TypeError when using ctypes.BigEndianUnion nested in ctypes.BigEndianStructure on a little endian system (or vice versa)

[Sh3idan]

Bug report

The classes ctypes.BigEndianUnion and ctypes.LittleEndianUnion was added in Python 3.11. In the case of the use of an Union in a Structure class that match the opposite system endianness, the Union class can't be used as shown below:

# coding: utf-8
import sys 
import ctypes as c

assert sys.byteorder == "little"

"""
struct MyStruct {
    unsigned char a;
    unsigned char b:
};
struct MainStruct {
    union {
        struct MyStruct my_struct;
        short field_ab;
    } my_union;
    unsigned char foo;
};
"""
class MyStruct(c.BigEndianStructure):
    _fields_ = [("a", c.c_byte), ("b", c.c_byte)]

    
class MyUnion(c.BigEndianUnion):
    _fields_ = [("my_struct", MyStruct), ("ab", c.c_short)]
    

class MainStruct(c.BigEndianStructure):
    _fields_ = [("my_union", MyUnion), ("foo", c.c_byte)]
    

The following traceback is given by the interpretor:

TypeError                                 Traceback (most recent call last)
Cell In[1], line 28
     24 class MyUnion(c.BigEndianUnion):
     25     _fields_ = [("my_struct", MyStruct), ("ab", c.c_short)]
---> 28 class MainStruct(c.BigEndianStructure):
     29     _fields_ = [("my_union", MyUnion), ("foo", c.c_byte)]

File ~\AppData\Local\Programs\Python\Python311\Lib\ctypes\_endian.py:31, in _swapped_meta.__setattr__(self, attrname, value)
     29         typ = desc[1]
     30         rest = desc[2:]
---> 31         fields.append((name, _other_endian(typ)) + rest)
     32     value = fields
     33 super().__setattr__(attrname, value)

File ~\AppData\Local\Programs\Python\Python311\Lib\ctypes\_endian.py:21, in _other_endian(typ)
     19 if issubclass(typ, Structure):
     20     return typ
---> 21 raise TypeError("This type does not support other endian: %s" % typ)

TypeError: This type does not support other endian: <class '__main__.MyUnion'>

Your environment

• CPython versions tested on: Interpreter compiled from the main branch in debug mode and also Python 3.11 (coming from the official release)
• Operating system and architecture: Windows 10 x64

Linked PRs

• gh-105102: fix nested unions in structures when the system byteorder is the opposite #105106
• [3.12] gh-105102: Fix nested unions in structures when the system byteorder is the opposite (GH-105106) #114204
• [3.11] gh-105102: Fix nested unions in structures when the system byteorder is the opposite (GH-105106) #114205