If you want to learn Python but have Java/Android experience, I've got a GREAT option for you: Task Coach http://www.taskcoach.org/
Task Coach is great GPL organizing program written in Python. However, the most requested feature is an Android client that would connect with the data from the main program. There is already an iOS client to model off of. The main desktop program is also very robust but has tons of room for improvement. The existing community is small and could use help but is very friendly. Unfortunately, I can't say you'd be working with a large team of people, there's only a couple of us who are pretty active plus some other volunteers around the world, but it's a project used by at least thousands of users and has been active for many years.
Coming a little late to this thread, but taskcoach. Its a todo list manager, open source and mostly free (the ios version is paid). Available everywhere but the Android version is pretty bad.
Like a lot of open source software its a mixed bag. It is, hands down, the most powerful todolist style software I've encountered (including omnifocus). It supports prerequsite tasks, extremely flexible recurrence scheduling, priority settings, color and image coding, tagging (via categories), projects, infinite(?) subtasking effort tracking, notes, attachments, completion percentage, and many other features. Essentially if the question is "can I do X" the answer is yes.
Now the bad. Its interface is a pretty clunky but once you learn it its totally functional. It has limited sync support but its very easy to use dropbox between multiple desktops and even the android app. It doesn't have the polish or cloud features a modern propriety app would. I still use Wunderlist for projects that my girlfriend is part of and for lists that I need to use my phone for (grocery shopping)
TL;DR: extremely powerful, open source software but with some classic open source jank and no cloud service backend.
Edit: I was wrong, it does have sync support but its off by default and requires setup, I'll stick with dropboxing the file.