#3071: tell how many values were expected when unpacking too many.

Lib/test/test_unpack.py

@@ -62,14 +62,14 @@ Unpacking tuple of wrong size
     >>> a, b = t
     Traceback (most recent call last):
       ...
-    ValueError: too many values to unpack
+    ValueError: too many values to unpack (expected 2)
 
 Unpacking tuple of wrong size
 
     >>> a, b = l
     Traceback (most recent call last):
       ...
-    ValueError: too many values to unpack
+    ValueError: too many values to unpack (expected 2)
 
 Unpacking sequence too short
 
@@ -83,7 +83,7 @@ Unpacking sequence too long
     >>> a, b = Seq()
     Traceback (most recent call last):
       ...
-    ValueError: too many values to unpack
+    ValueError: too many values to unpack (expected 2)
 
 Unpacking a sequence where the test for too long raises a different kind of
 error

Python/ceval.c

@@ -3464,7 +3464,8 @@ unpack_iterable(PyObject *v, int argcnt, int argcntafter, PyObject **sp)
             return 1;
         }
         Py_DECREF(w);
-        PyErr_SetString(PyExc_ValueError, "too many values to unpack");
+        PyErr_Format(PyExc_ValueError, "too many values to unpack "
+                     "(expected %d)", argcnt);
         goto Error;
     }