[Python-Dev] On the METH_FASTCALL calling convention

Stefan Behnel stefan_ml at behnel.de
Sat Jul 7 03:19:11 EDT 2018

Jeroen Demeyer schrieb am 05.07.2018 um 16:53:
> The only case when this handling of keywords is suboptimal is when using
> **kwargs. In that case, a dict must be converted to a tuple. It looks hard
> to me to support efficiently both the case of fixed keyword arguments
> (f(foo=x)) and a keyword dict (f(**kwargs)). Since the former is more
> common than the latter, the current choice is optimal.

Wrappers that adapt or add some arguments (think partial()) aren't all that
uncommon, even when speed is not irrelevant. But I agree that actual
keyword arguments should rarely be involved in those calls.

Typically, it's calls with 1 to ~3 positional arguments that occur in
performance critical situations. Often just one, rarely more, and zero
arguments is a fast case anyway. Keyword arguments will always suffer some
kind of penalty compared to positional arguments, regardless of how they
are implemented (at runtime). But they can easily be avoided in many cases,
and anyone designing a performance relevant API that *requires* keyword
arguments deserves to have their code forked away from them. :)

The current keyword calling conventions seem fine with me and I do not see
a reason why we should invest discussion time and distributed brain
capacity into "improving" them.


PS: Passing keyword arguments through wrappers unchanged might be a case to
consider in the future, but the calling PEPs don't seem the right place to
discuss those, as it's not just a call issue but also a compiler issue.

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