gh-111138: Add PyList_Extend() and PyList_Clear() functions (#111862)

* 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

@@ -128,6 +128,30 @@ List Objects
    list is not supported.
 
 
+.. c:function:: int PyList_Extend(PyObject *list, PyObject *iterable)
+
+   Extend *list* with the contents of *iterable*.  This is the same as
+   ``PyList_SetSlice(list, PY_SSIZE_T_MAX, PY_SSIZE_T_MAX, iterable)``
+   and analogous to ``list.extend(iterable)`` or ``list += iterable``.
+
+   Raise an exception and return ``-1`` if *list* is not a :class:`list`
+   object. Return 0 on success.
+
+   .. versionadded:: 3.13
+
+
+.. c:function:: int PyList_Clear(PyObject *list)
+
+   Remove all items from *list*.  This is the same as
+   ``PyList_SetSlice(list, 0, PY_SSIZE_T_MAX, NULL)`` and analogous to
+   ``list.clear()`` or ``del list[:]``.
+
+   Raise an exception and return ``-1`` if *list* is not a :class:`list`
+   object.  Return 0 on success.
+
+   .. versionadded:: 3.13
+
+
 .. c:function:: int PyList_Sort(PyObject *list)
 
    Sort the items of *list* in place.  Return ``0`` on success, ``-1`` on

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/cpython/listobject.h

@@ -44,3 +44,6 @@ PyList_SET_ITEM(PyObject *op, Py_ssize_t index, PyObject *value) {
 }
 #define PyList_SET_ITEM(op, index, value) \
     PyList_SET_ITEM(_PyObject_CAST(op), (index), _PyObject_CAST(value))
+
+PyAPI_FUNC(int) PyList_Extend(PyObject *self, PyObject *iterable);
+PyAPI_FUNC(int) PyList_Clear(PyObject *self);

Lib/test/test_capi/test_list.py

@@ -1,5 +1,6 @@
+import gc
+import weakref
 import unittest
-import sys
 from test.support import import_helper
 from collections import UserList
 _testcapi = import_helper.import_module('_testcapi')
@@ -12,6 +13,15 @@ class ListSubclass(list):
     pass
 
 
+class DelAppend:
+    def __init__(self, lst, item):
+        self.lst = lst
+        self.item = item
+
+    def __del__(self):
+        self.lst.append(self.item)
+
+
 class CAPITest(unittest.TestCase):
     def test_check(self):
         # Test PyList_Check()
@@ -196,10 +206,10 @@ class CAPITest(unittest.TestCase):
 
     def test_list_setslice(self):
         # Test PyList_SetSlice()
-        setslice = _testcapi.list_setslice
+        list_setslice = _testcapi.list_setslice
         def set_slice(lst, low, high, value):
             lst = lst.copy()
-            self.assertEqual(setslice(lst, low, high, value), 0)
+            self.assertEqual(list_setslice(lst, low, high, value), 0)
             return lst
 
         # insert items
@@ -231,8 +241,21 @@ class CAPITest(unittest.TestCase):
         self.assertEqual(set_slice(lst, 0, len(lst), NULL), [])
         self.assertEqual(set_slice(lst, 3, len(lst), NULL), list("abc"))
 
-        self.assertRaises(SystemError, setslice, (), 0, 0, [])
+        self.assertRaises(SystemError, list_setslice, (), 0, 0, [])
-        self.assertRaises(SystemError, setslice, 42, 0, 0, [])
+        self.assertRaises(SystemError, list_setslice, 42, 0, 0, [])
+
+        # Item finalizer modify the list (clear the list)
+        lst = []
+        lst.append(DelAppend(lst, 'zombie'))
+        self.assertEqual(list_setslice(lst, 0, len(lst), NULL), 0)
+        self.assertEqual(lst, ['zombie'])
+
+        # Item finalizer modify the list (remove an list item)
+        lst = []
+        lst.append(DelAppend(lst, 'zombie'))
+        lst.extend("abc")
+        self.assertEqual(list_setslice(lst, 0, 1, NULL), 0)
+        self.assertEqual(lst, ['a', 'b', 'c', 'zombie'])
 
