Ok, so I did it in Python because I encountered a ton of broken/fragile class machinery when I tried to do it in Rust: subclassing a built-in class but also being a PyValue itself weirdness, inconsistency with CPython in how __new__ vs __init__ works (it was calling __new__ with arguments intended for __init__, and I still had to work around that in Python), and the final straw, __missing__ not being detected from dict.__getitem__, while __missing__ did work in Python. In any case defaultdict being a native class is mostly just an implementation detail, and there's really no reason why it can't be implemented in Python. We can look at all that at some point later, but we're really close to unittest and I'm focusing on that at the moment.