SLOW_SUM flag in sum builtin

[sobolevn]

Bug report

Here's a very strange condition that we have in our sum definition:

cpython/Python/bltinmodule.c

Lines 2573 to 2577 in 0abf997

	#ifndef SLOW_SUM
	/* Fast addition by keeping temporary sums in C instead of new Python objects.
	Assumes all inputs are the same type. If the assumption fails, default
	to the more general routine.
	*/

But, here are the strange parts:

1. SLOW_SUM is never documented
2. There are no configure options to set it
3. There are no other mentions of it in CPython
4. I was only able to find this mention of it on the internet: https://www.reddit.com/r/Python/comments/18vv3aa/what_is_slow_sum_in_the_cpython_source_code/
5. I don't think that it should exist, because it might change the builtin if this identifier is somehow magically defined. This would be a bug, because this is never documented / promised by us. It would be quite cryptic bug also
6. It is very old and still not "official", added in 8ce8a78

I propose to remove it and always have a single way how sum behaves.
I will send a PR.

Linked PRs

• gh-119192: Remove #ifndef SLOW_SUM check from sum(), use #if 1 #119193

[rhettinger]

I wish you would leave this in. It helped greatly in testing, in proving that it really is an optimization, and in segregating the more complex optimization path from the core logic.

Also, as you sweep through the code, please keep in mind Chesterson's Fence and not just remove everything that isn't exactly what you would have done.

And no, this should not be a configuration option. There is no need to present end users with build options they won't benefit from. This symbol is for people who are maintaining this code (in this case, me).

[tim-one]

OTOH, does it actually serve a purpose to give it a name? That is, wouldn't #if 1 allow a 1-character edit to turn it off when that's wanted? You have to recompile to change the behavior anyway.

No strong opinion from me regardless. But if you haven't actually #defined it in, say, the last year, you shouldn't have a strong opinion either 😉.