Added defaultweakobjectmap type

application/python/weakref.py

@@ -10,7 +10,7 @@
 from threading import local
 
 
-__all__ = ["weakobjectmap"]
+__all__ = ["weakobjectmap", "defaultweakobjectmap"]
 
 
 class objectref(weakref.ref):
@@ -153,6 +153,15 @@ def popitem(self):
                 return object, value
 
 
+class defaultweakobjectmap(weakobjectmap):
+    def __init__(self, factory, *args, **kw):
+        self.default_factory = factory
+        super(defaultweakobjectmap, self).__init__(*args, **kw)
+
+    def __missing__(self, key):
+        return self.setdefault(key.object, self.default_factory())
+
+
 class _ReprGuard(object):
     __local__ = local()