pythongh-123909: PyType_From*: Disallow metaclasses with custom tp_new

Author: encukou

Doc/c-api/type.rst

@@ -345,8 +345,12 @@ The following functions and structs are used to create
       The :c:member:`~PyTypeObject.tp_new` of the metaclass is *ignored*.
       which may result in incomplete initialization.
       Creating classes whose metaclass overrides
-      :c:member:`~PyTypeObject.tp_new` is deprecated and in Python 3.14+ it
-      will be no longer allowed.
+      :c:member:`~PyTypeObject.tp_new` is deprecated.
+
+   .. versionchanged:: 3.14
+
+      Creating classes whose metaclass overrides
+      :c:member:`~PyTypeObject.tp_new` is no longer allowed.
 
 .. c:function:: PyObject* PyType_FromSpecWithBases(PyType_Spec *spec, PyObject *bases)
 
@@ -362,24 +366,22 @@ The following functions and structs are used to create
       The :c:member:`~PyTypeObject.tp_new` of the metaclass is *ignored*.
       which may result in incomplete initialization.
       Creating classes whose metaclass overrides
-      :c:member:`~PyTypeObject.tp_new` is deprecated and in Python 3.14+ it
-      will be no longer allowed.
+      :c:member:`~PyTypeObject.tp_new` is deprecated.
+
+   .. versionchanged:: 3.14
+
+      Creating classes whose metaclass overrides
+      :c:member:`~PyTypeObject.tp_new` is no longer allowed.
 
 .. c:function:: PyObject* PyType_FromSpec(PyType_Spec *spec)
 
    Equivalent to ``PyType_FromMetaclass(NULL, NULL, spec, NULL)``.
 
    .. versionchanged:: 3.12
+   .. versionchanged:: 3.14
 
-      The function now finds and uses a metaclass corresponding to the
-      base classes provided in *Py_tp_base[s]* slots.
-      Previously, only :class:`type` instances were returned.
-
-      The :c:member:`~PyTypeObject.tp_new` of the metaclass is *ignored*.
-      which may result in incomplete initialization.
       Creating classes whose metaclass overrides
-      :c:member:`~PyTypeObject.tp_new` is deprecated and in Python 3.14+ it
-      will be no longer allowed.
+      :c:member:`~PyTypeObject.tp_new` is no longer allowed.
 
 .. raw:: html
 

Lib/test/test_capi/test_misc.py

@@ -722,22 +722,16 @@ def test_heaptype_with_custom_metaclass_custom_new(self):
         with self.assertRaisesRegex(TypeError, msg):
             t = _testcapi.pytype_fromspec_meta(metaclass)
 
-    def test_heaptype_with_custom_metaclass_deprecation(self):
+    def test_heaptype_base_with_custom_metaclass(self):
         metaclass = _testcapi.HeapCTypeMetaclassCustomNew
 
-        # gh-103968: a metaclass with custom tp_new is deprecated, but still
-        # allowed for functions that existed in 3.11
-        # (PyType_FromSpecWithBases is used here).
         class Base(metaclass=metaclass):
             pass
 
         # Class creation from C
-        with warnings_helper.check_warnings(
-                ('.* _testcapi.Subclass .* custom tp_new.*in Python 3.14.*', DeprecationWarning),
-                ):
+        msg = "Metaclasses with custom tp_new are not supported."
+        with self.assertRaisesRegex(TypeError, msg):
             sub = _testcapi.make_type_with_base(Base)
-        self.assertTrue(issubclass(sub, Base))
-        self.assertIsInstance(sub, metaclass)
 
     def test_multiple_inheritance_ctypes_with_weakref_or_dict(self):
         for weakref_cls in (_testcapi.HeapCTypeWithWeakref,

Misc/NEWS.d/next/C_API/2024-09-10-16-54-27.gh-issue-123909.CTGxDR.rst

@@ -0,0 +1,3 @@
+:c:func:`PyType_FromSpec`, :c:func:`PyType_FromSpecWithBases` and
+:c:func:`PyType_FromModuleAndSpec` will now fail if the metaclass of the new
+type has custom :c:member:`~PyTypeObject.tp_new`.

Objects/typeobject.c

@@ -4668,10 +4668,10 @@ special_offset_from_member(
     return -1;
 }
 
-static PyObject *
-_PyType_FromMetaclass_impl(
+PyObject *
+PyType_FromMetaclass(
     PyTypeObject *metaclass, PyObject *module,
-    PyType_Spec *spec, PyObject *bases_in, int _allow_tp_new)
+    PyType_Spec *spec, PyObject *bases_in)
 {
     /* Invariant: A non-NULL value in one of these means this function holds
      * a strong reference or owns allocated memory.
@@ -4848,21 +4848,10 @@ _PyType_FromMetaclass_impl(
         goto finally;
     }
     if (metaclass->tp_new && metaclass->tp_new != PyType_Type.tp_new) {
-        if (_allow_tp_new) {
-            if (PyErr_WarnFormat(
-                    PyExc_DeprecationWarning, 1,
-                    "Type %s uses PyType_Spec with a metaclass that has custom "
-                    "tp_new. This is deprecated and will no longer be allowed in "
-                    "Python 3.14.", spec->name) < 0) {
-                goto finally;
-            }
-        }
-        else {
-            PyErr_SetString(
-                PyExc_TypeError,
-                "Metaclasses with custom tp_new are not supported.");
-            goto finally;
-        }
+        PyErr_SetString(
+            PyExc_TypeError,
+            "Metaclasses with custom tp_new are not supported.");
+        goto finally;
     }
 
     /* Calculate best base, and check that all bases are type objects */
@@ -5109,29 +5098,22 @@ _PyType_FromMetaclass_impl(
     return (PyObject*)res;
 }
 
-PyObject *
-PyType_FromMetaclass(PyTypeObject *metaclass, PyObject *module,
-                     PyType_Spec *spec, PyObject *bases_in)
-{
-    return _PyType_FromMetaclass_impl(metaclass, module, spec, bases_in, 0);
-}
-
 PyObject *
 PyType_FromModuleAndSpec(PyObject *module, PyType_Spec *spec, PyObject *bases)
 {
-    return _PyType_FromMetaclass_impl(NULL, module, spec, bases, 1);
+    return PyType_FromMetaclass(NULL, module, spec, bases);
 }
 
 PyObject *
 PyType_FromSpecWithBases(PyType_Spec *spec, PyObject *bases)
 {
-    return _PyType_FromMetaclass_impl(NULL, NULL, spec, bases, 1);
+    return PyType_FromMetaclass(NULL, NULL, spec, bases);
 }
 
 PyObject *
 PyType_FromSpec(PyType_Spec *spec)
 {
-    return _PyType_FromMetaclass_impl(NULL, NULL, spec, NULL, 1);
+    return PyType_FromMetaclass(NULL, NULL, spec, NULL);
 }
 
 PyObject *