Static Python is a bit different in that it’s not a subset language. It makes a few semantic changes, most notably that classes are slotified, so you can’t tack random extra attributes into instances. If you’re writing typed Python you probably don’t do much of this anyway since all type checkers will complain about it. But the approach is “normal dynamic Python with optimizations where allowed by types,” not subset.

How similar is it to RPython?

Unlike RPython, Static Python in cinder is not really a subset of Python, it can compile everything (although it will throw compile time errors if it sees mismatched types). If it cannot determine type information, it just assumes the type could be anything, and falls back to slower CPython behavior.