Check for a common user error with defaultdict().

Lib/test/test_defaultdict.py

@@ -47,6 +47,7 @@ class TestDefaultDict(unittest.TestCase):
             self.assertEqual(err.args, (15,))
         else:
             self.fail("d2[15] didn't raise KeyError")
+        self.assertRaises(TypeError, defaultdict, 1)
 
     def test_missing(self):
         d1 = defaultdict()
@@ -60,10 +61,10 @@ class TestDefaultDict(unittest.TestCase):
         self.assertEqual(repr(d1), "defaultdict(None, {})")
         d1[11] = 41
         self.assertEqual(repr(d1), "defaultdict(None, {11: 41})")
-        d2 = defaultdict(0)
-        self.assertEqual(d2.default_factory, 0)
+        d2 = defaultdict(int)
+        self.assertEqual(d2.default_factory, int)
         d2[12] = 42
-        self.assertEqual(repr(d2), "defaultdict(0, {12: 42})")
+        self.assertEqual(repr(d2), "defaultdict(<type 'int'>, {12: 42})")
         def foo(): return 43
         d3 = defaultdict(foo)
         self.assert_(d3.default_factory is foo)

Misc/NEWS

@@ -103,6 +103,8 @@ Core and builtins
 Extension Modules
 -----------------
 
+- collections.defaultdict() now verifies that the factory function is callable.
+
 - Bug #1486663: don't reject keyword arguments for subclasses of builtin
   types.
 

Modules/collectionsmodule.c

@@ -1252,8 +1252,14 @@ defdict_init(PyObject *self, PyObject *args, PyObject *kwds)
 		newargs = PyTuple_New(0);
 	else {
 		Py_ssize_t n = PyTuple_GET_SIZE(args);
-		if (n > 0)
+		if (n > 0) {
 			newdefault = PyTuple_GET_ITEM(args, 0);
+			if (!PyCallable_Check(newdefault)) {
+				PyErr_SetString(PyExc_TypeError,
+					"first argument must be callable");                           
+				return -1;
+			}
+		}
 		newargs = PySequence_GetSlice(args, 1, n);
 	}
 	if (newargs == NULL)