PyPy is python implemented in RPython, which is technically a python subset. It's so restricted it might as well be a different language though.
PyRPy is just less catchy sounding
> technically a python subset
So it can just run under CPython? If so, then that isn't too misleading.
It is restricted in a way that you would restrict yourself to write high speed software in most languages, and I found it is not that restrictive compared to C that you would have to use if you were to write a fast Python library.