Hello all,
As discussed in some other threads ([1], [2]), we should discuss the
METH_FASTCALL calling convention.
For passing only positional arguments, a C array of Python objects is
used, which is as fast as it can get. When the Python interpreter calls
a function, it builds that C array on the interpreter stack:
>>> from dis import dis
>>> def f(x, y): return g(x, y, 12)
>>> dis(f)
1 0 LOAD_GLOBAL 0 (g)
2 LOAD_FAST 0 (x)
4 LOAD_FAST 1 (y)
6 LOAD_CONST 1 (12)
8 CALL_FUNCTION 3
10 RETURN_VALUE
A C array can also easily and efficiently be handled by the C function
receiving it. So I consider this uncontroversial.
The convention for METH_FASTCALL|METH_KEYWORDS is that keyword *names*
are passed as a tuple and keyword *values* in the same C array with
positional arguments. An example:
>>> from dis import dis
>>> def f(x, y, z): return f(x, foo=y, bar=z)
>>> dis(f)
1 0 LOAD_GLOBAL 0 (f)
2 LOAD_FAST 0 (x)
4 LOAD_FAST 1 (y)
6 LOAD_FAST 2 (z)
8 LOAD_CONST 1 (('foo', 'bar'))
10 CALL_FUNCTION_KW 3
12 RETURN_VALUE
This is pretty clever: it exploits the fact that ('foo', 'bar') is a
constant tuple stored in f.__code__.co_consts. Also, a tuple can be
efficiently handled by the called code: it is essentially a thin wrapper
around a C array of Python objects. So this works well.
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.
In other words: I see nothing to improve in the calling convention of
METH_FASTCALL. I suggest to keep it and make it public as-is.
Jeroen.
[1] https://mail.python.org/pipermail/python-dev/2018-June/153945.html
[2] https://mail.python.org/pipermail/python-dev/2018-July/154251.html
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