Andrew (from Meetup) here. While the main web app is written in plain ole Java, the API is almost all Jython (the streaming stuff is Scala). The API team was 1 person until last December, and Jython was a big reason for this. The ability to glue all of the core logic and custom ORM into web.py on top of the modjy Servlet container is really powerful.

And, I concur entirely with what ibejoeb says.