During metaclass.__init__, super() of the constructed class does not work

Python/bltinmodule.c

 /* Built-in functions */
 
 #include "Python.h"
 #include "Python-ast.h"
 
 #include "node.h"
 #include "code.h"
 
 #include "asdl.h"
 #include "ast.h"
@@ skipping 36 matching lines @@
 _Py_IDENTIFIER(stdin);
 _Py_IDENTIFIER(stdout);
 _Py_IDENTIFIER(stderr);
 
 #include "clinic/bltinmodule.c.h"
 
 /* AC: cannot convert yet, waiting for *args support */
 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 */
 
     assert(args != NULL);
     if (!PyTuple_Check(args)) {
         PyErr_SetString(PyExc_TypeError,
                         "__build_class__: args is not a tuple");
         return NULL;
     }
     nargs = PyTuple_GET_SIZE(args);
@@ skipping 91 matching lines @@
         PyObject *pargs[2] = {name, bases};
         ns = _PyObject_FastCallDict(prep, pargs, 2, mkw);
         Py_DECREF(prep);
     }
     if (ns == NULL) {
         Py_DECREF(meta);
         Py_XDECREF(mkw);
         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);

[storchaka, 2016/12/04 11:23:35]

Is it worth to check that cls is a type?

[Nick Coghlan, 2016/12/04 13:39:05]

Essential, as it turns out - the missing test case you spotted shows that this
currently emits an incorrect deprecation warning if the metaclass doesn't call
type.__new__ at all.

-        Py_DECREF(none);
+        if (cls != NULL && PyCell_Check(cell)) {
+            PyObject *cell_cls = PyCell_GET(cell);
+            if (cell_cls != cls) {
+                int cell_error = 0;

[storchaka, 2016/12/04 11:23:35]

Initialization is not needed.

[Nick Coghlan, 2016/12/04 13:39:05]

Done.

+                /*TODO: In 3.7, deprecation warning becomes a RuntimeError */

[storchaka, 2016/12/04 11:23:35]

Missed space after "/*".

[Nick Coghlan, 2016/12/04 13:39:05]

Done.

+                if (cell_cls == NULL) {
+                    char *msg = "__class__ not set for %.200R. "

[storchaka, 2016/12/04 11:23:35]

It would be better to use "const char *".

[Nick Coghlan, 2016/12/04 13:39:05]

Done.

+                                "Was __classcell__ propagated to type.__new__?";
+                    cell_error = PyErr_WarnFormat(
+                        PyExc_DeprecationWarning, 1, msg, cls);
+                } else {
+                    char *msg = "__class__ set to %.200R for %.200R. ";

[storchaka, 2016/12/04 11:23:35]

Redundant space at the end. The dot perhaps is not needed too.

Is it worth to output also the name argument? It can be different from the name
of cls.

[Nick Coghlan, 2016/12/04 13:39:05]

Done.

+                    PyErr_Format(PyExc_TypeError, msg, cell_cls, 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);

[storchaka, 2016/12/04 11:23:35]

Leaks old value of the cell.

You can add Py_XDECREF(cell_cls), but it may be safer to use PyCell_Set().

[Nick Coghlan, 2016/12/04 13:39:05]

Done.

+                }
+            }
+        }
     }
+error:
+    Py_XDECREF(cell);
     Py_DECREF(ns);
     Py_DECREF(meta);
     Py_XDECREF(mkw);
     Py_DECREF(bases);
     return cls;
 }
 
 PyDoc_STRVAR(build_class_doc,
 "__build_class__(func, name, *bases, metaclass=None, **kwds) -> class\n\
 \n\
@@ skipping 2523 matching lines @@
     if (PyDict_SetItemString(dict, "__debug__", debug) < 0) {
         Py_DECREF(debug);
         return NULL;
     }
     Py_DECREF(debug);
 
     return mod;
 #undef ADD_TO_ALL
 #undef SETBUILTIN
 }