Add PyFrozenSetWriter_AddSteal() function

Author: vstinner

Doc/c-api/set.rst

@@ -278,6 +278,11 @@ See also the :c:func:`PyFrozenSet_New` function.
    On success, return ``0``.
    On error, set an exception and return ``-1``.
 
+.. c:function:: int PyFrozenSetWriter_AddSteal(PyFrozenSetWriter *writer, PyObject *item)
+
+   Similar to :c:func:`PyFrozenSetWriter_Add`, but :term:`steal` the *item*
+   reference.
+
 .. c:function:: int PyFrozenSetWriter_Update(PyFrozenSetWriter *writer, PyObject *iterable)
 
    Update the frozenset writer, adding elements from *iterable*.

Doc/whatsnew/3.16.rst

@@ -1302,6 +1302,7 @@ New features
   * :c:func:`PyFrozenSetWriter_Finish`
   * :c:func:`PyFrozenSetWriter_Discard`
   * :c:func:`PyFrozenSetWriter_Add`
+  * :c:func:`PyFrozenSetWriter_AddSteal`
   * :c:func:`PyFrozenSetWriter_Update`
 
   (Contributed by Victor Stinner in :gh:`160147`.)

Include/cpython/setobject.h

@@ -81,6 +81,9 @@ PyAPI_FUNC(void) PyFrozenSetWriter_Discard(PyFrozenSetWriter *writer);
 PyAPI_FUNC(int) PyFrozenSetWriter_Add(
     PyFrozenSetWriter *writer,
     PyObject *item);
+PyAPI_FUNC(int) PyFrozenSetWriter_AddSteal(
+    PyFrozenSetWriter *writer,
+    PyObject *item);
 PyAPI_FUNC(int) PyFrozenSetWriter_Update(
     PyFrozenSetWriter *writer,
     PyObject *iterable);

Lib/test/test_capi/test_set.py

@@ -313,7 +313,7 @@ def create_items(self, nitem):
         # Use mortal (and unique) objects to be able to detect reference leaks
         return tuple(object() for _ in range(nitem))
 
-    def test_append(self):
+    def test_add(self):
         # Test PyFrozenSetWriter_Append()
         items = self.create_items(self.PySet_MINSIZE * 2 - 1)
         table_size = self.compute_table_size(len(items))
@@ -324,6 +324,13 @@ def test_append(self):
                     writer.add(item)
                 self.frozenset_equal(writer.finish(), items)
 
+        # Test error (unhashable type)
+        writer = self.create_writer(0)
+        writer.update('abc')
+        with self.assertRaisesRegex(TypeError, "unhashable type: 'list'"):
+            writer.add([])
+        self.frozenset_equal(writer.finish(), frozenset('abc'))
+
     def test_update(self):
         # Test PyFrozenSetWriter_Update()
         items1 = self.create_items(self.PySet_MINSIZE - 1)

Misc/NEWS.d/next/C_API/2026-10-21-18-20-57.gh-issue-160147.7uo4pF.rst

@@ -5,6 +5,7 @@ Add a new :ref:`PyFrozenSetWriter <pyfrozensetwriter>` C API to create a
 * :c:func:`PyFrozenSetWriter_Finish`
 * :c:func:`PyFrozenSetWriter_Discard`
 * :c:func:`PyFrozenSetWriter_Add`
+* :c:func:`PyFrozenSetWriter_AddSteal`
 * :c:func:`PyFrozenSetWriter_Update`
 
 Patch by Victor Stinner.

Modules/_hashopenssl.c

@@ -2505,11 +2505,11 @@ _openssl_hash_name_mapper(const EVP_MD *md, const char *from,
     py_name = name == NULL ? NULL : PyUnicode_FromString(name);
     if (py_name == NULL) {
         state->error = 1;
-    } else {
-        if (PyFrozenSetWriter_Add(state->set, py_name) != 0) {
-            state->error = 1;
-        }
-        Py_DECREF(py_name);
+        return;
+    }
+
+    if (PyFrozenSetWriter_AddSteal(state->set, py_name) != 0) {
+        state->error = 1;
     }
 }
 

Modules/_ssl.c

@@ -6541,8 +6541,7 @@ parseKeyUsage(PCCERT_CONTEXT pCertCtx, DWORD flags)
             if (oid == NULL) {
                 goto error;
             }
-            err = PyFrozenSetWriter_Add(retval, oid);
-            Py_DECREF(oid);
+            err = PyFrozenSetWriter_AddSteal(retval, oid);
             if (err == -1) {
                 goto error;
             }

Modules/_testcapi/set.c

@@ -118,6 +118,22 @@ writer_add(PyObject *self_raw, PyObject *item)
 }
 
 
+// Test PyFrozenSetWriter_AddSteal()
+static PyObject*
+writer_add_steal(PyObject *self_raw, PyObject *item)
+{
+    WriterObject *self = (WriterObject *)self_raw;
+    if (writer_check(self) < 0) {
+        return NULL;
+    }
+
+    if (PyFrozenSetWriter_AddSteal(self->writer, Py_NewRef(item)) < 0) {
+        return NULL;
+    }
+    Py_RETURN_NONE;
+}
+
+
 // Test PyFrozenSetWriter_Update()
 static PyObject*
 writer_update(PyObject *self_raw, PyObject *iterable)
@@ -180,6 +196,7 @@ writer_discard(PyObject *self_raw, PyObject *Py_UNUSED(args))
 
 static PyMethodDef writer_methods[] = {
     {"add", _PyCFunction_CAST(writer_add), METH_O},
+    {"add_steal", _PyCFunction_CAST(writer_add_steal), METH_O},
     {"update", _PyCFunction_CAST(writer_update), METH_O},
     {"get_table_size", _PyCFunction_CAST(writer_get_table_size), METH_NOARGS},
     {"finish", _PyCFunction_CAST(writer_finish), METH_NOARGS},

Objects/setobject.c

@@ -3304,11 +3304,26 @@ PyFrozenSetWriter_Finish(PyFrozenSetWriter *writer)
 
 
 int
-PyFrozenSetWriter_Add(PyFrozenSetWriter *writer, PyObject *item)
+PyFrozenSetWriter_AddSteal(PyFrozenSetWriter *writer, PyObject *item)
 {
     assert(frozsetwriter_check_consistency(writer));
+
+    Py_hash_t hash = PyObject_Hash(item);
+    if (hash == -1) {
+        set_unhashable_type(item);
+        Py_DECREF(item);
+        return -1;
+    }
+
     PySetObject *frozenset = _PySet_CAST(writer);
-    return set_add_key(frozenset, item);
+    return set_add_entry_takeref(frozenset, item, hash);
+}
+
+
+int
+PyFrozenSetWriter_Add(PyFrozenSetWriter *writer, PyObject *item)
+{
+    return PyFrozenSetWriter_AddSteal(writer, Py_NewRef(item));
 }