diff --git a/lib/matplotlib/transforms.py b/lib/matplotlib/transforms.py
index 89942d87e319..9b1ab59f357f 100644
--- a/lib/matplotlib/transforms.py
+++ b/lib/matplotlib/transforms.py
@@ -111,7 +111,11 @@ def __getstate__(self):
def __setstate__(self, data_dict):
self.__dict__ = data_dict
# turn the normal dictionary back into a dictionary with weak values
- self._parents = {k: weakref.ref(v)
+ # The extra lambda is to provide a callback to remove dead
+ # weakrefs from the dictionary when garbage collection is done.
+ self._parents = {k: weakref.ref(v, lambda ref, sid=k,
+ target=self._parents:
+ target.pop(sid))
for k, v in self._parents.items() if v is not None}
def __copy__(self, *args):
@@ -169,7 +173,12 @@ def set_children(self, *children):
# parents are destroyed, references from the children won't
# keep them alive.
for child in children:
- child._parents[id(self)] = weakref.ref(self)
+ # Use weak references so this dictionary won't keep obsolete nodes
+ # alive; the callback deletes the dictionary entry. This is a
+ # performance improvement over using WeakValueDictionary.
+ ref = weakref.ref(self, lambda ref, sid=id(self),
+ target=child._parents: target.pop(sid))
+ child._parents[id(self)] = ref
if DEBUG:
_set_children = set_children
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