Even though it's private at 3.7, extension authors can start using it at their risk if we decide METH_FASTCALL is public in 3.8 without any change from 3.7.

People can still wait for 3.8. Waiting 1.5 years for a feature is nothing when the software you're talking about is already 28 years. :) It's simply not worth the risk.

​Of course.  My idea is providing information to "early adaptors" who writes C extension

​PEP ​580 is trying to expand METH_FASTCALL to custom function types in 3rd party
library written in tools like Cython.
But METH_FASTCALL cannot be used widely even for normal function types
in 3rd party library yet.

Without publicating METH_FASTCALL, PEP 580 is useful only for libraries using
private APIs.  That's unhealthy.

​So I think we should discuss about METH_FASTCALL​ publication before evaluating
PEP 580.  That's my main point, and "from 3.7" part is just a bonus, sorry.

INADA Naoki  <songofacandy@gmail.com>