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Oops, I forgot to attach decimal-2.patch, here you have.
> I would dissuade from direct using of FASTCALL, but Argument Clinic is enough reliable.
I wrote decimal-2.patch in a few minutes, when I was trying to find all functions still calling _PyStack_AsTuple() in my tp_fast{new,init,call} fork. I just wrote it to identify which parts of CPython can be optimized to make bm_telco faster.
I agree that Argument Clinic should be preferred over using directly METH_FASTCALL, especially for the _decimal module: Stefan already wrote that he wants to have the same code base on Python 3.5, 3.6 and 3.7.
> I would recommend to use Argument Clinic unless Stefan supports external fork of the decimal module and is limited by public API.
What is the status of Argument Clinic? Is it something "public" or not?
The status of _decimal is also unclear to me. Is it part of CPython or not? :-) I know that there is also a "backported" module for Python 2:
 https://pypi.python.org/pypi/cdecimal
 http://www.bytereef.org/mpdecimal/index.html
IMHO it's a great tool. Maybe it's time to make it usable outside CPython in Python 3.7? Or maybe we should wait until the remaining missing features are implemented? Issues #20291 and #29299 for example.
I'm now waiting for Stefan's feedback ;-)