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Commitbe76e3f

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gh-100980: ctypes: Test, document, and fix finalizing _fields_ (GH-124292)
- If setting `_fields_` fails, e.g. with AttributeError, don't set the attribute in `__dict__`- Document the “finalization” behaviour- Beef up tests: add `getattr`, test Union as well as Structure- Put common functionality in a common functionCo-authored-by: Peter Bierma <zintensitydev@gmail.com>
1 parente256a75 commitbe76e3f

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4 files changed

+67
-55
lines changed

4 files changed

+67
-55
lines changed

‎Doc/library/ctypes.rst‎

Lines changed: 14 additions & 7 deletions
Original file line numberDiff line numberDiff line change
@@ -2499,6 +2499,8 @@ Structured data types
24992499

25002500
Abstract base class for unions in native byte order.
25012501

2502+
Unions share common attributes and behavior with structures;
2503+
see:class:`Structure` documentation for details.
25022504

25032505
..class::BigEndianUnion(*args, **kw)
25042506

@@ -2558,14 +2560,19 @@ fields, or any other data types containing pointer type fields.
25582560
...
25592561
]
25602562

2561-
The:attr:`_fields_` class variable must, however, be defined before the
2562-
type is first used (an instance is created,:func:`sizeof` is called on it,
2563-
and so on). Later assignments to the:attr:`_fields_` class variable will
2564-
raise an AttributeError.
2563+
The:attr:`!_fields_` class variable can only be set once.
2564+
Later assignments will raise an:exc:`AttributeError`.
25652565

2566-
It is possible to define sub-subclasses of structure types, they inherit
2567-
the fields of the base class plus the:attr:`_fields_` defined in the
2568-
sub-subclass, if any.
2566+
Additionally, the:attr:`!_fields_` class variable must be defined before
2567+
the structure or union type is first used: an instance or subclass is
2568+
created,:func:`sizeof` is called on it, and so on.
2569+
Later assignments to:attr:`!_fields_` will raise an:exc:`AttributeError`.
2570+
If:attr:`!_fields_` has not been set before such use,
2571+
the structure or union will have no own fields, as if:attr:`!_fields_`
2572+
was empty.
2573+
2574+
Sub-subclasses of structure types inherit the fields of the base class
2575+
plus the:attr:`_fields_` defined in the sub-subclass, if any.
25692576

25702577

25712578
..attribute::_pack_

‎Lib/test/test_ctypes/test_struct_fields.py‎

Lines changed: 35 additions & 32 deletions
Original file line numberDiff line numberDiff line change
@@ -4,7 +4,9 @@
44
Py_TPFLAGS_IMMUTABLETYPE)
55

66

7-
classStructFieldsTestCase(unittest.TestCase):
7+
NOTHING=object()
8+
9+
classFieldsTestBase:
810
# Structure/Union classes must get 'finalized' sooner or
911
# later, when one of these things happen:
1012
#
@@ -14,42 +16,47 @@ class StructFieldsTestCase(unittest.TestCase):
1416
# 4. The type is subclassed
1517
#
1618
# When they are finalized, assigning _fields_ is no longer allowed.
19+
20+
defassert_final_fields(self,cls,expected=NOTHING):
21+
self.assertRaises(AttributeError,setattr,cls,"_fields_", [])
22+
self.assertEqual(getattr(cls,"_fields_",NOTHING),expected)
23+
1724
deftest_1_A(self):
18-
classX(Structure):
25+
classX(self.cls):
1926
pass
2027
self.assertEqual(sizeof(X),0)# not finalized
2128
X._fields_= []# finalized
22-
self.assertRaises(AttributeError,setattr,X,"_fields_",[])
29+
self.assert_final_fields(X,expected=[])
2330

2431
deftest_1_B(self):
25-
classX(Structure):
32+
classX(self.cls):
2633
_fields_= []# finalized
27-
self.assertRaises(AttributeError,setattr,X,"_fields_",[])
34+
self.assert_final_fields(X,expected=[])
2835

2936
deftest_2(self):
30-
classX(Structure):
37+
classX(self.cls):
3138
pass
3239
X()
33-
self.assertRaises(AttributeError,setattr,X,"_fields_", [])
40+
self.assert_final_fields(X)
3441

3542
deftest_3(self):
36-
classX(Structure):
43+
classX(self.cls):
3744
pass
38-
classY(Structure):
45+
classY(self.cls):
3946
_fields_= [("x",X)]# finalizes X
40-
self.assertRaises(AttributeError,setattr,X,"_fields_", [])
47+
self.assert_final_fields(X)
4148

4249
deftest_4(self):
43-
classX(Structure):
50+
classX(self.cls):
4451
pass
4552
classY(X):
4653
pass
47-
self.assertRaises(AttributeError,setattr,X,"_fields_", [])
54+
self.assert_final_fields(X)
4855
Y._fields_= []
49-
self.assertRaises(AttributeError,setattr,X,"_fields_", [])
56+
self.assert_final_fields(X)
5057

