Make C helper function more closely match the pure python version, and add tests.

Lib/test/test_collections.py

@@ -3,7 +3,7 @@
 import unittest, doctest, operator
 import inspect
 from test import support
-from collections import namedtuple, Counter, OrderedDict
+from collections import namedtuple, Counter, OrderedDict, _count_elements
 from test import mapping_tests
 import pickle, copy
 from random import randrange, shuffle
@@ -775,6 +775,19 @@ class TestCounter(unittest.TestCase):
         c.subtract('aaaabbcce')
         self.assertEqual(c, Counter(a=-1, b=0, c=-1, d=1, e=-1))
 
+    def test_helper_function(self):
+        # two paths, one for real dicts and one for other mappings
+        elems = list('abracadabra')
+
+        d = dict()
+        _count_elements(d, elems)
+        self.assertEqual(d, {'a': 5, 'r': 2, 'b': 2, 'c': 1, 'd': 1})
+
+        m = OrderedDict()
+        _count_elements(m, elems)
+        self.assertEqual(m,
+             OrderedDict([('a', 5), ('b', 2), ('r', 2), ('c', 1), ('d', 1)]))
+
 class TestOrderedDict(unittest.TestCase):
 
     def test_init(self):

Modules/_collectionsmodule.c

@@ -1536,25 +1536,23 @@ _count_elements(PyObject *self, PyObject *args)
     if (!PyArg_UnpackTuple(args, "_count_elements", 2, 2, &mapping, &iterable))
         return NULL;
 
-    if (!PyDict_Check(mapping)) {
-        PyErr_SetString(PyExc_TypeError,
-            "Expected mapping argument to be a dictionary");
-        return NULL;
-    }
-
     it = PyObject_GetIter(iterable);
     if (it == NULL)
         return NULL;
+
     one = PyLong_FromLong(1);
     if (one == NULL) {
         Py_DECREF(it);
         return NULL;
     }
+
+    if (PyDict_CheckExact(mapping)) {
         while (1) {
             key = PyIter_Next(it);
             if (key == NULL) {
                 if (PyErr_Occurred() && PyErr_ExceptionMatches(PyExc_StopIteration))
                     PyErr_Clear();
+                else
                     break;
             }
             oldval = PyDict_GetItem(mapping, key);
@@ -1571,6 +1569,35 @@ _count_elements(PyObject *self, PyObject *args)
             }
             Py_DECREF(key);
         }
+    } else {
+        while (1) {
+            key = PyIter_Next(it);
+            if (key == NULL) {
+                if (PyErr_Occurred() && PyErr_ExceptionMatches(PyExc_StopIteration))
+                    PyErr_Clear();
+                else
+                    break;
+            }
+            oldval = PyObject_GetItem(mapping, key);
+            if (oldval == NULL) {
+                if (!PyErr_Occurred() || !PyErr_ExceptionMatches(PyExc_KeyError))
+                    break;
+                PyErr_Clear();
+                Py_INCREF(one);
+                newval = one;
+            } else {
+                newval = PyNumber_Add(oldval, one);
+                Py_DECREF(oldval);
+                if (newval == NULL)
+                    break;
+            }
+            if (PyObject_SetItem(mapping, key, newval) == -1)
+                break;
+            Py_CLEAR(newval);
+            Py_DECREF(key);
+        }
+    }
+
     Py_DECREF(it);
     Py_XDECREF(key);
     Py_XDECREF(newval);