Index: Python/ceval.c
===================================================================
--- Python/ceval.c	(revision 69382)
+++ Python/ceval.c	(working copy)
@@ -1283,7 +1283,11 @@
 		case BINARY_MODULO:
 			w = POP();
 			v = TOP();
-			x = PyNumber_Remainder(v, w);
+			/* CheckExact is faster than Check. */
+			if (PyString_CheckExact(v) || PyString_Check(v))
+				x = PyString_Format(v, w);
+			else
+				x = PyNumber_Remainder(v, w);
 			Py_DECREF(v);
 			Py_DECREF(w);
 			SET_TOP(x);