has to conform to style-guidelines and go through extensive code-review, so it winds up pretty polished by the time it gets committed. On the other hand, Twisted is a very old project and there's code in there that pre-dates the modern quality requirements; such code is often... idiosyncratic, or at least perplexing.

I'd recommend poking around well-tested things like twisted.internet.defer for starters, then start to explore further afield. :)