The difference here is that on Windows y will never change, it will stay 1 forever while on Unix systems y will increment. Having done a bit more research it seems this is due to the way multiprocessing works on Windows vs Unix systems. In unix systems the new thread is a fork of the parent while in Windows it is a whole new process built from scratch (if I am understanding it correctly). I am not going to close it as I am unsure if it is by design that Windows and Unix python acts differently.