SF bug #1014215: Unspecific errors with metaclass

High level error message was stomping useful detailed messages from lower
level routines.

The new approach is to augment string error messages returned by the low
level routines.  The provides both high and low level information.  If
the exception value is not a string, no changes are made.

To see the improved messages in action, type:
   import random
   class R(random): pass
   class B(bool): pass

1 files changed, 16 insertions, 4 deletions

diff --git a/Python/ceval.c b/Python/ceval.c
index 186bc82..2223aba 100644
--- a/Python/ceval.c
+++ b/Python/ceval.c
@@ -4086,10 +4086,22 @@ build_class(PyObject *methods, PyObject *bases, PyObject *name)
 		/* A type error here likely means that the user passed 
 		   in a base that was not a class (such the random module
 		   instead of the random.random type).  Help them out with
-		   a more informative error message */
-		PyErr_SetString(PyExc_TypeError,
-			"Error when calling the metaclass.\n" \
-			"Make sure the base arguments are valid.");
+		   by augmenting the error message with more information.*/
+
+		PyObject *ptype, *pvalue, *ptraceback;
+
+		PyErr_Fetch(&ptype, &pvalue, &ptraceback);
+		if (PyString_Check(pvalue)) {
+			PyObject *newmsg;
+			newmsg = PyString_FromFormat(
+				"Error when calling the metaclass bases\n    %s",
+				PyString_AS_STRING(pvalue));
+			if (newmsg != NULL) {
+				Py_DECREF(pvalue);
+				pvalue = newmsg;
+			}
+		}
+		PyErr_Restore(ptype, pvalue, ptraceback);
 	}
 	return result;
 }