Index: Objects/stringobject.c
===================================================================
--- Objects/stringobject.c	(revision 50819)
+++ Objects/stringobject.c	(working copy)
@@ -4437,6 +4437,12 @@
 */
 #define FORMATBUFLEN (size_t)120
 
+/* Constants for the type of % argument being used -- not known, 
+   a mapping, or a tuple. */
+#define FORMAT_TYPE_UNKNOWN 0
+#define FORMAT_TYPE_MAPPING 1
+#define FORMAT_TYPE_TUPLE 2
+
 PyObject *
 PyString_Format(PyObject *format, PyObject *args)
 {
@@ -4444,6 +4450,7 @@
 	Py_ssize_t arglen, argidx;
 	Py_ssize_t reslen, rescnt, fmtcnt;
 	int args_owned = 0;
+	int format_type = FORMAT_TYPE_UNKNOWN;
 	PyObject *result, *orig_args;
 #ifdef Py_USING_UNICODE
 	PyObject *v, *w;
@@ -4516,6 +4523,12 @@
 						 "format requires a mapping");
 					goto error;
 				}
+				if (format_type == FORMAT_TYPE_TUPLE) {
+					PyErr_SetString(PyExc_ValueError,
+						 "both keyed and unkeyed format specifiers used");
+					goto error;
+				}
+				format_type = FORMAT_TYPE_MAPPING;
 				++fmt;
 				--fmtcnt;
 				keystart = fmt;
@@ -4550,6 +4563,14 @@
 				arglen = -1;
 				argidx = -2;
 			}
+			else if (format_type == FORMAT_TYPE_MAPPING) {
+			  PyErr_SetString(PyExc_ValueError,
+					  "both keyed and unkeyed format specifiers used");
+			  goto error;
+			}
+			else if (format_type == FORMAT_TYPE_UNKNOWN) {
+			  format_type = FORMAT_TYPE_TUPLE;
+			}
 			while (--fmtcnt >= 0) {
 				switch (c = *fmt++) {
 				case '-': flags |= F_LJUST; continue;