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gh-144995: Optimize memoryview == memoryview #144996

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Merged
vstinner merged 11 commits into from
Mar 3, 2026
Merged

gh-144995: Optimize memoryview == memoryview #144996

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gh-144995: Optimize memoryview == memoryview
vstinner Feb 19, 2026
0dd7911
Replace blocklist with allowlist
vstinner Feb 19, 2026
61e37e4
Dummy change to update GitHub
vstinner Feb 19, 2026
102f26d
Apply suggestions from code review
vstinner Feb 19, 2026
2316fab
Address review
vstinner Feb 21, 2026
800e85f
Test more formats: @b, @B, P and ?
vstinner Feb 21, 2026
a754be4
Fix for empty format string
vstinner Feb 21, 2026
1d72ed2
Add tests with optimization disabled
vstinner Feb 21, 2026
134f0de
Do not optimize "P" format
vstinner Mar 2, 2026
3272520
Add tests on "c", "n" and "N" formats
vstinner Mar 2, 2026
83ad213
Address Serhiy's review
vstinner Mar 3, 2026
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21 changes: 21 additions & 0 deletions Lib/test/test_memoryview.py
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Original file line number Diff line number Diff line change
Expand Up @@ -575,6 +575,27 @@ def test_array_assign(self):
m[:] = new_a
self.assertEqual(a, new_a)

def test_compare_equal(self):
# A memoryview is equal to itself: there is no need to compare
# individual values. This is not true for float values since they can
# be NaN, and NaN is not equal to itself.
for int_format in 'bBhHiIlLqQ':
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@serhiy-storchaka serhiy-storchaka Feb 20, 2026

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Can "?" be tested? Can format starting with "@" be tested? Can the null format be tested?

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@vstinner vstinner Feb 21, 2026

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I don't know how to test these formats. array.array doesn't support "P" and "?" formats and it doesn't support "@" byte order. Do you have an idea how to test these cases?

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@serhiy-storchaka serhiy-storchaka Feb 21, 2026

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memoryview.cast() supports them.

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@serhiy-storchaka serhiy-storchaka Feb 21, 2026

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Surprisingly:

>>> memoryview(b'\0\1').cast('?') == memoryview(b'\0\2').cast('?')
False

even if

>>> list(memoryview(b'\0\1').cast('?')) == list(memoryview(b'\0\2').cast('?'))
True

But this may be platform depending, so I would not test values different than 0 and 1. Or 1 is also not safe?

It may be undefined behavior to interpret random values except 0 as void* (even if it works on x86). Maybe there is a way to create an array of pointers in ctypes? Or it is not worth to bother?

with self.subTest(format=int_format):
a = array.array(int_format, [1, 2, 3])
m = memoryview(a)
self.assertTrue(m == m)
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for float_format in 'fd':
with self.subTest(format=int_format):
a = array.array(float_format, [1.0, 2.0, float('nan')])
m = memoryview(a)
# nan is not equal to nan
self.assertFalse(m == m)
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a = array.array(float_format, [1.0, 2.0, 3.0])
m = memoryview(a)
self.assertTrue(m == m)


class BytesMemorySliceTest(unittest.TestCase,
BaseMemorySliceTests, BaseBytesMemoryTests):
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2 changes: 2 additions & 0 deletions Misc/NEWS.d/next/Core_and_Builtins/2026-02-19-12-49-15.gh-issue-144995.Ob2oYJ.rst
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@@ -0,0 +1,2 @@
Optimize :class:`memoryview` comparison: a :class:`memoryview` is equal to
itself, there is no need to compare values. Patch by Victor Stinner.
27 changes: 27 additions & 0 deletions Objects/memoryobject.c
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Expand Up @@ -3101,6 +3101,25 @@ cmp_rec(const char *p, const char *q,
return 1;
}

static int
is_float_format(const char *format)
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@eendebakpt eendebakpt Feb 19, 2026

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Does this cover the complex types?

import numpy as np
a = np.array([1+2j, 3+4j, float('nan')], dtype=np.complex128)
mv = memoryview(a)
mv == mv # False

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@vstinner vstinner Feb 19, 2026

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This memory format is Zd. Oh, my change doesn't work for this memoryview. I should replace the blocklist with an allowlist. I'm not a memoryview/buffer expert. I didn't know that 3rd party projects can have their own format.

{
if (format == NULL) {
return 0;
}
if (strcmp("d", format) == 0) {
return 1;
}
if (strcmp("f", format) == 0) {
return 1;
}
if (strcmp("e", format) == 0) {
return 1;
}
return 0;
}


static PyObject *
memory_richcompare(PyObject *v, PyObject *w, int op)
{
Expand All @@ -3122,6 +3141,14 @@ memory_richcompare(PyObject *v, PyObject *w, int op)
}
vv = VIEW_ADDR(v);

// A memoryview is equal to itself: there is no need to compare individual
// values. This is not true for float values since they can be NaN, and NaN
// is not equal to itself.
if (v == w && !is_float_format(vv->format)) {
equal = 1;
goto result;
}

if (PyMemoryView_Check(w)) {
if (BASE_INACCESSIBLE(w)) {
equal = (v == w);
Expand Down
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