         # CRASHES setslice(NULL, 0, 0, [])
 
@@ -275,3 +298,47 @@ class CAPITest(unittest.TestCase):
         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(SystemError, list_clear, ())
+        self.assertRaises(SystemError, list_clear, object())
+
+        # Item finalizer modify the list
+        lst = []
+        lst.append(DelAppend(lst, 'zombie'))
+        list_clear(lst)
+        self.assertEqual(lst, ['zombie'])
+
+        # CRASHES list_clear(NULL)
+
+    def test_list_extend(self):
+        # Test PyList_Extend()
+        list_extend = _testcapi.list_extend
+
+        for other_type in (list, tuple, str, iter):
+            lst = list("ab")
+            arg = other_type("def")
+            self.assertEqual(list_extend(lst, arg), 0)
+            self.assertEqual(lst, list("abdef"))
+
+        # PyList_Extend(lst, lst)
+        lst = list("abc")
+        self.assertEqual(list_extend(lst, lst), 0)
+        self.assertEqual(lst, list("abcabc"))
+
+        self.assertRaises(TypeError, list_extend, [], object())
+        self.assertRaises(SystemError, list_extend, (), list("abc"))
+
+        # 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},
 };
 

Objects/clinic/listobject.c.h

@@ -50,22 +50,22 @@ exit:
     return return_value;
 }
 
-PyDoc_STRVAR(list_clear__doc__,
+PyDoc_STRVAR(py_list_clear__doc__,
 "clear($self, /)\n"
 "--\n"
 "\n"
 "Remove all items from list.");
 
-#define LIST_CLEAR_METHODDEF    \
+#define PY_LIST_CLEAR_METHODDEF    \
-    {"clear", (PyCFunction)list_clear, METH_NOARGS, list_clear__doc__},
+    {"clear", (PyCFunction)py_list_clear, METH_NOARGS, py_list_clear__doc__},
 
 static PyObject *
-list_clear_impl(PyListObject *self);
+py_list_clear_impl(PyListObject *self);
 
 static PyObject *
-list_clear(PyListObject *self, PyObject *Py_UNUSED(ignored))
+py_list_clear(PyListObject *self, PyObject *Py_UNUSED(ignored))
 {
-    return list_clear_impl(self);
+    return py_list_clear_impl(self);
 }
 
 PyDoc_STRVAR(list_copy__doc__,
@@ -95,14 +95,14 @@ PyDoc_STRVAR(list_append__doc__,
 #define LIST_APPEND_METHODDEF    \
     {"append", (PyCFunction)list_append, METH_O, list_append__doc__},
 
-PyDoc_STRVAR(list_extend__doc__,
+PyDoc_STRVAR(py_list_extend__doc__,
 "extend($self, iterable, /)\n"
 "--\n"
 "\n"
 "Extend list by appending elements from the iterable.");
 
-#define LIST_EXTEND_METHODDEF    \
+#define PY_LIST_EXTEND_METHODDEF    \
-    {"extend", (PyCFunction)list_extend, METH_O, list_extend__doc__},
+    {"extend", (PyCFunction)py_list_extend, METH_O, py_list_extend__doc__},
 
 PyDoc_STRVAR(list_pop__doc__,
 "pop($self, index=-1, /)\n"
@@ -384,4 +384,4 @@ list___reversed__(PyListObject *self, PyObject *Py_UNUSED(ignored))
 {
     return list___reversed___impl(self);
 }
-/*[clinic end generated code: output=5dea9dd3bb219a7f input=a9049054013a1b77]*/
+/*[clinic end generated code: output=f2d7b63119464ff4 input=a9049054013a1b77]*/

Objects/listobject.c

@@ -592,26 +592,33 @@ list_repeat(PyListObject *a, Py_ssize_t n)
     return (PyObject *) np;
 }
 