5158
deftest_5(self):
52-
classX(Structure):
59+
classX(self.cls):
5360
_fields_= (("char",c_char*5),)
5461

5562
x=X(b'#'*5)
@@ -59,14 +66,8 @@ class X(Structure):
5966
deftest_6(self):
6067
self.assertRaises(TypeError,CField)
6168

62-
deftest_cfield_type_flags(self):
63-
self.assertTrue(CField.__flags__&Py_TPFLAGS_IMMUTABLETYPE)
64-
65-
deftest_cfield_inheritance_hierarchy(self):
66-
self.assertEqual(CField.mro(), [CField,object])
67-
6869
deftest_gh99275(self):
69-
classBrokenStructure(Structure):
70+
classBrokenStructure(self.cls):
7071
def__init_subclass__(cls,**kwargs):
7172
cls._fields_= []# This line will fail, `stginfo` is not ready
7273

@@ -77,26 +78,28 @@ class Subclass(BrokenStructure): ...
7778
# __set__ and __get__ should raise a TypeError in case their self
7879
# argument is not a ctype instance.
7980
deftest___set__(self):
80-
classMyCStruct(Structure):
81+
classMyCStruct(self.cls):
8182
_fields_= (("field",c_int),)
8283
self.assertRaises(TypeError,
8384
MyCStruct.field.__set__,'wrong type self',42)
8485

85-
classMyCUnion(Union):
86-
_fields_= (("field",c_int),)
87-
self.assertRaises(TypeError,
88-
MyCUnion.field.__set__,'wrong type self',42)
89-
9086
deftest___get__(self):
91-
classMyCStruct(Structure):
87+
classMyCStruct(self.cls):
9288
_fields_= (("field",c_int),)
9389
self.assertRaises(TypeError,
9490
MyCStruct.field.__get__,'wrong type self',42)
9591

96-
classMyCUnion(Union):
97-
_fields_= (("field",c_int),)
98-
self.assertRaises(TypeError,
99-
MyCUnion.field.__get__,'wrong type self',42)
92+
classStructFieldsTestCase(unittest.TestCase,FieldsTestBase):
93+
cls=Structure
94+
95+
deftest_cfield_type_flags(self):
96+
self.assertTrue(CField.__flags__&Py_TPFLAGS_IMMUTABLETYPE)
97+
98+
deftest_cfield_inheritance_hierarchy(self):
99+
self.assertEqual(CField.mro(), [CField,object])
100+
101+
classUnionFieldsTestCase(unittest.TestCase,FieldsTestBase):
102+
cls=Union
100103

101104

102105
if__name__=="__main__":
Lines changed: 3 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -0,0 +1,3 @@
1+
The:attr:`~ctypes.Structure._fields_` attribute of
2+
:class:`ctypes.Structure` and:class:`~ctypes.Union` is no longer set if
3+
the setattr operation raises an error.

‎Modules/_ctypes/_ctypes.c‎

Lines changed: 15 additions & 16 deletions
Original file line numberDiff line numberDiff line change
@@ -1067,32 +1067,31 @@ CType_Type_repeat(PyObject *self, Py_ssize_t length)
10671067
returnPyCArrayType_from_ctype(st,self,length);
10681068
}
10691069

1070-
10711070
staticint
1072-
PyCStructType_setattro(PyObject*self,PyObject*key,PyObject*value)
1071+
_structunion_setattro(PyObject*self,PyObject*key,PyObject*value,intis_struct)
10731072
{
10741073
/* XXX Should we disallow deleting _fields_? */
1075-
if (-1==PyType_Type.tp_setattro(self,key,value))
1076-
return-1;
1074+
if (PyUnicode_Check(key)
1075+
&&_PyUnicode_EqualToASCIIString(key,"_fields_"))
1076+
{
1077+
if (PyCStructUnionType_update_stginfo(self,value,is_struct)<0) {
1078+
return-1;
1079+
}
1080+
}
10771081

1078-
if (value&&PyUnicode_Check(key)&&
1079-
_PyUnicode_EqualToASCIIString(key,"_fields_"))
1080-
returnPyCStructUnionType_update_stginfo(self,value,1);
1081-
return0;
1082+
returnPyType_Type.tp_setattro(self,key,value);
10821083
}
10831084

1085+
staticint
1086+
PyCStructType_setattro(PyObject*self,PyObject*key,PyObject*value)
1087+
{
1088+
return_structunion_setattro(self,key,value,1);
1089+
}
10841090

10851091
staticint
10861092
UnionType_setattro(PyObject*self,PyObject*key,PyObject*value)
10871093
{
1088-
/* XXX Should we disallow deleting _fields_? */
1089-
if (-1==PyType_Type.tp_setattro(self,key,value))
1090-
return-1;
1091-
1092-
if (PyUnicode_Check(key)&&
1093-
_PyUnicode_EqualToASCIIString(key,"_fields_"))
1094-
returnPyCStructUnionType_update_stginfo(self,value,0);
1095-
return0;
1094+
return_structunion_setattro(self,key,value,0);
10961095
}
10971096

10981097
staticPyType_Slotpycstruct_type_slots[]= {

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