Repository navigation
ctypes Stucture._fields_ cannot be set when Structure has been subclassed #100980
Description
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 finalStrangely, 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
Metadata
Metadata
Assignees
Projects
- Status
Activity
[-]ctypes._fields_ cannot be set when Structure has been subclassed[/-][+]ctypes Stucture._fields_ cannot be set when Structure has been subclassed[/+]TMP: pythongh-100980: Don't set _fields_ if update_stginfo fails
encukou commentedon Sep 20, 2024
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 despiteAttributeError. 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 -- likeStructureitself.TMP: pythongh-100980: Don't set _fields_ if update_stginfo fails
gh-100980: ctypes: Test, document, and fix finalizing _fields_ (GH-12…