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Commit 59f78d7

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encukouZeroIntensity
encukou
and
ZeroIntensity
authored
gh-131747: ctypes: Deprecate _pack_ implicitly setting _layout_ = 'ms' (GH-133205)
On non-Windows, warn when _pack_ implicitly changes default _layout_ to 'ms'. Co-authored-by: Peter Bierma <zintensitydev@gmail.com>
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‎Doc/deprecations/index.rst

Copy file name to clipboardExpand all lines: Doc/deprecations/index.rst
+2Lines changed: 2 additions & 0 deletions
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@@ -7,6 +7,8 @@ Deprecations
77

88
.. include:: pending-removal-in-3.17.rst
99

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.. include:: pending-removal-in-3.19.rst
11+
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.. include:: pending-removal-in-future.rst
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C API deprecations

‎Doc/deprecations/pending-removal-in-3.19.rst

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@@ -0,0 +1,8 @@
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Pending removal in Python 3.19
2+
------------------------------
3+
4+
* :mod:`ctypes`:
5+
6+
* Implicitly switching to the MSVC-compatible struct layout by setting
7+
:attr:`~ctypes.Structure._pack_` but not :attr:`~ctypes.Structure._layout_`
8+
on non-Windows platforms.

‎Doc/library/ctypes.rst

Copy file name to clipboardExpand all lines: Doc/library/ctypes.rst
+14-1Lines changed: 14 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -2754,6 +2754,16 @@ fields, or any other data types containing pointer type fields.
27542754
when :attr:`_fields_` is assigned, otherwise it will have no effect.
27552755
Setting this attribute to 0 is the same as not setting it at all.
27562756

2757+
This is only implemented for the MSVC-compatible memory layout.
2758+
2759+
.. deprecated-removed:: next 3.19
2760+
2761+
For historical reasons, if :attr:`!_pack_` is non-zero,
2762+
the MSVC-compatible layout will be used by default.
2763+
On non-Windows platforms, this default is deprecated and is slated to
2764+
become an error in Python 3.19.
2765+
If it is intended, set :attr:`~Structure._layout_` to ``'ms'``
2766+
explicitly.
27572767

27582768
.. attribute:: _align_
27592769

@@ -2782,12 +2792,15 @@ fields, or any other data types containing pointer type fields.
27822792
Currently the default will be:
27832793

27842794
- On Windows: ``"ms"``
2785-
- When :attr:`~Structure._pack_` is specified: ``"ms"``
2795+
- When :attr:`~Structure._pack_` is specified: ``"ms"``.
2796+
(This is deprecated; see :attr:`~Structure._pack_` documentation.)
27862797
- Otherwise: ``"gcc-sysv"``
27872798

27882799
:attr:`!_layout_` must already be defined when
27892800
:attr:`~Structure._fields_` is assigned, otherwise it will have no effect.
27902801

2802+
.. versionadded:: next
2803+
27912804
.. attribute:: _anonymous_
27922805

27932806
An optional sequence that lists the names of unnamed (anonymous) fields.

‎Doc/whatsnew/3.14.rst

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+8Lines changed: 8 additions & 0 deletions
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@@ -1874,6 +1874,12 @@ Deprecated
18741874
:func:`codecs.open` is now deprecated. Use :func:`open` instead.
18751875
(Contributed by Inada Naoki in :gh:`133036`.)
18761876

1877+
* :mod:`ctypes`:
1878+
On non-Windows platforms, setting :attr:`.Structure._pack_` to use a
1879+
MSVC-compatible default memory layout is deprecated in favor of setting
1880+
:attr:`.Structure._layout_` to ``'ms'``.
1881+
(Contributed by Petr Viktorin in :gh:`131747`.)
1882+
18771883
* :mod:`ctypes`:
18781884
Calling :func:`ctypes.POINTER` on a string is deprecated.
18791885
Use :ref:`ctypes-incomplete-types` for self-referential structures.
@@ -1948,6 +1954,8 @@ Deprecated
19481954

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.. include:: ../deprecations/pending-removal-in-3.17.rst
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.. include:: ../deprecations/pending-removal-in-3.19.rst
1958+
19511959
.. include:: ../deprecations/pending-removal-in-future.rst
19521960

19531961
Removed

‎Lib/ctypes/_layout.py

Copy file name to clipboardExpand all lines: Lib/ctypes/_layout.py
+19-2Lines changed: 19 additions & 2 deletions
Original file line numberDiff line numberDiff line change
@@ -5,6 +5,7 @@
55
"""
66

77
import sys
8+
import warnings
89

910
from _ctypes import CField, buffer_info
1011
import ctypes
@@ -66,9 +67,26 @@ def get_layout(cls, input_fields, is_struct, base):
6667

