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

Deprecate calling PyObject_SetAttr() and PyObject_SetAttrString()
with NULL value and an exception set.

Author: vstinner

Doc/c-api/object.rst

@@ -197,6 +197,10 @@ Object Protocol
    in favour of using :c:func:`PyObject_DelAttr`, but there are currently no
    plans to remove it.
 
+   .. deprecated:: next
+      Calling this function with ``NULL`` *v* and an exception set is now
+      deprecated.
+
 
 .. c:function:: int PyObject_SetAttrString(PyObject *o, const char *attr_name, PyObject *v)
 
@@ -215,6 +219,11 @@ Object Protocol
    For more details, see :c:func:`PyUnicode_InternFromString`, which may be
    used internally to create a key object.
 
+   .. deprecated:: next
+      Calling this function with ``NULL`` *v* and an exception set is now
+      deprecated.
+
+
 .. c:function:: int PyObject_GenericSetAttr(PyObject *o, PyObject *name, PyObject *value)
 
    Generic attribute setter and deleter function that is meant

Doc/whatsnew/3.15.rst

@@ -322,7 +322,9 @@ Porting to Python 3.15
 Deprecated C APIs
 -----------------
 
-* TODO
+* Calling :c:func:`PyObject_SetAttr` and :c:func:`PyObject_SetAttrString` with
+  ``NULL`` value and an exception set is now deprecated.
+  (Contributed by Victor Stinner in :gh:`135075`.)
 
 .. Add C API deprecations above alphabetically, not here at the end.
 

Lib/test/test_capi/test_object.py

@@ -247,5 +247,23 @@ def func(x):
 
         func(object())
 
+    def test_object_setattr_null_exc(self):
+        class Obj:
+            pass
+        obj = Obj()
+
+        obj.attr = 123
+        with self.assertWarns(DeprecationWarning):
+            with self.assertRaises(ValueError):
+                _testcapi.object_setattr_null_exc(obj, 'attr')
+        self.assertFalse(hasattr(obj, 'attr'))
+
+        obj.attr = 456
+        with self.assertWarns(DeprecationWarning):
+            with self.assertRaises(ValueError):
+                _testcapi.object_setattrstring_null_exc(obj, 'attr')
+        self.assertFalse(hasattr(obj, 'attr'))
+
+
 if __name__ == "__main__":
     unittest.main()

Misc/NEWS.d/next/C_API/2025-06-03-14-55-43.gh-issue-135075.fq1iqH.rst

@@ -0,0 +1,3 @@
+Deprecate calling :c:func:`PyObject_SetAttr` and
+:c:func:`PyObject_SetAttrString` with ``NULL`` value and an exception set is
+now deprecated. Patch by Victor Stinner.

Modules/_testcapi/object.c

@@ -485,6 +485,41 @@ is_uniquely_referenced(PyObject *self, PyObject *op)
 }
 
 
+static PyObject *
+object_setattr_null_exc(PyObject *self, PyObject *args)
+{
+    PyObject *obj, *name;
+    if (!PyArg_ParseTuple(args, "OO", &obj, &name)) {
+        return NULL;
+    }
+
+    PyErr_SetString(PyExc_ValueError, "error");
+    if (PyObject_SetAttr(obj, name, NULL) < 0) {
+        return NULL;
+    }
+    assert(PyErr_Occurred());
+    return NULL;
+}
+
+
+static PyObject *
+object_setattrstring_null_exc(PyObject *self, PyObject *args)
+{
+    PyObject *obj;
+    const char *name;
+    if (!PyArg_ParseTuple(args, "Os", &obj, &name)) {
+        return NULL;
+    }
+
+    PyErr_SetString(PyExc_ValueError, "error");
+    if (PyObject_SetAttrString(obj, name, NULL) < 0) {
+        return NULL;
+    }
+    assert(PyErr_Occurred());
+    return NULL;
+}
+
+
 static PyMethodDef test_methods[] = {
     {"call_pyobject_print", call_pyobject_print, METH_VARARGS},
     {"pyobject_print_null", pyobject_print_null, METH_VARARGS},
@@ -511,6 +546,8 @@ static PyMethodDef test_methods[] = {
     {"test_py_is_funcs", test_py_is_funcs, METH_NOARGS},
     {"clear_managed_dict", clear_managed_dict, METH_O, NULL},
     {"is_uniquely_referenced", is_uniquely_referenced, METH_O},
+    {"object_setattr_null_exc", object_setattr_null_exc, METH_VARARGS},
+    {"object_setattrstring_null_exc", object_setattrstring_null_exc, METH_VARARGS},
     {NULL},
 };
 

