bpo-32259: Make a TypeError message when unpack non-iterable more specific.

Lib/test/test_dataclasses.py

@@ -866,7 +866,7 @@ class Date:
         self.assertNotEqual(Point3D(1, 2, 3), (1, 2, 3))
 
         # Make sure we can't unpack
-        with self.assertRaisesRegex(TypeError, 'is not iterable'):
+        with self.assertRaisesRegex(TypeError, 'unpack'):
             x, y, z = Point3D(4, 5, 6)
 
         # Maka sure another class with the same field names isn't

Lib/test/test_unpack.py

@@ -55,7 +55,7 @@
     >>> a, b, c = 7
     Traceback (most recent call last):
       ...
-    TypeError: 'int' object is not iterable
+    TypeError: cannot unpack non-iterable int object
 
 Unpacking tuple of wrong size
 
@@ -129,7 +129,7 @@
     >>> () = 42
     Traceback (most recent call last):
       ...
-    TypeError: 'int' object is not iterable
+    TypeError: cannot unpack non-iterable int object
 
 Unpacking to an empty iterable should raise ValueError
 

Lib/test/test_unpack_ex.py

@@ -263,7 +263,7 @@
     >>> a, *b = 7
     Traceback (most recent call last):
       ...
-    TypeError: 'int' object is not iterable
+    TypeError: cannot unpack non-iterable int object
 
 Unpacking sequence too short
 

Misc/NEWS.d/next/Core and Builtins/2017-12-16-14-30-21.bpo-32259.GoOJiX.rst

@@ -0,0 +1,2 @@
+The error message of a TypeError raised when unpack non-iterable is now more
+specific.

Python/ceval.c

@@ -4137,8 +4137,16 @@ unpack_iterable(PyObject *v, int argcnt, int argcntafter, PyObject **sp)
     assert(v != NULL);
 
     it = PyObject_GetIter(v);
-    if (it == NULL)
-        goto Error;
+    if (it == NULL) {
+        if (PyErr_ExceptionMatches(PyExc_TypeError) &&
+            v->ob_type->tp_iter == NULL && !PySequence_Check(v))
+        {
+            PyErr_Format(PyExc_TypeError,
+                         "cannot unpack non-iterable %.200s object",
+                         v->ob_type->tp_name);
+        }
+        return 0;
+    }
 
     for (; i < argcnt; i++) {
         w = PyIter_Next(it);