gh-135385: do not reserve inline values for attributes stored in slots (#156656)

Author: serhiy-storchaka

Misc/NEWS.d/next/Core_and_Builtins/2026-08-30-16-30-00.gh-issue-135385.Wn7Rd4.rst

@@ -0,0 +1,3 @@
+Reduce the memory consumption of instances of classes which define
+``__slots__`` and also have a ``__dict__``.  Space for the attributes stored
+in slots is no longer reserved in the inline values of every instance.

Objects/dictobject.c

@@ -7305,6 +7305,11 @@ _PyDict_NewKeysForClass(PyHeapTypeObject *cls)
                 PyObject *key = PyTuple_GET_ITEM(attrs, i);
                 Py_hash_t hash;
                 if (PyUnicode_CheckExact(key) && (hash = unicode_get_hash(key)) != -1) {
+                    /* An attribute stored in a slot never uses the dict. */
+                    PyObject *descr = _PyType_Lookup((PyTypeObject *)cls, key);
+                    if (descr != NULL && Py_IS_TYPE(descr, &PyMemberDescr_Type)) {
+                        continue;
+                    }
                     if (insert_split_key(keys, key, hash) == DKIX_EMPTY) {
                         break;
                     }