6768
# For clarity, variables that count bits have `bit` in their names.
6869

70+
pack = getattr(cls, '_pack_', None)
71+
6972
layout = getattr(cls, '_layout_', None)
7073
if layout is None:
71-
if sys.platform == 'win32' or getattr(cls, '_pack_', None):
74+
if sys.platform == 'win32':
75+
gcc_layout = False
76+
elif pack:
77+
if is_struct:
78+
base_type_name = 'Structure'
79+
else:
80+
base_type_name = 'Union'
81+
warnings._deprecated(
82+
'_pack_ without _layout_',
83+
f"Due to '_pack_', the '{cls.__name__}' {base_type_name} will "
84+
+ "use memory layout compatible with MSVC (Windows). "
85+
+ "If this is intended, set _layout_ to 'ms'. "
86+
+ "The implicit default is deprecated and slated to become "
87+
+ "an error in Python {remove}.",
88+
remove=(3, 19),
89+
)
7290
gcc_layout = False
7391
else:
7492
gcc_layout = True
@@ -95,7 +113,6 @@ def get_layout(cls, input_fields, is_struct, base):
95113
else:
96114
big_endian = sys.byteorder == 'big'
97115

98-
pack = getattr(cls, '_pack_', None)
99116
if pack is not None:
100117
try:
101118
pack = int(pack)

‎Lib/test/test_ctypes/test_aligned_structures.py

Copy file name to clipboardExpand all lines: Lib/test/test_ctypes/test_aligned_structures.py
+1Lines changed: 1 addition & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -316,6 +316,7 @@ class Inner(sbase):
316316

317317
class Main(sbase):
318318
_pack_ = 1
319+
_layout_ = "ms"
319320
_fields_ = [
320321
("a", c_ubyte),
321322
("b", Inner),

‎Lib/test/test_ctypes/test_bitfields.py

Copy file name to clipboardExpand all lines: Lib/test/test_ctypes/test_bitfields.py
+4-1Lines changed: 4 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -430,6 +430,7 @@ class TestStruct(Structure):
430430
def test_gh_84039(self):
431431
class Bad(Structure):
432432
_pack_ = 1
433+
_layout_ = "ms"
433434
_fields_ = [
434435
("a0", c_uint8, 1),
435436
("a1", c_uint8, 1),
@@ -443,9 +444,9 @@ class Bad(Structure):
443444
("b1", c_uint16, 12),
444445
]
445446

446-
447447
class GoodA(Structure):
448448
_pack_ = 1
449+
_layout_ = "ms"
449450
_fields_ = [
450451
("a0", c_uint8, 1),
451452
("a1", c_uint8, 1),
@@ -460,6 +461,7 @@ class GoodA(Structure):
460461

461462
class Good(Structure):
462463
_pack_ = 1
464+
_layout_ = "ms"
463465
_fields_ = [
464466
("a", GoodA),
465467
("b0", c_uint16, 4),
@@ -475,6 +477,7 @@ class Good(Structure):
475477
def test_gh_73939(self):
476478
class MyStructure(Structure):
477479
_pack_ = 1
480+
_layout_ = "ms"
478481
_fields_ = [
479482
("P", c_uint16),
480483
("L", c_uint16, 9),

‎Lib/test/test_ctypes/test_byteswap.py

Copy file name to clipboardExpand all lines: Lib/test/test_ctypes/test_byteswap.py
+2Lines changed: 2 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -269,6 +269,7 @@ def test_unaligned_nonnative_struct_fields(self):
269269

270270
class S(base):
271271
_pack_ = 1
272+
_layout_ = "ms"
272273
_fields_ = [("b", c_byte),
273274
("h", c_short),
274275

@@ -296,6 +297,7 @@ def test_unaligned_native_struct_fields(self):
296297

297298
class S(Structure):
298299
_pack_ = 1
300+
_layout_ = "ms"
299301
_fields_ = [("b", c_byte),
300302

301303
("h", c_short),

‎Lib/test/test_ctypes/test_generated_structs.py

Copy file name to clipboardExpand all lines: Lib/test/test_ctypes/test_generated_structs.py
+10-1Lines changed: 10 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -125,18 +125,21 @@ class Nested(Structure):
125125
class Packed1(Structure):
126126
_fields_ = [('a', c_int8), ('b', c_int64)]
127127
_pack_ = 1
128+
_layout_ = 'ms'
128129

129130

130131
@register()
131132
class Packed2(Structure):
132133
_fields_ = [('a', c_int8), ('b', c_int64)]
133134
_pack_ = 2
135+
_layout_ = 'ms'
134136

135137

136138
@register()
137139
class Packed3(Structure):
138140
_fields_ = [('a', c_int8), ('b', c_int64)]
139141
_pack_ = 4
142+
_layout_ = 'ms'
140143

