Issue #23722: improve __classcell__ compatibility

Handling zero-argument super() in __init_subclass__ and
__set_name__ involved moving __class__ initialisation to
type.__new__. This requires cooperation from custom
metaclasses to ensure that the new __classcell__ entry
is passed along appropriately.

The initial implementation of that change resulted in abruptly
broken zero-argument super() support in metaclasses that didn't
adhere to the new requirements (such as Django's metaclass for
Model definitions).

The updated approach adopted here instead emits a deprecation
warning for those cases, and makes them work the same way they
did in Python 3.5.

This patch also improves the related class machinery documentation
to cover these details and to include more reader-friendly
cross-references and index entries.

1 files changed, 35 insertions, 5 deletions

diff --git a/Python/bltinmodule.c b/Python/bltinmodule.c
index a49cb9d..69e5f08 100644
--- a/Python/bltinmodule.c
+++ b/Python/bltinmodule.c
@@ -54,8 +54,8 @@ _Py_IDENTIFIER(stderr);
 static PyObject *
 builtin___build_class__(PyObject *self, PyObject *args, PyObject *kwds)
 {
-    PyObject *func, *name, *bases, *mkw, *meta, *winner, *prep, *ns, *none;
-    PyObject *cls = NULL;
+    PyObject *func, *name, *bases, *mkw, *meta, *winner, *prep, *ns;
+    PyObject *cls = NULL, *cell = NULL;
     Py_ssize_t nargs;
     int isclass = 0;   /* initialize to prevent gcc warning */
 
@@ -167,14 +167,44 @@ builtin___build_class__(PyObject *self, PyObject *args, PyObject *kwds)
         Py_DECREF(bases);
         return NULL;
     }
-    none = PyEval_EvalCodeEx(PyFunction_GET_CODE(func), PyFunction_GET_GLOBALS(func), ns,
+    cell = PyEval_EvalCodeEx(PyFunction_GET_CODE(func), PyFunction_GET_GLOBALS(func), ns,
                              NULL, 0, NULL, 0, NULL, 0, NULL,
                              PyFunction_GET_CLOSURE(func));
-    if (none != NULL) {
+    if (cell != NULL) {
         PyObject *margs[3] = {name, bases, ns};
         cls = _PyObject_FastCallDict(meta, margs, 3, mkw);
-        Py_DECREF(none);
+        if (cls != NULL && PyType_Check(cls) && PyCell_Check(cell)) {
+            PyObject *cell_cls = PyCell_GET(cell);
+            if (cell_cls != cls) {
+                /* TODO: In 3.7, DeprecationWarning will become RuntimeError.
+                 *       At that point, cell_error won't be needed.
+                 */
+                int cell_error;
+                if (cell_cls == NULL) {
+                    const char *msg =
+                        "__class__ not set defining %.200R as %.200R. "
+                        "Was __classcell__ propagated to type.__new__?";
+                    cell_error = PyErr_WarnFormat(
+                        PyExc_DeprecationWarning, 1, msg, name, cls);
+                } else {
+                    const char *msg =
+                        "__class__ set to %.200R defining %.200R as %.200R";
+                    PyErr_Format(PyExc_TypeError, msg, cell_cls, name, cls);
+                    cell_error = 1;
+                }
+                if (cell_error) {
+                    Py_DECREF(cls);
+                    cls = NULL;
+                    goto error;
+                } else {
+                    /* Fill in the cell, since type.__new__ didn't do it */
+                    PyCell_Set(cell, cls);
+                }
+            }
+        }
     }
+error:
+    Py_XDECREF(cell);
     Py_DECREF(ns);
     Py_DECREF(meta);
     Py_XDECREF(mkw);