Issue with `PyObject_DelAttr[String]` in stable ABI builds targeting older CPython versions

[wjakob]

Bug report

Bug description:

To my knowledge, one can use newer Python version (e.g. Python 3.13) to build C extensions for older Python versions (e.g. 3.12) when targeting the stable ABI. This is accomplished by setting Py_LIMITED_API to a suitable number like 0x030b0000, which simply disables all features beyond this cutoff.

If this understanding is correct, then I would like to report a bug:

Python 3.13 added an new C API function for PyObject_DelAttr(). In contrast, Python 3.12 realized this operation using a macro wrapping PyObject_SetAttr():

#define  PyObject_DelAttr(O, A) PyObject_SetAttr((O), (A), NULL)

This change is problematic: extensions built on 3.13 targeting 3.12 stable ABI will now call a function that does not exist on Python 3.12, and such extensions will fail to load with a linker error. The change to the new C API entry point should have been guarded with #ifdefs.

This issue also affects PyObject_DelAttrString().

CPython versions tested on:

3.13

Operating systems tested on:

Linux

Linked PRs

• gh-134989: Implement PyObject_DelAttr() as a macro in the limited C API #135021
• [3.14] gh-134989: Implement PyObject_DelAttr() as a macro in the limited C API (GH-135021) #135133
• [3.13] gh-134989: Implement PyObject_DelAttr() as a macro in the limited C API (GH-135021) #135134
• gh-134989: Fix Py_RETURN_NONE in the limited C API #135165
• [3.14] gh-134989: Fix Py_RETURN_NONE in the limited C API (GH-135165) #135178
• [3.13] gh-134989: Fix Py_RETURN_NONE in the limited C API (GH-135165) #135182

[ZeroIntensity]

cc @vstinner

[somik861]

Same behaviour also on Windows 11.

[vstinner]

I wrote PR #135021 to fix the regression.

@wjakob: Do you think that you can try the PR to see if it does fix your issue?

[wjakob]

This works! However, would it not be safer to also wrap the declaration of PyObject_DelAttr "elsewhere" so that it is entirely avoided if the following preprocessor condition holds: defined(Py_LIMITED_API) && Py_LIMITED_API+0 < 0x030d0000?

I am concerned that a user might include files in such a way that the macro rewrites the declaration. Maybe you already thought about this, and it is not possible.

[vstinner]

This works!

Cool. Thanks for testing!

However, would it not be safer to also wrap the declaration of PyObject_DelAttr "elsewhere" so that it is entirely avoided if the following preprocessor condition holds: defined(Py_LIMITED_API) && Py_LIMITED_API+0 < 0x030d0000?
I am concerned that a user might include files in such a way that the macro rewrites the declaration. Maybe you already thought about this, and it is not possible.

The Python C API requires to only include <Python.h>. Including other header files is not supported. My PR changes Include/abstract.h which is included by <Python.h>.