bpo-39573: Py_TYPE becomes a static inline function (GH-28128)

Convert the Py_TYPE() and Py_SIZE() macros to static inline
functions. The Py_SET_TYPE() and Py_SET_SIZE() functions must now be
used to set an object type and size.

Author: vstinner

Diff for: Doc/c-api/structures.rst

@@ -99,7 +99,10 @@ the definition of all other Python objects.
 
    Return a :term:`borrowed reference`.
 
-   The :c:func:`Py_SET_TYPE` function must be used to set an object type.
+   Use the :c:func:`Py_SET_TYPE` function to set an object type.
+
+   .. versionchanged:: 3.11
+      :c:func:`Py_TYPE()` is changed to an inline static function.
 
 
 .. c:function:: int Py_IS_TYPE(PyObject *o, PyTypeObject *type)
@@ -121,9 +124,10 @@ the definition of all other Python objects.
 
    Get the reference count of the Python object *o*.
 
+   Use the :c:func:`Py_SET_REFCNT()` function to set an object reference count.
+
    .. versionchanged:: 3.10
       :c:func:`Py_REFCNT()` is changed to the inline static function.
-      Use :c:func:`Py_SET_REFCNT()` to set an object reference count.
 
 
 .. c:function:: void Py_SET_REFCNT(PyObject *o, Py_ssize_t refcnt)
@@ -137,7 +141,10 @@ the definition of all other Python objects.
 
    Get the size of the Python object *o*.
 
-   The :c:func:`Py_SET_SIZE` function must be used to set an object size.
+   Use the :c:func:`Py_SET_SIZE` function to set an object size.
+
+   .. versionchanged:: 3.11
+      :c:func:`Py_SIZE()` is changed to an inline static function.
 
 
 .. c:function:: void Py_SET_SIZE(PyVarObject *o, Py_ssize_t size)

Diff for: Doc/whatsnew/3.11.rst

@@ -402,6 +402,34 @@ Porting to Python 3.11
   :ref:`static types <static-types>`.
   (Contributed by Erlend E. Aasland in :issue:`43908`)
 
+* Since :c:func:`Py_TYPE()` is changed to a inline static function,
+  ``Py_TYPE(obj) = new_type`` must be replaced with
+  ``Py_SET_TYPE(obj, new_type)``: see the :c:func:`Py_SET_TYPE()` function
+  (available since Python 3.9). For backward compatibility, this macro can be
+  used::
+
+      #if PY_VERSION_HEX < 0x030900A4 && !defined(Py_SET_TYPE)
+      static inline void _Py_SET_TYPE(PyObject *ob, PyTypeObject *type)
+      { ob->ob_type = type; }
+      #define Py_SET_TYPE(ob, type) _Py_SET_TYPE((PyObject*)(ob), type)
+      #endif
+
+  (Contributed by Victor Stinner in :issue:`39573`.)
+
+* Since :c:func:`Py_SIZE()` is changed to a inline static function,
+  ``Py_SIZE(obj) = new_size`` must be replaced with
+  ``Py_SET_SIZE(obj, new_size)``: see the :c:func:`Py_SET_SIZE()` function
+  (available since Python 3.9). For backward compatibility, this macro can be
+  used::
+
+      #if PY_VERSION_HEX < 0x030900A4 && !defined(Py_SET_SIZE)
+      static inline void _Py_SET_SIZE(PyVarObject *ob, Py_ssize_t size)
+      { ob->ob_size = size; }
+      #define Py_SET_SIZE(ob, size) _Py_SET_SIZE((PyVarObject*)(ob), size)
+      #endif
+
+  (Contributed by Victor Stinner in :issue:`39573`.)
+
 Deprecated
 ----------
 

Diff for: Include/object.h

@@ -134,10 +134,16 @@ static inline Py_ssize_t _Py_REFCNT(const PyObject *ob) {
 
 
 // bpo-39573: The Py_SET_TYPE() function must be used to set an object type.
-#define Py_TYPE(ob)             (_PyObject_CAST(ob)->ob_type)
+static inline PyTypeObject* _Py_TYPE(const PyObject *ob) {
+    return ob->ob_type;
+}
+#define Py_TYPE(ob) _Py_TYPE(_PyObject_CAST_CONST(ob))
 
 // bpo-39573: The Py_SET_SIZE() function must be used to set an object size.
-#define Py_SIZE(ob)             (_PyVarObject_CAST(ob)->ob_size)
+static inline Py_ssize_t _Py_SIZE(const PyVarObject *ob) {
+    return ob->ob_size;
+}
+#define Py_SIZE(ob) _Py_SIZE(_PyVarObject_CAST_CONST(ob))
 
 
 static inline int _Py_IS_TYPE(const PyObject *ob, const PyTypeObject *type) {

Diff for: Misc/NEWS.d/next/C API/2021-06-03-00-59-48.bpo-39573.-elHTJ.rst

@@ -0,0 +1,3 @@
+Convert the :c:func:`Py_TYPE` and :c:func:`Py_SIZE` macros to static inline
+functions. The :c:func:`Py_SET_TYPE` and :c:func:`Py_SET_SIZE` functions
+must now be used to set an object type and size. Patch by Victor Stinner.

Diff for: Modules/_testcapimodule.c

@@ -5511,10 +5511,9 @@ test_set_type_size(PyObject *self, PyObject *Py_UNUSED(ignored))
     assert(Py_TYPE(obj) == &PyList_Type);
     assert(Py_SIZE(obj) == 0);
 
-    // bpo-39573: Check that Py_TYPE() and Py_SIZE() can be used
-    // as l-values to set an object type and size.
-    Py_TYPE(obj) = &PyList_Type;
-    Py_SIZE(obj) = 0;
+    // bpo-39573: Test Py_SET_TYPE() and Py_SET_SIZE() functions.
+    Py_SET_TYPE(obj, &PyList_Type);
+    Py_SET_SIZE(obj, 0);
 
     Py_DECREF(obj);
     Py_RETURN_NONE;