While RPython is a restricted version of python used to build the PyPy python interpreters, the interpreters themselves are not restricted. Any deviation from CPython behavior, intended or not, is considered a bug. So Codon should be compared to the PyPy python interpreter, not to RPython. The advantage to writing the interpreter in RPython rather than C (CPython) or pre-compiling python code to LLVM IR and from there to creating and executable (Codon), is that RPython comes with a metaJIT (which can generate a JIT) and a mark-and-sweep garbage collector for any interpreter built on top of it.