[cptwunderlich]

Isn't the biggest issue that alternative Python implementations (pyston, pypy) have to implement all the quirks of CPython and compatibility with C-Extensions? Since there is no written standard and libraries rely on CPython behavior. Making things like Numpy etc. work, that heavily use C, seems really hard for jitted implementations.

[chrisseaton]

This is why Graal languages virtualise C extensions by interpreting them, allowing them to appear to meet a fixed C interface with quirks but really being implemented more efficiently.