Putting on my "Python teacher" hat for a moment (I also work at Nylas), our codebase is interesting for learning purposes because it's a fairly homogenous, 'live' product managed by a commercial engineering team. Definitely on the complex side of things, though -- let me know if you want any help figuring it out (we also have a community Slack at http://slack-invite.nylas.com/).

On the testing side, I was chatting to a fellow Pyladies attendee about how we do test fixture setup and teardown (especially around databases) -- you might find that interesting to look at too.