Issue 6637: defaultdict.copy() failed with an empty factory.

Lib/test/test_defaultdict.py

@@ -60,6 +60,7 @@ class TestDefaultDict(unittest.TestCase):
         d1 = defaultdict()
         self.assertEqual(d1.default_factory, None)
         self.assertEqual(repr(d1), "defaultdict(None, {})")
+        self.assertEqual(eval(repr(d1)), d1)
         d1[11] = 41
         self.assertEqual(repr(d1), "defaultdict(None, {11: 41})")
         d2 = defaultdict(int)
@@ -112,6 +113,12 @@ class TestDefaultDict(unittest.TestCase):
         d4[12]
         self.assertEqual(d4, {42: [], 12: []})
 
+        # Issue 6637: Copy fails for empty default dict
+        d = defaultdict()
+        d['a'] = 42
+        e = d.copy()
+        self.assertEqual(e['a'], 42)
+
     def test_shallow_copy(self):
         d1 = defaultdict(foobar, {1: 1})
         d2 = copy.copy(d1)

Modules/_collectionsmodule.c

@@ -1170,6 +1170,9 @@ defdict_copy(defdictobject *dd)
 	   whose class constructor has the same signature.  Subclasses that
 	   define a different constructor signature must override copy().
 	*/
+
+	if (dd->default_factory == NULL)
+		return PyObject_CallFunctionObjArgs((PyObject*)Py_TYPE(dd), Py_None, dd, NULL);
 	return PyObject_CallFunctionObjArgs((PyObject*)Py_TYPE(dd),
 					    dd->default_factory, dd, NULL);
 }
@@ -1316,7 +1319,7 @@ defdict_init(PyObject *self, PyObject *args, PyObject *kwds)
 		Py_ssize_t n = PyTuple_GET_SIZE(args);
 		if (n > 0) {
 			newdefault = PyTuple_GET_ITEM(args, 0);
-			if (!PyCallable_Check(newdefault)) {
+			if (!PyCallable_Check(newdefault) && newdefault != Py_None) {
 				PyErr_SetString(PyExc_TypeError,
 					"first argument must be callable");                           
 				return -1;