build_class(): one more (hopefully the last) step on the way to

backwards compatibility.  When using the class of the first base as
the metaclass, use its __class__ attribute in preference over its
ob_type slot.  This ensures that we can still use classic classes as
metaclasse, as shown in the original "Metaclasses" essay.  This also
makes all the examples in Demo/metaclasses/ work again (maybe these
should be turned into a test suite?).

diff --git a/Python/ceval.c b/Python/ceval.c
index 030afe60e2cf11500bac6da40d06318a0b9305ad..f9489428095d369a16edddb0c4fd0da7540c1279 100644 (file)
--- a/Python/ceval.c
+++ b/Python/ceval.c
@@ -3500,25 +3500,32 @@ import_all_from(PyObject *locals, PyObject *v)
 static PyObject *
 build_class(PyObject *methods, PyObject *bases, PyObject *name)
 {
-       PyObject *metaclass = NULL;
+       PyObject *metaclass = NULL, *result, *base;
 
        if (PyDict_Check(methods))
                metaclass = PyDict_GetItemString(methods, "__metaclass__");
-
-       if (metaclass == NULL) {
-               if (PyTuple_Check(bases) && PyTuple_GET_SIZE(bases) > 0)
-                       metaclass = (PyObject *)
-                               PyTuple_GET_ITEM(bases, 0)->ob_type;
-               else {
-                       PyObject *g = PyEval_GetGlobals();
-                       if (g != NULL && PyDict_Check(g))
-                               metaclass = PyDict_GetItemString(
-                                       g, "__metaclass__");
-                       if (metaclass == NULL)
-                               metaclass = (PyObject *) &PyClass_Type;
+       if (metaclass != NULL)
+               Py_INCREF(methods);
+       else if (PyTuple_Check(bases) && PyTuple_GET_SIZE(bases) > 0) {
+               base = PyTuple_GET_ITEM(bases, 0);
+               metaclass = PyObject_GetAttrString(base, "__class__");
+               if (metaclass == NULL) {
+                       PyErr_Clear();
+                       metaclass = (PyObject *)base->ob_type;
+                       Py_INCREF(metaclass);
                }
        }
-       return PyObject_CallFunction(metaclass, "OOO", name, bases, methods);
+       else {
+               PyObject *g = PyEval_GetGlobals();
+               if (g != NULL && PyDict_Check(g))
+                       metaclass = PyDict_GetItemString(g, "__metaclass__");
+               if (metaclass == NULL)
+                       metaclass = (PyObject *) &PyClass_Type;
+               Py_INCREF(metaclass);
+       }
+       result = PyObject_CallFunction(metaclass, "OOO", name, bases, methods);
+       Py_DECREF(metaclass);
+       return result;
 }
 
 static int