Cython is both differently defined and serves a different purpose. First of all, RPython is runnable, Cython is not. RPython has type inference, but more importantly, because it works from live objects, it has Python as a metaprogramming language. It's also much easier to extend than Cython.
Purpose wise cython is for optimizing pieces of code, rpython is for writing interpreters. RPython is quite faster than Cython, but it's also much more restricted, making it a bit unusable for the purpose of "just" speeding up pieces of code (you can't have PyObject equivalents).
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Cython is both differently defined and serves a different purpose. First of all, RPython is runnable, Cython is not. RPython has type inference, but more importantly, because it works from live objects, it has Python as a metaprogramming language. It's also much easier to extend than Cython.
Purpose wise cython is for optimizing pieces of code, rpython is for writing interpreters. RPython is quite faster than Cython, but it's also much more restricted, making it a bit unusable for the purpose of "just" speeding up pieces of code (you can't have PyObject equivalents).