In Python, you can get at a variable, or even code, in another thread with "getattr()". You can monkey-patch another thread while it is running. This is not very useful, but it's easy to implement in a naive interpreter such as CPython. Part of the price for this is the Global Interpreter Lock, so you don't really have two threads running at once. PyPy has a huge amount of machinery so that stuff will work.

Grumpy doesn't even seem to try to implement that. That's a good thing. If you restrict Python a little, it's much easier to compile.