[cyberdrunk]

I'm not sure that working on an open source project is good way to learn a new subfield of coding. I've done it and, in retrospect, this time could've been better spent elsewhere.

When working on an opensource project, you'll become an expert on the few functionalities (and code that realises it) of that single project that you will be working on - while you'll be oblivious about everything else in the subfield. IMO just doing a project that's typical for the subfield will allow you to learn much more. So, for example, when learning ML, don't go do obscure work on R codebase, but instead try some simple challenges from Kaggle.