bytes should be verboten in sum() (fixes #12654)

Lib/test/test_builtin.py

@@ -1128,6 +1128,9 @@ class BuiltinTest(unittest.TestCase):
         self.assertRaises(TypeError, sum, 42)
         self.assertRaises(TypeError, sum, ['a', 'b', 'c'])
         self.assertRaises(TypeError, sum, ['a', 'b', 'c'], '')
+        self.assertRaises(TypeError, sum, [b'a', b'c'], b'')
+        values = [bytearray(b'a'), bytearray(b'b')]
+        self.assertRaises(TypeError, sum, values, bytearray(b''))
         self.assertRaises(TypeError, sum, [[1], [2], [3]])
         self.assertRaises(TypeError, sum, [{2:3}])
         self.assertRaises(TypeError, sum, [{2:3}]*2, {2:3})

Misc/NEWS

@@ -10,6 +10,8 @@ What's New in Python 3.3 Alpha 1?
 Core and Builtins
 -----------------
 
+- Forbid summing bytes in sum().
+
 - Verify the types of AST strings and identifiers provided by the user before
   compiling them.
 

Python/bltinmodule.c

@@ -1888,6 +1888,11 @@ builtin_sum(PyObject *self, PyObject *args)
             Py_DECREF(iter);
             return NULL;
         }
+        if (PyBytes_Check(result)) {
+            PyErr_SetString(PyExc_TypeError,
+                "sum() can't sum bytes [use b''.join(seq) instead]");
+            return NULL;
+        }
         if (PyByteArray_Check(result)) {
             PyErr_SetString(PyExc_TypeError,
                 "sum() can't sum bytes [use b''.join(seq) instead]");