[the_mitsuhiko]

And PyPy breaks some Python code (eg: most C extensions are very slow) in the process. PyPy is a different dialect of Python.

[overgard]

Slow != Breaks. I've run plenty of production python code in pypy. I'm sure it's not appropriate everywhere, but I wouldn't go so far as to call it a separate dialect.

[Rochus]

CPython could implement an alternative, more efficient FFI (such as e.g. the one by LuaJIT) which would not slow down PyPy. So people could gradually migrate.