gh-139888: Add PyTupleWriter C API (stack flavor)

* Add _PyTuple_NewNoTrack() and _PyTuple_ResizeNoTrack() helper
  functions.
* Modify PySequence_Tuple() to use PyTupleWriter API.
* Soft deprecate _PyTuple_Resize().

Author: vstinner

Doc/c-api/tuple.rst

@@ -142,6 +142,83 @@ Tuple Objects
    ``*p`` is destroyed.  On failure, returns ``-1`` and sets ``*p`` to ``NULL``, and
    raises :exc:`MemoryError` or :exc:`SystemError`.
 
+   .. deprecated:: 3.15
+      Use the :ref:`PyTupleWriter API <pytuplewriter>` instead.
+
+
+.. _pytuplewriter:
+
+PyTupleWriter
+-------------
+
+The :c:type:`PyTupleWriter` API can be used to create a Python :class:`tuple`
+object.
+
+.. versionadded:: next
+
+.. c:type:: PyTupleWriter
+
+   A tuple writer instance.
+
+   The API is **not thread safe**: a writer should only be used by a single
+   thread at the same time.
+
+   The instance must be destroyed by :c:func:`PyTupleWriter_Finish` on
+   success, or :c:func:`PyTupleWriter_Discard` on error.
+
+
+.. c:function:: int PyTupleWriter_Init(PyTupleWriter *writer, Py_ssize_t size)
+
+   Initialize a :c:type:`PyTupleWriter` to add *size* items.
+
+   If *size* is greater than zero, preallocate *size* items. The caller is
+   responsible to add *size* items using :c:func:`PyTupleWriter_Add`.
+
+   On success, return ``0``.
+   On error, set an exception and return ``-1``.
+
+   *size* must be positive or zero.
+
+
+.. c:function:: PyObject* PyTupleWriter_Finish(PyTupleWriter *writer)
+
+   Finish a :c:type:`PyTupleWriter` created by
+   :c:func:`PyTupleWriter_Init`.
+
+   On success, return a Python :class:`tuple` object.
+   On error, set an exception and return ``NULL``.
+
+   The writer instance is invalid after the call in any case.
+   No API can be called on the writer after :c:func:`PyTupleWriter_Finish`.
+
+.. c:function:: void PyTupleWriter_Discard(PyTupleWriter *writer)
+
+   Discard a :c:type:`PyTupleWriter` created by :c:func:`PyTupleWriter_Init`.
+
+   Do nothing if *writer* is ``NULL``.
+
+   The writer instance is invalid after the call.
+   No API can be called on the writer after :c:func:`PyTupleWriter_Discard`.
+
+.. c:function:: int PyTupleWriter_Add(PyTupleWriter *writer, PyObject *item)
+
+   Add an item to *writer*.
+
+   * On success, return ``0``.
+   * If *item* is ``NULL`` with an exception set, return ``-1``.
+   * On error, set an exception and return ``-1``.
+
+.. c:function:: int PyTupleWriter_AddSteal(PyTupleWriter *writer, PyObject *item)
+
+   Similar to :c:func:`PyTupleWriter_Add`, but take the ownership of *item*.
+
+.. c:function:: int PyTupleWriter_AddArray(PyTupleWriter *writer, PyObject *const *array, Py_ssize_t size)
+
+   Add items from an array to *writer*.
+
+   On success, return ``0``.
+   On error, set an exception and return ``-1``.
+
 
 .. _struct-sequence-objects:
 

Doc/whatsnew/3.15.rst

@@ -887,6 +887,17 @@ New features
 * Add :c:func:`PyTuple_FromArray` to create a :class:`tuple` from an array.
   (Contributed by Victor Stinner in :gh:`111489`.)
 
+* Add a :ref:`PyTupleWriter API <pytuplewriter>` to create :class:`tuple`:
+
+  * :c:func:`PyTupleWriter_Init`
+  * :c:func:`PyTupleWriter_Add`
+  * :c:func:`PyTupleWriter_AddSteal`
+  * :c:func:`PyTupleWriter_AddArray`
+  * :c:func:`PyTupleWriter_Finish`
+  * :c:func:`PyTupleWriter_Discard`
+
+  (Contributed by Victor Stinner in :gh:`139888`.)
+
 
 Porting to Python 3.15
 ----------------------
@@ -1017,6 +1028,9 @@ Deprecated C APIs
   since 3.15 and will be removed in 3.17.
   (Contributed by Nikita Sobolev in :gh:`136355`.)
 
+* Deprecate the :c:func:`_PyTuple_Resize` function:
+  use the new :ref:`PyTupleWriter API <pytuplewriter>` instead.
+  (Contributed by Victor Stinner in :gh:`139888`.)
 
 .. Add C API deprecations above alphabetically, not here at the end.
 

