CC @rhettinger for the second part (integer values). Is this an issue or a feature?

Actually, even without the sum, we have:

>>> x = 10 ** 100
>>> 1.0 + x + 1.0 - x
0.0

@picnixz, that's a feature.

Ok, it seems that in the second case PyLong_AsLongAndOverflow() just overflows and we fallback to the generic sum. That's something copied from the specialization for integers (in 8ce8a78) and it wasn't changed in #100426. Maybe it should?

Hmm, with PyLong_AsDouble() + compensated summation it's even faster!

./python -m timeit -r11 -s 'xs=[1.0, 10**100, 1.0, -10**100]' 'sum(xs)'
500000 loops, best of 11: 634 nsec per loop

while in the main:

$ ./python -m timeit -r11 -s 'xs=[1.0, 10**100, 1.0, -10**100]' 'sum(xs)'
500000 loops, best of 11: 801 nsec per loop

PR is ready for review: #122236

It combines fixes for floats and integers. If second case requires more discussion or is "a feature", I can quickly revert that part.

The added error returns leak references. This was caught by the noGIL refleak buildbot (which runs more often than refleaks for regular-builds): https://buildbot.python.org/#/builders/1226/builds/2352/steps/6/logs/stdio

The added error returns leak references. This was caught by the noGIL refleak buildbot (which runs more often than refleaks for regular-builds): https://buildbot.python.org/#/builders/1226/builds/2352/steps/6/logs/stdio

PR is ready: #122405

My PR #122406 looks exactly the same :)
IMO, it's better to associate the PR with this issue, so the fix-up PR is kept together with the original.

Closing since the refleak is now fixed.