PyPy is a language for writing interpreters. It tends to be similar to Python in a sense that it's a restricted subset of Python but this is largely irrelevant for the architectural question. The other part is essentially the translation tool chain or a compiler for our Python. We implement the Python semantics once in the Python interpreter and then it gets either directly executed or compiled to assembly. If you're trying to encode all the protocol or how actually functions and parameters are called, chances are you'll make a mistake.