-static int
+static void
-_list_clear(PyListObject *a)
+list_clear(PyListObject *a)
 {
-    Py_ssize_t i;
+    PyObject **items = a->ob_item;
-    PyObject **item = a->ob_item;
+    if (items == NULL) {
-    if (item != NULL) {
+        return;
+    }
+
     /* Because XDECREF can recursively invoke operations on
        this list, we make it empty first. */
-        i = Py_SIZE(a);
+    Py_ssize_t i = Py_SIZE(a);
     Py_SET_SIZE(a, 0);
     a->ob_item = NULL;
     a->allocated = 0;
     while (--i >= 0) {
-            Py_XDECREF(item[i]);
+        Py_XDECREF(items[i]);
     }
-        PyMem_Free(item);
+    PyMem_Free(items);
+
+    // Note that there is no guarantee that the list is actually empty
+    // at this point, because XDECREF may have populated it indirectly again!
 }
-    /* Never fails; the return value can be ignored.
+
-       Note that there is no guarantee that the list is actually empty
+static int
-       at this point, because XDECREF may have populated it again! */
+list_clear_slot(PyListObject *self)
+{
+    list_clear(self);
     return 0;
 }
 
@@ -675,7 +682,8 @@ list_ass_slice(PyListObject *a, Py_ssize_t ilow, Py_ssize_t ihigh, PyObject *v)
     d = n - norig;
     if (Py_SIZE(a) + d == 0) {
         Py_XDECREF(v_as_SF);
-        return _list_clear(a);
+        list_clear(a);
+        return 0;
     }
     item = a->ob_item;
     /* recycle the items that we are about to remove */
@@ -745,7 +753,7 @@ list_inplace_repeat(PyListObject *self, Py_ssize_t n)
     }
 
     if (n < 1) {
-        (void)_list_clear(self);
+        list_clear(self);
         return Py_NewRef(self);
     }
 
@@ -801,16 +809,16 @@ list_insert_impl(PyListObject *self, Py_ssize_t index, PyObject *object)
 }
 
 /*[clinic input]
-list.clear
+list.clear as py_list_clear
 
 Remove all items from list.
 [clinic start generated code]*/
 
 static PyObject *
-list_clear_impl(PyListObject *self)
+py_list_clear_impl(PyListObject *self)
-/*[clinic end generated code: output=67a1896c01f74362 input=ca3c1646856742f6]*/
+/*[clinic end generated code: output=83726743807e3518 input=378711e10f545c53]*/
 {
-    _list_clear(self);
+    list_clear(self);
     Py_RETURN_NONE;
 }
 
@@ -846,83 +854,61 @@ list_append(PyListObject *self, PyObject *object)
     Py_RETURN_NONE;
 }
 
-/*[clinic input]
+static int
-list.extend
+list_extend_fast(PyListObject *self, PyObject *iterable)
-
-     iterable: object
-     /
-
-Extend list by appending elements from the iterable.
-[clinic start generated code]*/
-
-static PyObject *
-list_extend(PyListObject *self, PyObject *iterable)
-/*[clinic end generated code: output=630fb3bca0c8e789 input=9ec5ba3a81be3a4d]*/
 {
-    PyObject *it;      /* iter(v) */
+    Py_ssize_t n = PySequence_Fast_GET_SIZE(iterable);
-    Py_ssize_t m;                  /* size of self */
-    Py_ssize_t n;                  /* guess for size of iterable */
-    Py_ssize_t i;
-    PyObject *(*iternext)(PyObject *);
-
-    /* Special cases:
-       1) lists and tuples which can use PySequence_Fast ops
-       2) extending self to self requires making a copy first
-    */
-    if (PyList_CheckExact(iterable) || PyTuple_CheckExact(iterable) ||
-                (PyObject *)self == iterable) {
-        PyObject **src, **dest;
-        iterable = PySequence_Fast(iterable, "argument must be iterable");
-        if (!iterable)
-            return NULL;
-        n = PySequence_Fast_GET_SIZE(iterable);
     if (n == 0) {
         /* short circuit when iterable is empty */
-            Py_DECREF(iterable);
+        return 0;
-            Py_RETURN_NONE;
     }
-        m = Py_SIZE(self);
+
-        /* It should not be possible to allocate a list large enough to cause
+    Py_ssize_t m = Py_SIZE(self);
-        an overflow on any relevant platform */
+    // It should not be possible to allocate a list large enough to cause
+    // an overflow on any relevant platform.
     assert(m < PY_SSIZE_T_MAX - n);
     if (self->ob_item == NULL) {
         if (list_preallocate_exact(self, n) < 0) {
-                return NULL;
+            return -1;
         }
         Py_SET_SIZE(self, n);
     }
     else if (list_resize(self, m + n) < 0) {
-            Py_DECREF(iterable);
+        return -1;
-            return NULL;
     }
-        /* note that we may still have self == iterable here for the
+
-         * situation a.extend(a), but the following code works
+    // note that we may still have self == iterable here for the
-         * in that case too.  Just make sure to resize self
+    // situation a.extend(a), but the following code works
-         * before calling PySequence_Fast_ITEMS.
+    // in that case too.  Just make sure to resize self
-         */
+    // before calling PySequence_Fast_ITEMS.
-        /* populate the end of self with iterable's items */
+    //
-        src = PySequence_Fast_ITEMS(iterable);
+    // populate the end of self with iterable's items.
-        dest = self->ob_item + m;
+    PyObject **src = PySequence_Fast_ITEMS(iterable);
-        for (i = 0; i < n; i++) {
+    PyObject **dest = self->ob_item + m;
+    for (Py_ssize_t i = 0; i < n; i++) {
         PyObject *o = src[i];
         dest[i] = Py_NewRef(o);
     }
-        Py_DECREF(iterable);
+    return 0;
-        Py_RETURN_NONE;
 }
 
