gh-119109: functool.partial vectorcall supports pto->kw & fallback to tp_call removed

[dg-pb]

Although comment stated that "merging keyword arguments for vector call is messy" (not exact quote), but I have found that it is fairly straight forward.

This achieves 2 goals:

1. Solves the raised issue of partial being stuck in tp_call after keywords are deleted even if function supports vectorcall.
2. Improves performance when len(pto-kw) != 0 and function supports vectorcall.

Number 1. is solved better than it was in #119125 as there are no more switching between methods and faster vectorcall is used whenever appropriate. Also, this does not require additional variable in class struct.

Comparison of performance:

def sub(a, b):
    return a - b

p = partial(sub, b=2)
# Before
%timeit p(1)    # 240 ns
# After
%timeit p(1)    # 180 ns
# Using simpler but slower approach with `_PyObject_VectorcallDictTstate`
%timeit p(1)    # 200 ns

This is only initial attempt and I think the code can be made simpler. (much simpler if decide to use _PyObject_VectorcallDictTstate).

This will need to be adjusted to #119827 so do not merge.

• Issue: functools.partial does not re-set vector call. #119109

[vstinner]

Is the PR a draft or is it ready for review?

[dg-pb]

Is the PR a draft or is it ready for review?

This one is a draft. Just an example of concept that works for current functools.partial.

Reason being that #119827 will need a different implementation, which I have, but still waiting for reply how to go about it (#119827 (comment)):
a) Make it part of #119827
b) Wait after it is merged and issue a new PR

Here is implementation that works for partial with Placeholders:
https://gist.github.com/dg-pb/45f6d3af262056aaea1acd4ce29ee4dd

It can be reviewed - sooner or later it will become PR. It is contained within one function - no documentation updates or anything additional is needed.

Let me know if there is a good way to issue a separate PR on top of the one which is not yet merged - I am not aware of such.

[dg-pb]

Will issue a new PR for partial with placeholders