For one thing, Cython isn't a Python implementation. As for the rest, of course not. Especially not those two in particular. But PyPy and Pyston are doing pret

For one thing, Cython isn't a Python implementation. As for the rest, of course not. Especially not those two in particular. But PyPy and Pyston are doing pretty well on compatibility. Not sure about Stackless, but I think it's mostly been subsumed by PyPy anyway (even though it's still being developed independently). The main problem for alternative implementations of Python is the C extensions, much like with Ruby. But the Python community has started to take ownership of the problem and a new C interface is being developed, called HPy, that is easier to implement for non-CPython implementations, and library developers are taking an interest in targeting HPy instead of the traditional CPython interface. With that in place, Python will be in a pretty good place when it comes to standardization.