@@ -111,7 +111,11 @@ def __getstate__(self):
111111def __setstate__ (self ,data_dict ):
112112self .__dict__ = data_dict
113113# turn the normal dictionary back into a dictionary with weak values
114- self ._parents = {k :weakref .ref (v )
114+ # The extra lambda is to provide a callback to remove dead
115+ # weakrefs from the dictionary when garbage collection is done.
116+ self ._parents = {k :weakref .ref (v ,lambda ref ,sid = k ,
117+ target = self ._parents :
118+ target .pop (sid ))
115119for k ,v in self ._parents .items ()if v is not None }
116120
117121def __copy__ (self ,* args ):
@@ -169,7 +173,12 @@ def set_children(self, *children):
169173# parents are destroyed, references from the children won't
170174# keep them alive.
171175for child in children :
172- child ._parents [id (self )]= weakref .ref (self )
176+ # Use weak references so this dictionary won't keep obsolete nodes
177+ # alive; the callback deletes the dictionary entry. This is a
178+ # performance improvement over using WeakValueDictionary.
179+ ref = weakref .ref (self ,lambda ref ,sid = id (self ),
180+ target = child ._parents :target .pop (sid ))
181+ child ._parents [id (self )]= ref
173182
174183if DEBUG :
175184_set_children = set_children