Objects/object.c

@@ -1212,8 +1212,18 @@ PyObject_SetAttrString(PyObject *v, const char *name, PyObject *w)
     PyObject *s;
     int res;
 
-    if (Py_TYPE(v)->tp_setattr != NULL)
+    if (Py_TYPE(v)->tp_setattr != NULL) {
+        if (w == NULL && PyErr_Occurred()) {
+            if (PyErr_WarnFormat(PyExc_DeprecationWarning, 0,
+                    "calling PyObject_SetAttrString() with NULL value "
+                    "and an exception set is deprecated; "
+                    "use PyObject_DelAttrString() instead")) {
+                return -1;
+            }
+        }
+
         return (*Py_TYPE(v)->tp_setattr)(v, (char*)name, w);
+    }
     s = PyUnicode_InternFromString(name);
     if (s == NULL)
         return -1;
@@ -1437,7 +1447,7 @@ int
 PyObject_SetAttr(PyObject *v, PyObject *name, PyObject *value)
 {
     PyTypeObject *tp = Py_TYPE(v);
-    int err;
+    int res;
 
     if (!PyUnicode_Check(name)) {
         PyErr_Format(PyExc_TypeError,
@@ -1447,25 +1457,37 @@ PyObject_SetAttr(PyObject *v, PyObject *name, PyObject *value)
     }
     Py_INCREF(name);
 
-    PyInterpreterState *interp = _PyInterpreterState_GET();
-    _PyUnicode_InternMortal(interp, &name);
+    PyThreadState *tstate = _PyThreadState_GET();
+    PyObject *exc = NULL;
+    if (value == NULL && _PyErr_Occurred(tstate)) {
+        exc = _PyErr_GetRaisedException(tstate);
+        res = PyErr_WarnFormat(PyExc_DeprecationWarning, 0,
+                "calling PyObject_SetAttr() with NULL value "
+                "and an exception set is deprecated; "
+                "use PyObject_DelAttr() instead");
+        if (res) {
+            res = -1;
+            goto done;
+        }
+    }
+
+    _PyUnicode_InternMortal(tstate->interp, &name);
+
     if (tp->tp_setattro != NULL) {
-        err = (*tp->tp_setattro)(v, name, value);
-        Py_DECREF(name);
-        return err;
+        res = (*tp->tp_setattro)(v, name, value);
+        goto done;
     }
+
     if (tp->tp_setattr != NULL) {
         const char *name_str = PyUnicode_AsUTF8(name);
         if (name_str == NULL) {
-            Py_DECREF(name);
-            return -1;
+            res = -1;
+            goto done;
         }
-        err = (*tp->tp_setattr)(v, (char *)name_str, value);
-        Py_DECREF(name);
-        return err;
+        res = (*tp->tp_setattr)(v, (char *)name_str, value);
+        goto done;
     }
-    Py_DECREF(name);
-    _PyObject_ASSERT(name, Py_REFCNT(name) >= 1);
+
     if (tp->tp_getattr == NULL && tp->tp_getattro == NULL)
         PyErr_Format(PyExc_TypeError,
                      "'%.100s' object has no attributes "
@@ -1480,7 +1502,15 @@ PyObject_SetAttr(PyObject *v, PyObject *name, PyObject *value)
                      tp->tp_name,
                      value==NULL ? "del" : "assign to",
                      name);
-    return -1;
+    res = -1;
+    goto done;
+
+done:
+    if (exc) {
+        _PyErr_ChainExceptions1Tstate(tstate, exc);
+    }
+    Py_DECREF(name);
+    return res;
 }
 
 int