141144

142145
@register()
@@ -155,6 +158,7 @@ def _maybe_skip():
155158

156159
_fields_ = [('a', c_int8), ('b', c_int64)]
157160
_pack_ = 8
161+
_layout_ = 'ms'
158162

159163
@register()
160164
class X86_32EdgeCase(Structure):
@@ -366,6 +370,7 @@ class Example_gh_95496(Structure):
366370
@register()
367371
class Example_gh_84039_bad(Structure):
368372
_pack_ = 1
373+
_layout_ = 'ms'
369374
_fields_ = [("a0", c_uint8, 1),
370375
("a1", c_uint8, 1),
371376
("a2", c_uint8, 1),
@@ -380,6 +385,7 @@ class Example_gh_84039_bad(Structure):
380385
@register()
381386
class Example_gh_84039_good_a(Structure):
382387
_pack_ = 1
388+
_layout_ = 'ms'
383389
_fields_ = [("a0", c_uint8, 1),
384390
("a1", c_uint8, 1),
385391
("a2", c_uint8, 1),
@@ -392,13 +398,15 @@ class Example_gh_84039_good_a(Structure):
392398
@register()
393399
class Example_gh_84039_good(Structure):
394400
_pack_ = 1
401+
_layout_ = 'ms'
395402
_fields_ = [("a", Example_gh_84039_good_a),
396403
("b0", c_uint16, 4),
397404
("b1", c_uint16, 12)]
398405

399406
@register()
400407
class Example_gh_73939(Structure):
401408
_pack_ = 1
409+
_layout_ = 'ms'
402410
_fields_ = [("P", c_uint16),
403411
("L", c_uint16, 9),
404412
("Pro", c_uint16, 1),
@@ -419,6 +427,7 @@ class Example_gh_86098(Structure):
419427
@register()
420428
class Example_gh_86098_pack(Structure):
421429
_pack_ = 1
430+
_layout_ = 'ms'
422431
_fields_ = [("a", c_uint8, 8),
423432
("b", c_uint8, 8),
424433
("c", c_uint32, 16)]
@@ -528,7 +537,7 @@ def dump_ctype(tp, struct_or_union_tag='', variable_name='', semi=''):
528537
pushes.append(f'#pragma pack(push, {pack})')
529538
pops.append(f'#pragma pack(pop)')
530539
layout = getattr(tp, '_layout_', None)
531-
if layout == 'ms' or pack:
540+
if layout == 'ms':
532541
# The 'ms_struct' attribute only works on x86 and PowerPC
533542
requires.add(
534543
'defined(MS_WIN32) || ('

‎Lib/test/test_ctypes/test_pep3118.py

Copy file name to clipboardExpand all lines: Lib/test/test_ctypes/test_pep3118.py
+3Lines changed: 3 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -81,20 +81,23 @@ class Point(Structure):
8181

8282
class PackedPoint(Structure):
8383
_pack_ = 2
84+
_layout_ = 'ms'
8485
_fields_ = [("x", c_long), ("y", c_long)]
8586

8687
class PointMidPad(Structure):
8788
_fields_ = [("x", c_byte), ("y", c_uint)]
8889

8990
class PackedPointMidPad(Structure):
9091
_pack_ = 2
92+
_layout_ = 'ms'
9193
_fields_ = [("x", c_byte), ("y", c_uint64)]
9294

9395
class PointEndPad(Structure):
9496
_fields_ = [("x", c_uint), ("y", c_byte)]
9597

9698
class PackedPointEndPad(Structure):
9799
_pack_ = 2
100+
_layout_ = 'ms'
98101
_fields_ = [("x", c_uint64), ("y", c_byte)]
99102

100103
class Point2(Structure):

‎Lib/test/test_ctypes/test_structunion.py

Copy file name to clipboardExpand all lines: Lib/test/test_ctypes/test_structunion.py
+18Lines changed: 18 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -11,6 +11,8 @@
1111
Py_TPFLAGS_DISALLOW_INSTANTIATION,
1212
Py_TPFLAGS_IMMUTABLETYPE)
1313
from struct import calcsize
14+
import contextlib
15+
from test.support import MS_WINDOWS
1416

1517

1618
class StructUnionTestBase:
@@ -335,6 +337,22 @@ def test_methods(self):
335337
self.assertIn("from_address", dir(type(self.cls)))
336338
self.assertIn("in_dll", dir(type(self.cls)))
337339

340+
def test_pack_layout_switch(self):
341+
# Setting _pack_ implicitly sets default layout to MSVC;
342+
# this is deprecated on non-Windows platforms.
343+
if MS_WINDOWS:
344+
warn_context = contextlib.nullcontext()
345+
else:
346+
warn_context = self.assertWarns(DeprecationWarning)
347+
with warn_context:
348+
class X(self.cls):
349+
_pack_ = 1
350+
# _layout_ missing
351+
_fields_ = [('a', c_int8, 1), ('b', c_int16, 2)]
352+
353+
# Check MSVC layout (bitfields of different types aren't combined)
354+
self.check_sizeof(X, struct_size=3, union_size=2)
355+
338356

