So, a little background: I've done some programming for a few years, but it's mostly on my own or with friends. Something that's always been at the back of my mind for a while has been contributing to an open source project. But I've always been worried I wasn't capable enough.
Of the languages I know, the two I have enough skills in to not make everyone go "Oh my god! Why did you do it THAT way? _function x_ is in the standard library!" are Python or PHP. Preferably, I'd like to work on a Python project.
The two big ones that keep coming up are both PHP, Wordpress and phpBB, but as large projects, they're both able to attract and avail of developers far better than me, and their codebase would require a good amount of time to get acquainted to. Further, wordpress at least seems to be more in need of translators, testers, etc than actual coders.
So, can anyone suggest a mid-sized (preferably Python) open source project actively looking for new contributors?
You're not looking for a project "actively looking for new contributors". You're looking for a project you want to contribute to. You don't ask/interview for a spot on the team; you come up with a feature you will use, write it, and send diffs.
Even a project like Wordpress, which has a small army of PHP contributors, is "open to contributions". If you write something genuinely useful, send diffs, and are willing to transfer your rights to the code, chances are they'll seriously consider it. Of course, Wordpress probably "wants" you to build a plugin, not a core modification. But if your best idea is a core modification, just go write it.
You're overthinking this. Consider whether that's a sign that you're procrastinating.