diff -r ab4432daf69f Modules/_struct.c
--- a/Modules/_struct.c	Mon Oct 22 21:50:27 2012 -0700
+++ b/Modules/_struct.c	Tue Oct 23 13:52:33 2012 +0300
@@ -1661,7 +1661,7 @@
     if (PyTuple_GET_SIZE(args) != soself->s_len)
     {
         PyErr_Format(StructError,
-            "pack requires exactly %zd arguments", soself->s_len);
+            "pack expected %zd items for packing (got %zd)", soself->s_len, PyTuple_GET_SIZE(args));
         return NULL;
     }
 
@@ -1701,8 +1701,8 @@
     if (PyTuple_GET_SIZE(args) != (soself->s_len + 2))
     {
         PyErr_Format(StructError,
-                     "pack_into requires exactly %zd arguments",
-                     (soself->s_len + 2));
+                     "pack_into expected %zd items for packing (got %zd)",
+                     (soself->s_len + 2), PyTuple_GET_SIZE(args));
         return NULL;
     }