[C API] Deprecate calling PyObject_SetAttr(obj, attr, NULL) with an exception set

[vstinner]

The issue gh-106572 changed PyObject_DelAttr() and PyObject_DelAttrString() implementation from a macro to a function (commit 1f2921b).

Now I propose deprecating calls PyObject_SetAttr(obj, attr, NULL) and PyObject_SetAttrString(obj, attr, NULL) with an exception set to help detecting bugs. When the code creating the attribute value failed, it should be handled before calling PyObject_SetAttr[String)() to avoid deleting the attribute by mistake.

The change affects also the stable ABI.

I also propose to treat this deprecation as an error in Python 3.17.

Example of buggy code:

PyObject *module = PyUnicode_FromString(module_name);
int res = PyObject_SetAttrString(ns, "__module__", module);
Py_DECREF(module);

Correct code:

PyObject *module = PyUnicode_FromString(module_name);
if (module == NULL) { // <=== CHECK VALUE ===
    goto error;
}
int res = PyObject_SetAttrString(ns, "__module__", module);
Py_DECREF(module);

Linked PRs

• gh-135075: Deprecate PyObject_SetAttr(obj, name, NULL) with exc #135082

[vstinner]

It seems like currently, it's not really possible to call PyObject_SetAttr() with NULL and an exception set. For example, if type->tp_setattro is PyObject_GenericSetAttr(), it fails with an assertion error:

python: Objects/typeobject.c:6080: _PyType_LookupStackRefAndVersion: Assertion `!PyErr_Occurred()' failed.

[vstinner]

Maybe we should directly treat this case (calling PyObject_SetAttr() with NULL value and an exception set) as an error, rather than trying to emit a DeprecationWarning.