bpo-43413: Revert changes in set.__init__ (GH-28403)

Co-authored-by: Jason R. Coombs <[email protected]>

Author: serhiy-storchaka

Lib/test/test_set.py

@@ -667,10 +667,13 @@ def __new__(cls, arg, newarg=None):
                 self = super().__new__(cls, arg)
                 self.newarg = newarg
                 return self
-        u = subclass_with_new([1, 2], newarg=3)
+        u = subclass_with_new([1, 2])
         self.assertIs(type(u), subclass_with_new)
         self.assertEqual(set(u), {1, 2})
-        self.assertEqual(u.newarg, 3)
+        self.assertIsNone(u.newarg)
+        # disallow kwargs in __new__ only (https://bugs.python.org/issue43413#msg402000)
+        with self.assertRaises(TypeError):
+            subclass_with_new([1, 2], newarg=3)
 
 
 class TestFrozenSet(TestJointOps, unittest.TestCase):

Misc/NEWS.d/3.11.0a1.rst

@@ -1140,7 +1140,7 @@ blocks. Patch by Pablo Galindo.
 
 Constructors of subclasses of some builtin classes (e.g. :class:`tuple`,
 :class:`list`, :class:`frozenset`) no longer accept arbitrary keyword
-arguments. Subclass of :class:`set` can now define a ``__new__()`` method
+arguments. [reverted in 3.11a4] Subclass of :class:`set` can now define a ``__new__()`` method
 with additional keyword parameters without overriding also ``__init__()``.
 
 ..

Misc/NEWS.d/next/Core and Builtins/2021-05-30-16-37-47.bpo-43413.vYFPPC1.rst

@@ -0,0 +1 @@
+Revert changes in ``set.__init__``. Subclass of :class:`set` needs to define a ``__init__()`` method if it defines a ``__new__()`` method with additional keyword parameters.

Objects/setobject.c

@@ -1946,9 +1946,7 @@ set_init(PySetObject *self, PyObject *args, PyObject *kwds)
 {
     PyObject *iterable = NULL;
 
-     if ((Py_IS_TYPE(self, &PySet_Type) ||
-          Py_TYPE(self)->tp_new == PySet_Type.tp_new) &&
-         !_PyArg_NoKeywords("set", kwds))
+     if (!_PyArg_NoKeywords("set", kwds))
         return -1;
     if (!PyArg_UnpackTuple(args, Py_TYPE(self)->tp_name, 0, 1, &iterable))
         return -1;