diff --git a/Lib/weakref.py b/Lib/weakref.py
index 88c60e7..787c885 100644
--- a/Lib/weakref.py
+++ b/Lib/weakref.py
@@ -20,7 +20,7 @@
      ProxyType,
      ReferenceType)
 
-from _weakrefset import WeakSet
+from _weakrefset import WeakSet, _IterationGuard
 
 from exceptions import ReferenceError
 
@@ -48,10 +48,24 @@
         def remove(wr, selfref=ref(self)):
             self = selfref()
             if self is not None:
-                del self.data[wr.key]
+                if self._iterating:
+                    self._pending_removals.append(wr.key)
+                else:
+                    del self.data[wr.key]
         self._remove = remove
+        # A list of keys to be removed
+        self._pending_removals = []
+        self._iterating = set()
         UserDict.UserDict.__init__(self, *args, **kw)
 
+    def _commit_removals(self):
+        l = self._pending_removals
+        d = self.data
+        # We shouldn't encounter any KeyError, because this method should
+        # always be called *before* mutating the dict.
+        while l:
+            del d[l.pop()]
+
     def __getitem__(self, key):
         o = self.data[key]()
         if o is None:
@@ -59,6 +73,11 @@
         else:
             return o
 
+    def __delitem__(self, key):
+        if self._pending_removals:
+            self._commit_removals()
+        del self.data[key]
+
     def __contains__(self, key):
         try:
             o = self.data[key]()
@@ -77,8 +96,15 @@
         return "<WeakValueDictionary at %s>" % id(self)
 
     def __setitem__(self, key, value):
+        if self._pending_removals:
+            self._commit_removals()
         self.data[key] = KeyedRef(value, self._remove, key)
 
+    def clear(self):
+        if self._pending_removals:
+            self._commit_removals()
+        self.data.clear()
+
     def copy(self):
         new = WeakValueDictionary()
         for key, wr in self.data.items():
@@ -120,16 +146,18 @@
         return L
 
     def iteritems(self):
-        for wr in self.data.itervalues():
-            value = wr()
-            if value is not None:
-                yield wr.key, value
+        with _IterationGuard(self):
+            for wr in self.data.itervalues():
+                value = wr()
+                if value is not None:
+                    yield wr.key, value
 
     def iterkeys(self):
-        return self.data.iterkeys()
+        with _IterationGuard(self):
+            for k in self.data.iterkeys():
+                yield k
 
-    def __iter__(self):
-        return self.data.iterkeys()
+    __iter__ = iterkeys
 
     def itervaluerefs(self):
         """Return an iterator that yields the weak references to the values.
@@ -141,15 +169,20 @@
         keep the values around longer than needed.
 
         """
-        return self.data.itervalues()
+        with _IterationGuard(self):
+            for wr in self.data.itervalues():
+                yield wr
 
     def itervalues(self):
-        for wr in self.data.itervalues():
-            obj = wr()
-            if obj is not None:
-                yield obj
+        with _IterationGuard(self):
+            for wr in self.data.itervalues():
+                obj = wr()
+                if obj is not None:
+                    yield obj
 
     def popitem(self):
+        if self._pending_removals:
+            self._commit_removals()
         while 1:
             key, wr = self.data.popitem()
             o = wr()
@@ -157,6 +190,8 @@
                 return key, o
 
     def pop(self, key, *args):
+        if self._pending_removals:
+            self._commit_removals()
         try:
             o = self.data.pop(key)()
         except KeyError:
@@ -172,12 +207,16 @@
         try:
             wr = self.data[key]
         except KeyError:
+            if self._pending_removals:
+                self._commit_removals()
             self.data[key] = KeyedRef(default, self._remove, key)
             return default
         else:
             return wr()
 
     def update(self, dict=None, **kwargs):
+        if self._pending_removals:
+            self._commit_removals()
         d = self.data
         if dict is not None:
             if not hasattr(dict, "items"):
@@ -245,9 +284,29 @@
         def remove(k, selfref=ref(self)):
             self = selfref()
             if self is not None:
-                del self.data[k]
+                if self._iterating:
+                    self._pending_removals.append(k)
+                else:
+                    del self.data[k]
         self._remove = remove
-        if dict is not None: self.update(dict)
+        # A list of dead weakrefs (keys to be removed)
+        self._pending_removals = []
+        self._iterating = set()
+        if dict is not None:
+            self.update(dict)
+
+    def _commit_removals(self):
+        # NOTE: We don't need to call this method before mutating the dict,
+        # because a dead weakref never compares equal to a live weakref,
+        # even if they happened to refer to equal objects.
+        # However, it means keys may already have been removed.
+        l = self._pending_removals
+        d = self.data
+        while l:
+            try:
+                del d[l.pop()]
+            except KeyError:
+                pass
 
     def __delitem__(self, key):
         del self.data[ref(key)]
@@ -306,10 +365,11 @@
         return L
 
     def iteritems(self):
-        for wr, value in self.data.iteritems():
-            key = wr()
-            if key is not None:
-                yield key, value
+        with _IterationGuard(self):
+            for wr, value in self.data.iteritems():
+                key = wr()
+                if key is not None:
+                    yield key, value
 
     def iterkeyrefs(self):
         """Return an iterator that yields the weak references to the keys.
@@ -321,19 +381,23 @@
         keep the keys around longer than needed.
 
         """
-        return self.data.iterkeys()
+        with _IterationGuard(self):
+            for wr in self.data.iterkeys():
+                yield wr
 
     def iterkeys(self):
-        for wr in self.data.iterkeys():
-            obj = wr()
-            if obj is not None:
-                yield obj
+        with _IterationGuard(self):
+            for wr in self.data.iterkeys():
+                obj = wr()
+                if obj is not None:
+                    yield obj
 
-    def __iter__(self):
-        return self.iterkeys()
+    __iter__ = iterkeys
 
     def itervalues(self):
-        return self.data.itervalues()
+        with _IterationGuard(self):
+            for value in self.data.itervalues():
+                yield value
 
     def keyrefs(self):
         """Return a list of weak references to the keys.