Add Py_SETREF/Py_XSETREF to old limited C API versions

Author: vstinner

docs/changelog.rst

@@ -1,6 +1,8 @@
 Changelog
 =========
 
+* 2026-11-04: Add ``Py_SETREF()`` and ``Py_XSETREF()`` to the limited C API
+  older than Python 3.16.
 * 2026-10-12: Add functions:
 
   * ``PyEval_GetFrameBuiltins()``

pythoncapi_compat.h

@@ -169,6 +169,34 @@ static inline void _Py_SET_REFCNT(PyObject *ob, Py_ssize_t refcnt)
 #endif
 
 
+// Python 3.6 added Py_SETREF() and Py_XSETREF() macros.
+// Python 3.16 adds them to the limited C API.
+// Don't use the more advanced typeof()/__typeof__() or C++ (auto)
+// implementation, but only the memcpy() implementation to keep the code short.
+#ifndef Py_SETREF
+#define Py_SETREF(dst, src) \
+    do { \
+        PyObject **_tmp_dst_ptr = _Py_CAST(PyObject**, &(dst)); \
+        PyObject *_tmp_old_dst; \
+        PyObject *_tmp_src = _PyObject_CAST(src); \
+        memcpy(&_tmp_old_dst, _tmp_dst_ptr, sizeof(PyObject*)); \
+        memcpy(_tmp_dst_ptr, &_tmp_src, sizeof(PyObject*)); \
+        Py_DECREF(_tmp_old_dst); \
+    } while (0)
+#endif
+#ifndef Py_XSETREF
+#define Py_XSETREF(dst, src) \
+    do { \
+        PyObject **_tmp_dst_ptr = _Py_CAST(PyObject**, &(dst)); \
+        PyObject *_tmp_old_dst; \
+        PyObject *_tmp_src = _PyObject_CAST(src); \
+        memcpy(&_tmp_old_dst, _tmp_dst_ptr, sizeof(PyObject*)); \
+        memcpy(_tmp_dst_ptr, &_tmp_src, sizeof(PyObject*)); \
+        Py_XDECREF(_tmp_old_dst); \
+    } while (0)
+#endif
+
+
 // bpo-39573 added Py_SET_TYPE() to Python 3.9
 #if !defined(Py_SET_TYPE) && PY_VERSION_HEX < _Py_PACK_VERSION(3, 9)
 static inline void _Py_SET_TYPE(PyObject *ob, PyTypeObject *type)

tests/test_pythoncapi_compat_cext.c

@@ -75,7 +75,6 @@ test_object(PyObject *Py_UNUSED(module), PyObject* Py_UNUSED(ignored))
 
     assert(Py_XNewRef(_Py_NULL) == _Py_NULL);
 
-#if !defined(Py_LIMITED_API) || Py_LIMITED_API+0 >= _Py_PACK_VERSION(3, 16)
     // Py_SETREF()
     PyObject *setref = Py_NewRef(obj);
     PyObject *none = Py_None;
@@ -100,7 +99,6 @@ test_object(PyObject *Py_UNUSED(module), PyObject* Py_UNUSED(ignored))
     Py_XSETREF(xsetref, _Py_NULL);
     assert(Py_REFCNT(obj) == refcnt);
     assert(xsetref == _Py_NULL);
-#endif
 
     // Py_SET_REFCNT
     Py_SET_REFCNT(obj, Py_REFCNT(obj));