only accept AttributeError as indicating no __prepare__ attribute on a...

only accept AttributeError as indicating no __prepare__ attribute on a metaclass, allowing lookup errors to propogate

Lib/test/test_metaclass.py

@@ -230,6 +230,20 @@ Make sure it works with subclassing.
     42
     >>>
 
+Test failures in looking up the __prepare__ method work.
+    >>> class ObscureException(Exception):
+    ...     pass
+    >>> class FailDescr:
+    ...     def __get__(self, instance, owner):
+    ...        raise ObscureException
+    >>> class Meta(type):
+    ...     __prepare__ = FailDescr()
+    >>> class X(metaclass=Meta):
+    ...     pass
+    Traceback (most recent call last):
+    [...]
+    test.test_metaclass.ObscureException
+
 """
 
 __test__ = {'doctests' : doctests}

Misc/NEWS

@@ -12,6 +12,8 @@ What's New in Python 3.2 Alpha 1?
 Core and Builtins
 -----------------
 
+- Handle errors from looking up __prepare__ correctly.
+
 - Issue #5939: Add additional runtime checking to ensure a valid capsule
   in Modules/_ctypes/callproc.c.
 

Python/bltinmodule.c

@@ -108,9 +108,17 @@ builtin___build_class__(PyObject *self, PyObject *args, PyObject *kwds)
 	}
 	prep = PyObject_GetAttrString(meta, "__prepare__");
 	if (prep == NULL) {
+		if (PyErr_ExceptionMatches(PyExc_AttributeError)) {
 			PyErr_Clear();
 			ns = PyDict_New();
 		}
+		else {
+			Py_DECREF(meta);
+			Py_XDECREF(mkw);
+			Py_DECREF(bases);
+			return NULL;
+		}
+	}
 	else {
 		PyObject *pargs = PyTuple_Pack(2, name, bases);
 		if (pargs == NULL) {