Include/cpython/tupleobject.h

@@ -12,7 +12,7 @@ typedef struct {
     PyObject *ob_item[1];
 } PyTupleObject;
 
-PyAPI_FUNC(int) _PyTuple_Resize(PyObject **, Py_ssize_t);
+_Py_DEPRECATED_EXTERNALLY(3.15) PyAPI_FUNC(int) _PyTuple_Resize(PyObject **, Py_ssize_t);
 
 /* Cast argument to PyTupleObject* type. */
 #define _PyTuple_CAST(op) \
@@ -42,3 +42,23 @@ PyTuple_SET_ITEM(PyObject *op, Py_ssize_t index, PyObject *value) {
 PyAPI_FUNC(PyObject*) PyTuple_FromArray(
     PyObject *const *array,
     Py_ssize_t size);
+
+// --- Public PyUnicodeWriter API --------------------------------------------
+
+typedef struct PyTupleWriter {
+    char _reserved[sizeof(Py_uintptr_t) * 20];
+} PyTupleWriter;
+
+PyAPI_FUNC(int) PyTupleWriter_Init(PyTupleWriter *writer, Py_ssize_t size);
+PyAPI_FUNC(int) PyTupleWriter_Add(
+    PyTupleWriter *writer,
+    PyObject *item);
+PyAPI_FUNC(int) PyTupleWriter_AddSteal(
+    PyTupleWriter *writer,
+    PyObject *item);
+PyAPI_FUNC(int) PyTupleWriter_AddArray(
+    PyTupleWriter *writer,
+    PyObject *const *array,
+    Py_ssize_t size);
+PyAPI_FUNC(PyObject*) PyTupleWriter_Finish(PyTupleWriter *writer);
+PyAPI_FUNC(void) PyTupleWriter_Discard(PyTupleWriter *writer);

Include/internal/pycore_tuple.h

@@ -68,6 +68,10 @@ _PyTuple_Recycle(PyObject *op)
 #endif
 #define _PyTuple_HASH_EMPTY (_PyTuple_HASH_XXPRIME_5 + (_PyTuple_HASH_XXPRIME_5 ^ 3527539UL))
 
+extern PyObject** _PyTupleWriter_GetItems(
+    PyTupleWriter *writer,
+    Py_ssize_t *size);
+
 #ifdef __cplusplus
 }
 #endif

Lib/test/test_capi/test_tuple.py

@@ -302,5 +302,71 @@ def my_iter():
         self.assertEqual(tuples, [])
 
 
+class TupleWriterTest(unittest.TestCase):
+    def create_writer(self, size):
+        return _testcapi.PyTupleWriter(size)
+
+    def test_create(self):
+        # Test PyTupleWriter_Init()
+        writer = self.create_writer(0)
+        self.assertIs(writer.finish(), ())
+
+        writer = self.create_writer(123)
+        self.assertIs(writer.finish(), ())
+
+        with self.assertRaises(SystemError):
+            self.create_writer(-2)
+
+    def check_add(self, name):
+         writer = self.create_writer(3)
+         add = getattr(writer, name)
+         for ch in 'abc':
+             add(ch)
+         self.assertEqual(writer.finish(), ('a', 'b', 'c'))
+
+         writer = self.create_writer(0)
+         add = getattr(writer, name)
+         for i in range(1024):
+             add(i)
+         self.assertEqual(writer.finish(), tuple(range(1024)))
+
+         writer = self.create_writer(1)
+         add = getattr(writer, name)
+         with self.assertRaises(SystemError):
+             add(NULL)
+
+    def test_add(self):
+         # Test PyTupleWriter_Add()
+         self.check_add('add')
+
+    def test_add_steal(self):
+         # Test PyTupleWriter_AddSteal()
+         self.check_add('add_steal')
+
+    def test_add_array(self):
+         writer = self.create_writer(0)
+         writer.add_array(('a', 'b', 'c'))
+         writer.add_array(('d', 'e'))
+         self.assertEqual(writer.finish(), ('a', 'b', 'c', 'd', 'e'))
+
+         writer = self.create_writer(0)
+         writer.add_array(tuple(range(1024)))
+         self.assertEqual(writer.finish(), tuple(range(1024)))
+
+    def test_discard(self):
+         # test the small_tuple buffer (16 items)
+         writer = self.create_writer(3)
+         writer.add(object())
+         writer.add(object())
+         # must not leak references
+         writer.discard()
+
+         # test the tuple code path (17 items or more)
+         writer = self.create_writer(1024)
+         writer.add_array(tuple(range(1024)))
+         # must not leak references
+         writer.discard()
+
+
 if __name__ == "__main__":
     unittest.main()

Misc/NEWS.d/next/C_API/2025-10-10-09-54-28.gh-issue-139888.Z4Px61.rst

@@ -0,0 +1,10 @@
+Add a :ref:`PyTupleWriter API <pytuplewriter>` to create :class:`tuple`:
+
+* :c:func:`PyTupleWriter_Init`
+* :c:func:`PyTupleWriter_Add`
+* :c:func:`PyTupleWriter_AddSteal`
+* :c:func:`PyTupleWriter_AddArray`
+* :c:func:`PyTupleWriter_Finish`
+* :c:func:`PyTupleWriter_Discard`
+
+Patch by Victor Stinner.

Misc/NEWS.d/next/C_API/2025-10-10-09-58-57.gh-issue-139888.xCfJ3L.rst

@@ -0,0 +1,2 @@
+Deprecate the :c:func:`_PyTuple_Resize` function: use the :ref:`PyTupleWriter
+API <pytuplewriter>` instead.