ctypes Stucture._fields_ cannot be set when Structure has been subclassed

[ionite34]

Bug report

Currently the method for defining Structure fields that cannot be defined within the class, is to define the class with pass and then set _fields_ outside. But ctypes.Structure incorrectly raises AttributeError, even when no class has ever set a _fields_.

from ctypes import *

class PyObject(Structure):
    pass

class PyVarObject(PyObject):
    pass

class PyTypeObject(PyVarObject):
    pass

PyObject._fields_ = [
    ("ob_refcnt", c_long),
    ("ob_type", POINTER(PyTypeObject)),
]

    PyObject._fields_ = [
    ^^^^^^^^^^^^^^^^^
AttributeError: _fields_ is final

Strangely, suppressing this Attribute error reveals the _fields_ attribute does actually get set?

with suppress(AttributeError):
    PyObject._fields_ = [
        ("ob_refcnt", c_long),
        ("ob_type", POINTER(PyTypeObject)),
    ]

print(PyObject._fields_)
>> [('ob_refcnt', <class 'ctypes.c_long'>), ('ob_type', <class '__main__.LP_PyTypeObject'>)]

Your environment

• CPython versions tested on: 3.11.1, main
• Operating system and architecture: macOS ARM, ubuntu x64

Linked PRs

• gh-100980: ctypes: Test, document, and fix finalizing _fields_ #124292

[encukou]

Subclassing a Structure finalizes it, which prevents setting _fields_. That is the intended behaviour: the superclass's fields are “inherited” (included in the subclass), so they must be known rather early, and they can't change when a new subclass is made. (AFAIK we could delay the finalization a bit more, but that would add hard-to-test edge cases, so let's not.)
The docs could be clearer here.

The bug here is that the _fields_ attribute is set despite AttributeError. This should not happen. (However, the setattr is just “cosmetic”: the internals aren't updated, in fact the attribute is never consulted again after finalization.)

It's confusing that it's possible can finalize a class that doesn't have _fields_ set. However, this feature is useful for superclasses that don't add/affect C fields -- like Structure itself.