Reddit has opened their source up and we can now see just what they've been up to. It's been known for some time that Reddit was (re)built using Pylons and Mako templates, contrary to their FAQ which still states that they use web.py. As it turns out, they've also built something of their own database layer, which seems to include a homegrown caching layer and ultimately is built on top of SQLAlchemy, using the SQLA expression language to generate queries. Connections are served with the QueuePool, and they use the threadlocal setting, so that they can get implicit access to transactions in progress. They vertically partition their database access among four separate engines across four distinct areas of functionality on the site.
This is currently the highest volume website I'm aware of using SQLAlchemy and Pylons, and is a testament to the stability of our core components (I hope). Python in general is not too prominent in New York City where I work; Java, PHP and .NET are still the default "goto" platforms, and most developers here look at you kind of funny when you mention Python. Look how well-known Java advocate Ted Neward says even Python!, as though we're the most fringe Java alternative imaginable. I hope examples like Reddit continue to illustrate that Python presents the best mix of performance, stability, and rapid development for web development today, not to mention one of the broadest software ecosystems in the field (which I've always maintained is a good thing).