diff -r bf7329190ca6 Lib/test/test_dict.py
--- a/Lib/test/test_dict.py	Sun Dec 11 22:31:09 2011 -0800
+++ b/Lib/test/test_dict.py	Sun Dec 18 18:56:26 2011 +0100
@@ -299,6 +299,18 @@
         x.fail = True
         self.assertRaises(Exc, d.setdefault, x, [])
 
+    def test_setdefault_atomic(self):
+        class Hashed(object):
+            def __init__(self):
+                self.count = 0
+            def __hash__(self):
+                self.count += 1
+                return 42
+        hashed = Hashed()
+        x = {}
+        x.setdefault(hashed, [])
+        self.assertEqual(hashed.count, 1)
+
     def test_popitem(self):
         # dict.popitem()
         for copymode in -1, +1:
diff -r bf7329190ca6 Objects/dictobject.c
--- a/Objects/dictobject.c	Sun Dec 11 22:31:09 2011 -0800
+++ b/Objects/dictobject.c	Sun Dec 18 18:56:26 2011 +0100
@@ -738,36 +738,16 @@
     return ep->me_value;
 }
 
-/* CAUTION: PyDict_SetItem() must guarantee that it won't resize the
- * dictionary if it's merely replacing the value for an existing key.
- * This means that it's safe to loop over a dictionary with PyDict_Next()
- * and occasionally replace a value -- but you can't insert new keys or
- * remove them.
- */
-int
-PyDict_SetItem(register PyObject *op, PyObject *key, PyObject *value)
+static int
+dict_set_item_using_hash(register PyObject *op, PyObject *key, long hash,
+                         PyObject *value)
 {
     register PyDictObject *mp;
-    register long hash;
     register Py_ssize_t n_used;
 
-    if (!PyDict_Check(op)) {
-        PyErr_BadInternalCall();
-        return -1;
-    }
     assert(key);
     assert(value);
     mp = (PyDictObject *)op;
-    if (PyString_CheckExact(key)) {
-        hash = ((PyStringObject *)key)->ob_shash;
-        if (hash == -1)
-            hash = PyObject_Hash(key);
-    }
-    else {
-        hash = PyObject_Hash(key);
-        if (hash == -1)
-            return -1;
-    }
     assert(mp->ma_fill <= mp->ma_mask);  /* at least one empty slot */
     n_used = mp->ma_used;
     Py_INCREF(value);
@@ -793,6 +773,36 @@
     return dictresize(mp, (mp->ma_used > 50000 ? 2 : 4) * mp->ma_used);
 }
 
+/* CAUTION: PyDict_SetItem() must guarantee that it won't resize the
+ * dictionary if it's merely replacing the value for an existing key.
+ * This means that it's safe to loop over a dictionary with PyDict_Next()
+ * and occasionally replace a value -- but you can't insert new keys or
+ * remove them.
+ */
+int
+PyDict_SetItem(register PyObject *op, PyObject *key, PyObject *value)
+{
+    register long hash;
+
+    if (!PyDict_Check(op)) {
+        PyErr_BadInternalCall();
+        return -1;
+    }
+    assert(key);
+    assert(value);
+    if (PyString_CheckExact(key)) {
+        hash = ((PyStringObject *)key)->ob_shash;
+        if (hash == -1)
+            hash = PyObject_Hash(key);
+    }
+    else {
+        hash = PyObject_Hash(key);
+        if (hash == -1)
+            return -1;
+    }
+    return dict_set_item_using_hash(op, key, hash, value);
+}
+
 int
 PyDict_DelItem(PyObject *op, PyObject *key)
 {
@@ -1958,7 +1968,7 @@
     val = ep->me_value;
     if (val == NULL) {
         val = failobj;
-        if (PyDict_SetItem((PyObject*)mp, key, failobj))
+        if (dict_set_item_using_hash((PyObject*)mp, key, hash, failobj))
             val = NULL;
     }
     Py_XINCREF(val);