diff -r 17334c1d9245 Lib/weakref.py
--- a/Lib/weakref.py	Tue Oct 25 10:38:07 2016 +0300
+++ b/Lib/weakref.py	Tue Oct 25 12:48:51 2016 +0300
@@ -86,6 +86,12 @@ class WeakMethod(ref):
     __hash__ = ref.__hash__
 
 
+def _atomic_del_ref(d, key):
+    if d[key]() is None:
+        wr = d.pop(key)
+        if wr() is not None:
+            print('BOO!!!')
+
 class WeakValueDictionary(collections.MutableMapping):
     """Mapping class that references values weakly.
 
@@ -111,7 +117,7 @@ class WeakValueDictionary(collections.Mu
                 if self._iterating:
                     self._pending_removals.append(wr.key)
                 else:
-                    del self.data[wr.key]
+                    _atomic_del_ref(self.data, wr.key)
         self._remove = remove
         # A list of keys to be removed
         self._pending_removals = []
@@ -125,7 +131,7 @@ class WeakValueDictionary(collections.Mu
         # We shouldn't encounter any KeyError, because this method should
         # always be called *before* mutating the dict.
         while l:
-            del d[l.pop()]
+            _atomic_del_ref(d, l.pop())
 
     def __getitem__(self, key):
         o = self.data[key]()
@@ -137,10 +143,17 @@ class WeakValueDictionary(collections.Mu
     def __delitem__(self, key):
         if self._pending_removals:
             self._commit_removals()
-        del self.data[key]
+        o = self.data.pop(key)()
+        if o is None:
+            raise KeyError(key)
 
     def __len__(self):
-        return len(self.data) - len(self._pending_removals)
+        count = len(self.data)
+        if self._pending_removals:
+            for key in set(self._pending_removals):
+                if self.data[key]() is None:
+                    count -= 1
+        return count
 
     def __contains__(self, key):
         try: