pythongh-111138: Add PyList_Extend() and PyList_Clear() functions

* Split list_extend() into two sub-functions: list_extend_fast() and
  list_extend_iter().
* list_inplace_concat() no longer has to call Py_DECREF() on the
  list_extend() result, since list_extend() now returns an int.

Doc/c-api/list.rst

@@ -51,6 +51,17 @@ List Objects
    ``len(list)`` on a list object.
 
 
+.. c:function:: int PyList_Clear(PyObject *list)
+
+   Remove all items from *list*. Similar to:
+   ``PyList_SetSlice(L, 0, PY_SSIZE_T_MAX, NULL)``.
+
+   Raise an exception and return ``-1`` if *list* is not a :class:`list`
+   object.  Return 0 on success.
+
+   .. versionadded:: 3.13
+
+
 .. c:function:: Py_ssize_t PyList_GET_SIZE(PyObject *list)
 
    Similar to :c:func:`PyList_Size`, but without error checking.
@@ -112,6 +123,17 @@ List Objects
    to ``list.append(item)``.
 
 
+.. c:function:: int PyList_Extend(PyObject *self, PyObject *iterable)
+
+   Extend *list* with the contents of *iterable*. Similar to:
+   ``PyList_SetSlice(L, PY_SSIZE_T_MAX, PY_SSIZE_T_MAX, iterable)``.
+
+   Raise an exception and return ``-1`` if *list* is not a :class:`list`
+   object. Return 0 on success.
+
+   .. versionadded:: 3.13
+
+
 .. c:function:: PyObject* PyList_GetSlice(PyObject *list, Py_ssize_t low, Py_ssize_t high)
 
    Return a list of the objects in *list* containing the objects *between* *low*

Doc/whatsnew/3.13.rst

@@ -1164,6 +1164,10 @@ New Features
   :c:func:`PyErr_WriteUnraisable`, but allow to customize the warning mesage.
   (Contributed by Serhiy Storchaka in :gh:`108082`.)
 
+* Add :c:func:`PyList_Extend` and :c:func:`PyList_Clear` functions: similar to
+  Python ``list.extend()`` and ``list.clear()`` methods.
+  (Contributed by Victor Stinner in :gh:`111138`.)
+
 
 Porting to Python 3.13
 ----------------------

Include/listobject.h

@@ -35,6 +35,8 @@ PyAPI_FUNC(int) PyList_Append(PyObject *, PyObject *);
 
 PyAPI_FUNC(PyObject *) PyList_GetSlice(PyObject *, Py_ssize_t, Py_ssize_t);
 PyAPI_FUNC(int) PyList_SetSlice(PyObject *, Py_ssize_t, Py_ssize_t, PyObject *);
+PyAPI_FUNC(int) PyList_Extend(PyObject *self, PyObject *iterable);
+PyAPI_FUNC(int) PyList_Clear(PyObject *self);
 
 PyAPI_FUNC(int) PyList_Sort(PyObject *);
 PyAPI_FUNC(int) PyList_Reverse(PyObject *);

Lib/test/test_capi/test_list.py

@@ -275,3 +275,39 @@ def test_list_astuple(self):
         self.assertRaises(SystemError, astuple, ())
         self.assertRaises(SystemError, astuple, object())
         self.assertRaises(SystemError, astuple, NULL)
+
+    def test_list_clear(self):
+        # Test PyList_Clear()
+        list_clear = _testcapi.list_clear
+
+        lst = [1, 2, 3]
+        self.assertEqual(list_clear(lst), 0)
+        self.assertEqual(lst, [])
+
+        lst = []
+        self.assertEqual(list_clear(lst), 0)
+        self.assertEqual(lst, [])
+
+        self.assertRaises(TypeError, list_clear, ())
+        self.assertRaises(TypeError, list_clear, object())
+
+        # CRASHES list_clear(NULL)
+
+    def test_list_extend(self):
+        # Test PyList_Extend()
+        list_extend = _testcapi.list_extend
+
+        lst = [1]
+        arg = [2, 3]
+        self.assertEqual(list_extend(lst, arg), 0)
+        self.assertEqual(lst, [1, 2, 3])
+
+        lst = []
+        arg = (2, 3)
+        self.assertEqual(list_extend(lst, arg), 0)
+        self.assertEqual(lst, [2, 3])
+
+        self.assertRaises(TypeError, list_extend, [], object())
+
+        # CRASHES list_extend(NULL, [])
+        # CRASHES list_extend([], NULL)

Misc/NEWS.d/next/C API/2023-11-08-18-37-19.gh-issue-111138.3Ypq8h.rst

@@ -0,0 +1,2 @@
+Add :c:func:`PyList_Extend` and :c:func:`PyList_Clear` functions: similar to
+Python ``list.extend()`` and ``list.clear()`` methods. Patch by Victor Stinner.

Modules/_testcapi/list.c

@@ -162,6 +162,25 @@ list_astuple(PyObject* Py_UNUSED(module), PyObject *obj)
 }
 
 
+static PyObject *
+list_clear(PyObject* Py_UNUSED(module), PyObject *obj)
+{
+    NULLABLE(obj);
+    RETURN_INT(PyList_Clear(obj));
+}
+
+
+static PyObject *
+list_extend(PyObject* Py_UNUSED(module), PyObject *args)
+{
+    PyObject *obj, *arg;
+    if (!PyArg_ParseTuple(args, "OO", &obj, &arg)) {
+        return NULL;
+    }
+    NULLABLE(obj);
+    NULLABLE(arg);
+    RETURN_INT(PyList_Extend(obj, arg));
+}
 
 
 static PyMethodDef test_methods[] = {
@@ -181,6 +200,8 @@ static PyMethodDef test_methods[] = {
     {"list_sort", list_sort, METH_O},
     {"list_reverse", list_reverse, METH_O},
     {"list_astuple", list_astuple, METH_O},
+    {"list_clear", list_clear, METH_O},
+    {"list_extend", list_extend, METH_VARARGS},
     {NULL},
 };