Index: Objects/stringlib/string_format.h
===================================================================
--- Objects/stringlib/string_format.h	(révision 78814)
+++ Objects/stringlib/string_format.h	(copie de travail)
@@ -578,6 +578,12 @@
        this optimization for unicode.  This could be fixed, but it's a
        hassle. */
 #else
+    if (PyUnicode_CheckExact(fieldobj))
+    {
+	/* raise an UnicodeEncodeError */
+	(void)PyUnicode_Decode("\xff", 1, "ascii", "strict");
+	return 0;
+    }
     if (PyString_CheckExact(fieldobj))
 	formatter = _PyBytes_FormatAdvanced;
     else if (PyInt_CheckExact(fieldobj))
Index: Objects/stringobject.c
===================================================================
--- Objects/stringobject.c	(révision 78814)
+++ Objects/stringobject.c	(copie de travail)
@@ -3617,6 +3617,35 @@
 \n\
 ");
 
+static PyObject *
+string_format(PyObject *self, PyObject *args, PyObject *kwargs)
+{
+    PyObject *result, *unicode, *format;
+
+    result = do_string_format(self, args, kwargs);
+    if (result != NULL)
+	return result;
+
+    if (!PyErr_ExceptionMatches(PyExc_UnicodeError))
+	return NULL;
+
+    PyErr_Clear();
+    unicode = PyUnicode_Decode(PyString_AS_STRING(self), PyString_GET_SIZE(self), NULL, NULL);
+    if (unicode == NULL)
+	return NULL;
+
+    format = PyObject_GetAttrString(unicode, "format");
+    if (format == NULL) {
+	Py_DECREF(unicode);
+	return NULL;
+    }
+
+    result = PyObject_Call(format, args, kwargs);
+
+    Py_DECREF(format);
+    Py_DECREF(unicode);
+    return result;
+}
 
 static PyMethodDef
 string_methods[] = {
@@ -3661,7 +3690,7 @@
 	{"rjust", (PyCFunction)string_rjust, METH_VARARGS, rjust__doc__},
 	{"center", (PyCFunction)string_center, METH_VARARGS, center__doc__},
 	{"zfill", (PyCFunction)string_zfill, METH_VARARGS, zfill__doc__},
-	{"format", (PyCFunction) do_string_format, METH_VARARGS | METH_KEYWORDS, format__doc__},
+	{"format", (PyCFunction) string_format, METH_VARARGS | METH_KEYWORDS, format__doc__},
 	{"__format__", (PyCFunction) string__format__, METH_VARARGS, p_format__doc__},
 	{"_formatter_field_name_split", (PyCFunction) formatter_field_name_split, METH_NOARGS},
 	{"_formatter_parser", (PyCFunction) formatter_parser, METH_NOARGS},
Index: Lib/test/test_unicode.py
===================================================================
--- Lib/test/test_unicode.py	(révision 78814)
+++ Lib/test/test_unicode.py	(copie de travail)
@@ -1143,13 +1143,12 @@
         self.assertRaises(ValueError, u"{0:=s}".format, u'')
 
         # test combining string and unicode
-        self.assertEqual(u"foo{0}".format('bar'), u'foobar')
-        # This will try to convert the argument from unicode to str, which
-        #  will succeed
-        self.assertEqual("foo{0}".format(u'bar'), 'foobar')
-        # This will try to convert the argument from unicode to str, which
-        #  will fail
-        self.assertRaises(UnicodeEncodeError, "foo{0}".format, u'\u1000bar')
+        def check(a, b):
+            self.assertEqual(a, b)
+            self.assertEqual(type(a), type(b))
+        check(u"foo{0}".format('bar'), u'foobar')
+        check("foo{0}".format(u'bar'), u'foobar')
+        check("foo{0}".format(u'\u1000bar'), u'foo\u1000bar')
 
     def test_format_auto_numbering(self):
         class C: