Index: Objects/unicodeobject.c
===================================================================
--- Objects/unicodeobject.c	(revision 85697)
+++ Objects/unicodeobject.c	(working copy)
@@ -9214,6 +9214,13 @@
     return -1;
 }
 
+
+/* 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
+
 /* fmt%(v1,v2,...) is roughly equivalent to sprintf(fmt, v1, v2, ...)
    FORMATBUFLEN is the length of the buffer in which chars are formatted.
 */
@@ -9225,6 +9232,7 @@
     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;
@@ -9290,7 +9298,13 @@
                 Py_ssize_t keylen;
                 PyObject *key;
                 int pcount = 1;
-
+                if (format_type == FORMAT_TYPE_TUPLE) {
+                    PyErr_SetString(PyExc_ValueError,
+                            "both keyed and unkeyed format "
+                            "specifiers used");
+                    goto onError;
+                }
+                format_type = FORMAT_TYPE_MAPPING;
                 if (dict == NULL) {
                     PyErr_SetString(PyExc_TypeError,
                                     "format requires a mapping");
@@ -9339,6 +9353,18 @@
                 arglen = -1;
                 argidx = -2;
             }
+            /* Check if it's really format specifier and set format_type*/
+            else if (*fmt != '%') {
+                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;