339357
class StructureTestCase(unittest.TestCase, StructUnionTestBase):
340358
cls = Structure

‎Lib/test/test_ctypes/test_structures.py

Copy file name to clipboardExpand all lines: Lib/test/test_ctypes/test_structures.py
+20-11Lines changed: 20 additions & 11 deletions
Original file line numberDiff line numberDiff line change
@@ -25,6 +25,7 @@ class X(Structure):
2525
_fields_ = [("a", c_byte),
2626
("b", c_longlong)]
2727
_pack_ = 1
28+
_layout_ = 'ms'
2829
self.check_struct(X)
2930

3031
self.assertEqual(sizeof(X), 9)
@@ -34,6 +35,7 @@ class X(Structure):
3435
_fields_ = [("a", c_byte),
3536
("b", c_longlong)]
3637
_pack_ = 2
38+
_layout_ = 'ms'
3739
self.check_struct(X)
3840
self.assertEqual(sizeof(X), 10)
3941
self.assertEqual(X.b.offset, 2)
@@ -45,6 +47,7 @@ class X(Structure):
4547
_fields_ = [("a", c_byte),
4648
("b", c_longlong)]
4749
_pack_ = 4
50+
_layout_ = 'ms'
4851
self.check_struct(X)
4952
self.assertEqual(sizeof(X), min(4, longlong_align) + longlong_size)
5053
self.assertEqual(X.b.offset, min(4, longlong_align))
@@ -53,27 +56,33 @@ class X(Structure):
5356
_fields_ = [("a", c_byte),
5457
("b", c_longlong)]
5558
_pack_ = 8
59+
_layout_ = 'ms'
5660
self.check_struct(X)
5761

5862
self.assertEqual(sizeof(X), min(8, longlong_align) + longlong_size)
5963
self.assertEqual(X.b.offset, min(8, longlong_align))
6064

61-
62-
d = {"_fields_": [("a", "b"),
63-
("b", "q")],
64-
"_pack_": -1}
65-
self.assertRaises(ValueError, type(Structure), "X", (Structure,), d)
65+
with self.assertRaises(ValueError):
66+
class X(Structure):
67+
_fields_ = [("a", "b"), ("b", "q")]
68+
_pack_ = -1
69+
_layout_ = "ms"
6670

6771
@support.cpython_only
6872
def test_packed_c_limits(self):
6973
# Issue 15989
7074
import _testcapi
71-
d = {"_fields_": [("a", c_byte)],
72-
"_pack_": _testcapi.INT_MAX + 1}
73-
self.assertRaises(ValueError, type(Structure), "X", (Structure,), d)
74-
d = {"_fields_": [("a", c_byte)],
75-
"_pack_": _testcapi.UINT_MAX + 2}
76-
self.assertRaises(ValueError, type(Structure), "X", (Structure,), d)
75+
with self.assertRaises(ValueError):
76+
class X(Structure):
77+
_fields_ = [("a", c_byte)]
78+
_pack_ = _testcapi.INT_MAX + 1
79+
_layout_ = "ms"
80+
81+
with self.assertRaises(ValueError):
82+
class X(Structure):
83+
_fields_ = [("a", c_byte)]
84+
_pack_ = _testcapi.UINT_MAX + 2
85+
_layout_ = "ms"
7786

7887
def test_initializers(self):
7988
class Person(Structure):

‎Lib/test/test_ctypes/test_unaligned_structures.py

Copy file name to clipboardExpand all lines: Lib/test/test_ctypes/test_unaligned_structures.py
+2Lines changed: 2 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -19,10 +19,12 @@
1919
c_ushort, c_uint, c_ulong, c_ulonglong]:
2020
class X(Structure):
2121
_pack_ = 1
22+
_layout_ = 'ms'
2223
_fields_ = [("pad", c_byte),
2324
("value", typ)]
2425
class Y(SwappedStructure):
2526
_pack_ = 1
27+
_layout_ = 'ms'
2628
_fields_ = [("pad", c_byte),
2729
("value", typ)]
2830
structures.append(X)

‎Misc/NEWS.d/next/C_API/2025-04-28-18-26-37.gh-issue-131747.2AiQ9n.rst

Copy file name to clipboard
+4Lines changed: 4 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -0,0 +1,4 @@
1+
On non-Windows platforms, deprecate using :attr:`ctypes.Structure._pack_` to
2+
use a Windows-compatible layout on non-Windows platforms. The layout should
3+
be specified explicitly by setting :attr:`ctypes.Structure._layout_` to
4+
``'ms'``.

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