-    it = PyObject_GetIter(iterable);
+static int
-    if (it == NULL)
+list_extend_iter(PyListObject *self, PyObject *iterable)
-        return NULL;
+{
-    iternext = *Py_TYPE(it)->tp_iternext;
+    PyObject *it = PyObject_GetIter(iterable);
+    if (it == NULL) {
+        return -1;
+    }
+    PyObject *(*iternext)(PyObject *) = *Py_TYPE(it)->tp_iternext;
 
     /* Guess a result list size. */
-    n = PyObject_LengthHint(iterable, 8);
+    Py_ssize_t n = PyObject_LengthHint(iterable, 8);
     if (n < 0) {
         Py_DECREF(it);
-        return NULL;
+        return -1;
     }
-    m = Py_SIZE(self);
+
+    Py_ssize_t m = Py_SIZE(self);
     if (m > PY_SSIZE_T_MAX - n) {
         /* m + n overflowed; on the chance that n lied, and there really
          * is enough room, ignore it.  If n was telling the truth, we'll
@@ -935,8 +921,10 @@ list_extend(PyListObject *self, PyObject *iterable)
     }
     else {
         /* Make room. */
-        if (list_resize(self, m + n) < 0)
+        if (list_resize(self, m + n) < 0) {
             goto error;
+        }
+
         /* Make the list sane again. */
         Py_SET_SIZE(self, m);
     }
@@ -953,10 +941,10 @@ list_extend(PyListObject *self, PyObject *iterable)
             }
             break;
         }
+
         if (Py_SIZE(self) < self->allocated) {
-            /* steals ref */
             Py_ssize_t len = Py_SIZE(self);
-            PyList_SET_ITEM(self, len, item);
+            PyList_SET_ITEM(self, len, item);  // steals item ref
             Py_SET_SIZE(self, len + 1);
         }
         else {
@@ -972,28 +960,95 @@ list_extend(PyListObject *self, PyObject *iterable)
     }
 
     Py_DECREF(it);
-    Py_RETURN_NONE;
+    return 0;
 
