Adding a ctypes.Union to a ctypes.BigEndianStructure results in an error

[MrSurly]

BPO	43073
Nosy	@MrSurly

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2021-01-30.04:38:58.603>
labels = ['ctypes', '3.8']
title = 'Adding a ctypes.Union to a ctypes.BigEndianStructure results in an error'
updated_at = <Date 2021-11-07.14:24:54.167>
user = 'https://github.com/MrSurly'

bugs.python.org fields:

activity = <Date 2021-11-07.14:24:54.167>
actor = 'rindeal'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['ctypes']
creation = <Date 2021-01-30.04:38:58.603>
creator = 'MrSurly'
dependencies = []
files = []
hgrepos = []
issue_num = 43073
keywords = []
message_count = 2.0
messages = ['385970', '405906']
nosy_count = 2.0
nosy_names = ['MrSurly', 'rindeal']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = None
url = 'https://bugs.python.org/issue43073'
versions = ['Python 3.8']

[MrSurly]

Placing a ctypes.Union inside of a ctypes.BigEndianStructure results in "TypeError: This type does not support other endian". I believe this is a similar problem to issue bpo-4376 (https://bugs.python.org/issue4376)

Minimum repro test case:

import ctypes as ct

class U(ct.Union):
    _pack_=True
    _fields_=[
        ('a', ct.c_int),
        ('b', ct.c_int),
    ]

class S(ct.BigEndianStructure):
    _pack_=True
    _fields_=[
        ('x', ct.c_int),
        ('y', U),
    ]

I believe the fix is similar to that issue, though I admit I don't know enough about this code to be sure.

diff --git a/Lib/ctypes/_endian.py b/Lib/ctypes/_endian.py
index 37444bd6a7..525c5e58c9 100644
--- a/Lib/ctypes/_endian.py
+++ b/Lib/ctypes/_endian.py
@@ -18,6 +18,9 @@ def _other_endian(typ):
     # if typ is structure
     if issubclass(typ, Structure):
         return typ
+    # if typ is union:
+    if issubclass(typ, Union):
+        return typ
     raise TypeError("This type does not support other endian: %s" % typ)
 
 class _swapped_meta(type(Structure)):

[rindeal]

I have created a workaround, since it might take years to fix this in master. Hope it'll come in useful.

For the example in https://bugs.python.org/issue43073#msg385970, but probably any combination of Unions and BigEndianStructures can be constructed this way.

class U_a(ct.BigEndianStructure):
    _pack_ = True
    _fields_ = [('a', ct.c_int)]

class U_b(ct.BigEndianStructure):
    _pack_ = True
    _fields_ = [('b', ct.c_int)]

class U(ct.Union):
    _pack_ = True
    _fields_ = [
        ('_a', U_a),
        ('_b', U_b),
    ]
    _anonymous_ = ['_a', '_b']

class _S_be_fields_only(ct.Structure):
    _pack_ = True
    _fields_ = [
        ('_x', ct.c_int),
        ('y', U),
    ]
class _S_2be_fields_only(ct.BigEndianStructure):
    _pack_ = True
    _fields_ = [
        ('x', ct.c_int),
        ('_y', ct.c_byte * ct.sizeof(U)),
    ]

class _S_U(ct.Union):
    _pack_ = True
    _fields_ = [
        ('_be_fields_only', _S_be_fields_only),
        ('_2be_fields_only', _S_2be_fields_only),
    ]
    _anonymous_ = [f[0] for f in _fields_]

class S(ct.Structure):
    _pack_ = True
    _fields_ = [('_s_u', _S_U)]
    _anonymous_ = [_fields_[0][0]]


issubclass(S, ct.Structure) == True
s = S(x=0x11223344, y=U(a=0xaabbccdd))
s.y.a == s.y.b
bytes(s).hex() == "11223344aabbccdd"

[encukou]

This works in Python 3.11; presumably due to #25480.