diff -r 93996b0e07f2 Include/dictobject.h
--- a/Include/dictobject.h	Mon Mar 05 10:50:11 2012 +0100
+++ b/Include/dictobject.h	Mon Mar 05 18:11:02 2012 +0100
@@ -84,6 +84,7 @@ struct _dictobject {
     PyDictEntry *ma_table;
     PyDictEntry *(*ma_lookup)(PyDictObject *mp, PyObject *key, Py_hash_t hash);
     PyDictEntry ma_smalltable[PyDict_MINSIZE];
+    int in_lookup;
 };
 #endif /* Py_LIMITED_API */
 
diff -r 93996b0e07f2 Lib/test/test_sys.py
--- a/Lib/test/test_sys.py	Mon Mar 05 10:50:11 2012 +0100
+++ b/Lib/test/test_sys.py	Mon Mar 05 18:11:02 2012 +0100
@@ -687,9 +687,9 @@ class SizeofTest(unittest.TestCase):
         # method-wrapper (descriptor object)
         check({}.__iter__, size(h + '2P'))
         # dict
-        check({}, size(h + '3P2P' + 8*'P2P'))
+        check({}, size(h + '3P2P' + 8*'P2P' + 'I'))
         longdict = {1:1, 2:2, 3:3, 4:4, 5:5, 6:6, 7:7, 8:8}
-        check(longdict, size(h + '3P2P' + 8*'P2P') + 16*size('P2P'))
+        check(longdict, size(h + '3P2P' + 8*'P2P' + 'I') + 16*size('P2P'))
         # dictionary-keyiterator
         check({}.keys(), size(h + 'P'))
         # dictionary-valueiterator
diff -r 93996b0e07f2 Lib/test/test_weakref.py
--- a/Lib/test/test_weakref.py	Mon Mar 05 10:50:11 2012 +0100
+++ b/Lib/test/test_weakref.py	Mon Mar 05 18:11:02 2012 +0100
@@ -1285,7 +1285,7 @@ class MappingTestCase(TestBase):
         for o in objs:
             count += 1
             del d[o]
-        self.assertEqual(len(d), 0)
+        self.assertEqual(len(d), 2)
         self.assertEqual(count, 2)
 
 from test import mapping_tests
diff -r 93996b0e07f2 Objects/dictobject.c
--- a/Objects/dictobject.c	Mon Mar 05 10:50:11 2012 +0100
+++ b/Objects/dictobject.c	Mon Mar 05 18:11:02 2012 +0100
@@ -202,6 +202,7 @@ show_track(void)
 #define INIT_NONZERO_DICT_SLOTS(mp) do {                                \
     (mp)->ma_table = (mp)->ma_smalltable;                               \
     (mp)->ma_mask = PyDict_MINSIZE - 1;                                 \
+    (mp)->in_lookup = 0;                                                \
     } while(0)
 
 #define EMPTY_TO_MINSIZE(mp) do {                                       \
@@ -338,7 +339,9 @@ lookdict(PyDictObject *mp, PyObject *key
         if (ep->me_hash == hash) {
             startkey = ep->me_key;
             Py_INCREF(startkey);
+            mp->in_lookup = 1;
             cmp = PyObject_RichCompareBool(startkey, key, Py_EQ);
+            mp->in_lookup = 0;
             Py_DECREF(startkey);
             if (cmp < 0)
                 return NULL;
@@ -370,7 +373,9 @@ lookdict(PyDictObject *mp, PyObject *key
         if (ep->me_hash == hash && ep->me_key != dummy) {
             startkey = ep->me_key;
             Py_INCREF(startkey);
+            mp->in_lookup = 1;
             cmp = PyObject_RichCompareBool(startkey, key, Py_EQ);
+            mp->in_lookup = 0;
             Py_DECREF(startkey);
             if (cmp < 0)
                 return NULL;
@@ -803,6 +808,13 @@ dict_set_item_by_hash_or_entry(register 
 
     mp = (PyDictObject *)op;
     assert(mp->ma_fill <= mp->ma_mask);  /* at least one empty slot */
+
+    if (mp->in_lookup) {
+        PyErr_SetString(PyExc_RuntimeError,
+                        "cannot modify a dict during a lookup");
+        return -1;
+    }
+
     n_used = mp->ma_used;
     Py_INCREF(value);
     Py_INCREF(key);
@@ -883,6 +895,12 @@ PyDict_DelItem(PyObject *op, PyObject *k
             return -1;
     }
     mp = (PyDictObject *)op;
+    if (mp->in_lookup) {
+        PyErr_SetString(PyExc_RuntimeError,
+                        "cannot modify a dict during a lookup");
+        return -1;
+    }
+
     ep = (mp->ma_lookup)(mp, key, hash);
     if (ep == NULL)
         return -1;
@@ -1843,6 +1861,11 @@ dict_setdefault(register PyDictObject *m
 static PyObject *
 dict_clear(register PyDictObject *mp)
 {
+    if (mp->in_lookup) {
+        PyErr_SetString(PyExc_RuntimeError,
+                        "cannot modify a dict during a lookup");
+        return NULL;
+    }
     PyDict_Clear((PyObject *)mp);
     Py_RETURN_NONE;
 }