Issue #23722: Initialize __class__ from type.__new__()

The __class__ cell used by zero-argument super() is now initialized
from type.__new__ rather than __build_class__, so class methods
relying on that will now work correctly when called from metaclass
methods during class creation.

Patch by Martin Teichmann.

Python/bltinmodule.c

@@ -54,7 +54,7 @@ _Py_IDENTIFIER(stderr);
 static PyObject *
 builtin___build_class__(PyObject *self, PyObject *args, PyObject *kwds)
 {
-    PyObject *func, *name, *bases, *mkw, *meta, *winner, *prep, *ns, *cell;
+    PyObject *func, *name, *bases, *mkw, *meta, *winner, *prep, *ns, *none;
     PyObject *cls = NULL;
     Py_ssize_t nargs;
     int isclass = 0;   /* initialize to prevent gcc warning */
@@ -167,15 +167,13 @@ builtin___build_class__(PyObject *self, PyObject *args, PyObject *kwds)
         Py_DECREF(bases);
         return NULL;
     }
-    cell = PyEval_EvalCodeEx(PyFunction_GET_CODE(func), PyFunction_GET_GLOBALS(func), ns,
+    none = PyEval_EvalCodeEx(PyFunction_GET_CODE(func), PyFunction_GET_GLOBALS(func), ns,
                              NULL, 0, NULL, 0, NULL, 0, NULL,
                              PyFunction_GET_CLOSURE(func));
-    if (cell != NULL) {
+    if (none != NULL) {
         PyObject *margs[3] = {name, bases, ns};
         cls = _PyObject_FastCallDict(meta, margs, 3, mkw);
-        if (cls != NULL && PyCell_Check(cell))
-            PyCell_Set(cell, cls);
-        Py_DECREF(cell);
+        Py_DECREF(none);
     }
     Py_DECREF(ns);
     Py_DECREF(meta);

Python/compile.c

@@ -1968,7 +1968,7 @@ compiler_class(struct compiler *c, stmt_ty s)
             return 0;
         }
         if (c->u->u_ste->ste_needs_class_closure) {
-            /* return the (empty) __class__ cell */
+            /* store __classcell__ into class namespace */
             str = PyUnicode_InternFromString("__class__");
             if (str == NULL) {
                 compiler_exit_scope(c);
@@ -1981,15 +1981,20 @@ compiler_class(struct compiler *c, stmt_ty s)
                 return 0;
             }
             assert(i == 0);
-            /* Return the cell where to store __class__ */
+
             ADDOP_I(c, LOAD_CLOSURE, i);
+            str = PyUnicode_InternFromString("__classcell__");
+            if (!str || !compiler_nameop(c, str, Store)) {
+                Py_XDECREF(str);
+                compiler_exit_scope(c);
+                return 0;
+            }
+            Py_DECREF(str);
         }
         else {
+            /* This happens when nobody references the cell. */
             assert(PyDict_Size(c->u->u_cellvars) == 0);
-            /* This happens when nobody references the cell. Return None. */
-            ADDOP_O(c, LOAD_CONST, Py_None, consts);
         }
-        ADDOP_IN_SCOPE(c, RETURN_VALUE);
         /* create the code object */
         co = assemble(c, 1);
     }