Index: Objects/unicodeobject.c
===================================================================
--- Objects/unicodeobject.c	(revision 57477)
+++ Objects/unicodeobject.c	(working copy)
@@ -7730,12 +7730,19 @@
 */
 #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 *PyUnicode_Format(PyObject *format,
 			   PyObject *args)
 {
     Py_UNICODE *fmt, *res;
     Py_ssize_t fmtcnt, rescnt, reslen, arglen, argidx;
     int args_owned = 0;
+    int format_type = FORMAT_TYPE_UNKNOWN;
     PyUnicodeObject *result = NULL;
     PyObject *dict = NULL;
     PyObject *uformat;
@@ -7806,6 +7813,13 @@
 				    "format requires a mapping");
 		    goto onError;
 		}
+		if (format_type == FORMAT_TYPE_TUPLE) {
+		    PyErr_SetString(PyExc_ValueError,
+				    "both keyed and unkeyed format "
+				    "specifiers used");
+		    goto onError;
+		}
+		format_type = FORMAT_TYPE_MAPPING;
 		++fmt;
 		--fmtcnt;
 		keystart = fmt;
@@ -7849,6 +7863,15 @@
 		arglen = -1;
 		argidx = -2;
 	    }
+	    else if (format_type == FORMAT_TYPE_MAPPING) {
+		PyErr_SetString(PyExc_ValueError,
+				"both keyed and unkeyed format specifiers used"
+				);
+		goto onError;
+	    }
+	    else if (format_type == FORMAT_TYPE_UNKNOWN) {
+		format_type = FORMAT_TYPE_TUPLE;
+	    }
 	    while (--fmtcnt >= 0) {
 		switch (c = *fmt++) {
 		case '-': flags |= F_LJUST; continue;
Index: Objects/stringobject.c
===================================================================
--- Objects/stringobject.c	(revision 57477)
+++ Objects/stringobject.c	(working copy)
@@ -4455,6 +4455,12 @@
 */
 #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)
 {
@@ -4462,6 +4468,7 @@
 	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;
@@ -4534,6 +4541,12 @@
 						 "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;
@@ -4568,6 +4581,14 @@
 				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;