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GH-100425: Improve accuracy of builtin sum() for float inputs #100426

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Merged
rhettinger merged 13 commits into from
Dec 23, 2022
Merged

GH-100425: Improve accuracy of builtin sum() for float inputs #100426

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916ab7e
Neumaier compensated summation
rhettinger Dec 22, 2022
787f2e5
Kahan summation
rhettinger Dec 22, 2022
bc308aa
Revert "Kahan summation"
rhettinger Dec 22, 2022
0c2ba04
Restore the bogus sign tests
rhettinger Dec 22, 2022
89e46c8
Add tests and comments
rhettinger Dec 22, 2022
a9cb7d2
Add tests and docs
rhettinger Dec 22, 2022
839cc05
Add blurb
rhettinger Dec 22, 2022
66db937
Remove out of date example for fsum()
rhettinger Dec 22, 2022
af7613e
Update floating point tutorial
rhettinger Dec 22, 2022
4f4b731
Correct handling of overflowed or infinite sums
rhettinger Dec 23, 2022
4da0b8f
Move local declarations inside the block where they are used.
rhettinger Dec 23, 2022
6c7894c
For consistent results, apply compensation on the slow path as well.
rhettinger Dec 23, 2022
3cf9536
Mark as an implementation detail.
rhettinger Dec 23, 2022
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Add tests and docs
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@rhettinger
rhettinger committed Dec 22, 2022
commit a9cb7d2ab06374c22de09b70cc4a1317a7b8c24d
4 changes: 4 additions & 0 deletions Doc/library/functions.rst
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Expand Up @@ -1733,6 +1733,10 @@ are always available. They are listed here in alphabetical order.
.. versionchanged:: 3.8
The *start* parameter can be specified as a keyword argument.

.. versionchanged:: 3.12 Summation of floats switched to an algorithm
that gives higher accuracy on most builds.


.. class:: super()
super(type, object_or_type=None)

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18 changes: 14 additions & 4 deletions Lib/test/test_builtin.py
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Original file line number Diff line number Diff line change
Expand Up @@ -31,13 +31,20 @@
from test.support.os_helper import (EnvironmentVarGuard, TESTFN, unlink)
from test.support.script_helper import assert_python_ok
from test.support.warnings_helper import check_warnings
from test.support import requires_IEEE_754
from unittest.mock import MagicMock, patch
try:
import pty, signal
except ImportError:
pty = signal = None


# Detect evidence of double-rounding: sum() does not always
# get improved accuracy on machines that suffer from double rounding.
x, y = 1e16, 2.9999 # use temporary values to defeat peephole optimizer
HAVE_DOUBLE_ROUNDING = (x + y == 1e16 + 4)


class Squares:

def __init__(self, max):
Expand Down Expand Up @@ -1618,10 +1625,6 @@ def test_sum(self):
self.assertEqual(repr(sum([-0.0], -0.0)), '-0.0')
self.assertEqual(repr(sum([], -0.0)), '-0.0')

# Accuracy tests
self.assertEqual(sum([0.1] * 10), 1.0)
self.assertEqual(sum([1.0, 10E100, 1.0, -10E100]), 2.0)

self.assertRaises(TypeError, sum)
self.assertRaises(TypeError, sum, 42)
self.assertRaises(TypeError, sum, ['a', 'b', 'c'])
Expand All @@ -1645,6 +1648,13 @@ def __getitem__(self, index):
sum(([x] for x in range(10)), empty)
self.assertEqual(empty, [])

@requires_IEEE_754
@unittest.skipIf(HAVE_DOUBLE_ROUNDING,
"sum accuracy not guaranteed on machines with double rounding")
def test_sum_accuracy(self):
self.assertEqual(sum([0.1] * 10), 1.0)
self.assertEqual(sum([1.0, 10E100, 1.0, -10E100]), 2.0)

def test_type(self):
self.assertEqual(type(''), type('123'))
self.assertNotEqual(type(''), type(()))
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