   error:
     Py_DECREF(it);
-    return NULL;
+    return -1;
 }
 
+
+static int
+list_extend(PyListObject *self, PyObject *iterable)
+{
+    // Special cases:
+    // 1) lists and tuples which can use PySequence_Fast ops
+    // 2) extending self to self requires making a copy first
+    if (PyList_CheckExact(iterable)
+        || PyTuple_CheckExact(iterable)
+        || (PyObject *)self == iterable)
+    {
+        iterable = PySequence_Fast(iterable, "argument must be iterable");
+        if (!iterable) {
+            return -1;
+        }
+
+        int res = list_extend_fast(self, iterable);
+        Py_DECREF(iterable);
+        return res;
+    }
+    else {
+        return list_extend_iter(self, iterable);
+    }
+}
+
+
 PyObject *
 _PyList_Extend(PyListObject *self, PyObject *iterable)
 {
-    return list_extend(self, iterable);
+    if (list_extend(self, iterable) < 0) {
+        return NULL;
     }
+    Py_RETURN_NONE;
+}
+
+
+/*[clinic input]
+list.extend as py_list_extend
+
+     iterable: object
+     /
+
+Extend list by appending elements from the iterable.
+[clinic start generated code]*/
+
+static PyObject *
+py_list_extend(PyListObject *self, PyObject *iterable)
+/*[clinic end generated code: output=b8e0bff0ceae2abd input=9a8376a8633ed3ba]*/
+{
+    return _PyList_Extend(self, iterable);
+}
+
+
+int
+PyList_Extend(PyObject *self, PyObject *iterable)
+{
+    if (!PyList_Check(self)) {
+        PyErr_BadInternalCall();
+        return -1;
+    }
+    return list_extend((PyListObject*)self, iterable);
+}
+
+
+int
+PyList_Clear(PyObject *self)
+{
+    if (!PyList_Check(self)) {
+        PyErr_BadInternalCall();
+        return -1;
+    }
+    list_clear((PyListObject*)self);
+    return 0;
+}
+
 
 static PyObject *
 list_inplace_concat(PyListObject *self, PyObject *other)
 {
-    PyObject *result;
+    if (list_extend(self, other) < 0) {
-
+        return NULL;
-    result = list_extend(self, other);
+    }
-    if (result == NULL)
-        return result;
-    Py_DECREF(result);
     return Py_NewRef(self);
 }
 
@@ -1032,7 +1087,8 @@ list_pop_impl(PyListObject *self, Py_ssize_t index)
     const Py_ssize_t size_after_pop = Py_SIZE(self) - 1;
     if (size_after_pop == 0) {
         Py_INCREF(v);
-        status = _list_clear(self);
+        list_clear(self);
+        status = 0;
     }
     else {
         if ((size_after_pop - index) > 0) {
@@ -2501,7 +2557,7 @@ keyfunc_fail:
     self->ob_item = saved_ob_item;
     self->allocated = saved_allocated;
     if (final_ob_item != NULL) {
-        /* we cannot use _list_clear() for this because it does not
+        /* we cannot use list_clear() for this because it does not
            guarantee that the list is really empty when it returns */
         while (--i >= 0) {
             Py_XDECREF(final_ob_item[i]);
@@ -2789,13 +2845,12 @@ list___init___impl(PyListObject *self, PyObject *iterable)
 
     /* Empty previous contents */
     if (self->ob_item != NULL) {
-        (void)_list_clear(self);
+        list_clear(self);
     }
     if (iterable != NULL) {
-        PyObject *rv = list_extend(self, iterable);
+        if (list_extend(self, iterable) < 0) {
-        if (rv == NULL)
             return -1;
-        Py_DECREF(rv);
+        }
     }
     return 0;
 }
@@ -2849,11 +2904,11 @@ static PyMethodDef list_methods[] = {
      PyDoc_STR("__getitem__($self, index, /)\n--\n\nReturn self[index].")},
     LIST___REVERSED___METHODDEF
     LIST___SIZEOF___METHODDEF
-    LIST_CLEAR_METHODDEF
+    PY_LIST_CLEAR_METHODDEF
     LIST_COPY_METHODDEF
     LIST_APPEND_METHODDEF
     LIST_INSERT_METHODDEF
-    LIST_EXTEND_METHODDEF
+    PY_LIST_EXTEND_METHODDEF
     LIST_POP_METHODDEF
     LIST_REMOVE_METHODDEF
     LIST_INDEX_METHODDEF
@@ -3124,7 +3179,7 @@ PyTypeObject PyList_Type = {
         _Py_TPFLAGS_MATCH_SELF | Py_TPFLAGS_SEQUENCE,  /* tp_flags */
     list___init____doc__,                       /* tp_doc */
     (traverseproc)list_traverse,                /* tp_traverse */
-    (inquiry)_list_clear,                       /* tp_clear */
+    (inquiry)list_clear_slot,                   /* tp_clear */
     list_richcompare,                           /* tp_richcompare */
     0,                                          /* tp_weaklistoffset */
     list_iter,                                  /* tp_iter */