Ask HN: Best Open-Source Python Project to Contribute To

I realize the most satisfying way to contribute to open source software is to discover a project you're interested in and work with already, and then contribute back to it. Unfortunately, sometimes that first step of discovery isn't as easy as it sounds.

I'm a mid-level python developer looking to dedicate some spare cycles to challenge myself and also work with some other talented devs on a project that can help me take my skills to the next level.

What are some projects you'd recommend? Maybe something you work on personally, or would if you had spare time yourself. Preferably a project actually in need of some help.