diff -r e16ec3b468d1 Lib/test/test_unicode.py
--- a/Lib/test/test_unicode.py	Sat Oct 06 23:48:20 2012 +0200
+++ b/Lib/test/test_unicode.py	Sat Oct 06 23:48:40 2012 +0200
@@ -1746,7 +1746,6 @@ class UnicodeTest(string_tests.CommonTes
         self.assertEqual(PyUnicode_FromFormat(b'%c', c_int(0x10ffff)), '\U0010ffff')
 
         # test "%"
-        self.assertEqual(PyUnicode_FromFormat(b'%'), '%')
         self.assertEqual(PyUnicode_FromFormat(b'%%'), '%')
         self.assertEqual(PyUnicode_FromFormat(b'%%s'), '%s')
         self.assertEqual(PyUnicode_FromFormat(b'[%%]'), '[%]')
@@ -1813,12 +1812,12 @@ class UnicodeTest(string_tests.CommonTes
         text = PyUnicode_FromFormat(b'repr=%V', None, b'abc\xff')
         self.assertEqual(text, 'repr=abc\ufffd')
 
-        # not supported: copy the raw format string. these tests are just here
-        # to check for crashs and should not be considered as specifications
-        self.assertEqual(PyUnicode_FromFormat(b'%1%s', b'abc'), '%s')
-        self.assertEqual(PyUnicode_FromFormat(b'%1abc'), '%1abc')
-        self.assertEqual(PyUnicode_FromFormat(b'%+i', c_int(10)), '%+i')
-        self.assertEqual(PyUnicode_FromFormat(b'%.%s', b'abc'), '%.%s')
+        # invalid format string
+        self.assertRaises(ValueError, PyUnicode_FromFormat, b'%')
+        self.assertRaises(ValueError, PyUnicode_FromFormat, b'%1%s', b'abc')
+        self.assertRaises(ValueError, PyUnicode_FromFormat, b'%1abc')
+        self.assertRaises(ValueError, PyUnicode_FromFormat, b'%+i', c_int(10))
+        self.assertRaises(ValueError, PyUnicode_FromFormat, b'%.%s', b'abc')
 
     # Test PyUnicode_AsWideChar()
     def test_aswidechar(self):
diff -r e16ec3b468d1 Objects/unicodeobject.c
--- a/Objects/unicodeobject.c	Sat Oct 06 23:48:20 2012 +0200
+++ b/Objects/unicodeobject.c	Sat Oct 06 23:48:40 2012 +0200
@@ -2374,14 +2374,6 @@ unicode_fromformat_arg(_PyUnicodeWriter 
             precision = (precision*10) + (*f - '0');
             f++;
         }
-        if (*f == '%') {
-            /* "%.3%s" => f points to "3" */
-            f--;
-        }
-    }
-    if (*f == '\0') {
-        /* bogus format "%.123" => go backward, f points to "3" */
-        f--;
     }
 
     /* Handle %ld, %lu, %lld and %llu. */
@@ -2407,6 +2399,11 @@ unicode_fromformat_arg(_PyUnicodeWriter 
         ++f;
     }
 
+    if (f != p+1 && *f == '%')
+        goto invalid_format;
+    if (*f == '\0')
+        goto invalid_format;
+
     if (f[1] == '\0')
         writer->overallocate = 0;
 
@@ -2630,19 +2627,17 @@ unicode_fromformat_arg(_PyUnicodeWriter 
         break;
 
     default:
-        /* if we stumble upon an unknown formatting code, copy the rest
-           of the format string to the output string. (we cannot just
-           skip the code, since there's no way to know what's in the
-           argument list) */
-        len = strlen(p);
-        if (_PyUnicodeWriter_WriteCstr(writer, p, len) == -1)
-            return NULL;
-        f = p+len;
-        return f;
+        goto invalid_format;
     }
 
     f++;
     return f;
+
+invalid_format:
+    PyErr_Format(PyExc_ValueError,
+        "PyUnicode_FromFormatV() got an invalid format string near \"%.10s\"",
+        p);
+    return NULL;
 }
 
 PyObject *