Index: Objects/unicodeobject.c
===================================================================
--- Objects/unicodeobject.c	(revision 62728)
+++ Objects/unicodeobject.c	(working copy)
@@ -8276,10 +8276,27 @@
 static PyObject *
 unicode_mod(PyObject *v, PyObject *w)
 {
+       /* Used to ensure we don't apply the warning when called
+          recursively.  Needed because the warning system will
+          call back into this function. */
+       static int level = 0;
+
        if (!PyUnicode_Check(v)) {
                Py_INCREF(Py_NotImplemented);
                return Py_NotImplemented;
        }
+
+       if (level == 0) {
+               int err;
+               ++level;
+               err = PyErr_WarnEx(PyExc_PendingDeprecationWarning,
+                                  "% formatting will be deprecated, "
+                                  "use .format() instead", 1);
+               --level;
+               if (err < 0)
+                   return NULL;
+       }
+
        return PyUnicode_Format(v, w);
 }