Issue #14159: Fix the len() of weak containers (WeakSet, WeakKeyDictionary, WeakValueDictionary) to return a better approximation when some objects are dead or dying.

Moreover, the implementation is now O(1) rather than O(n).
Thanks to Yury Selivanov for reporting.

1 files changed, 2 insertions, 2 deletions

diff --git a/Lib/weakref.py b/Lib/weakref.py
index 468f8f1029..fcb6b74d1b 100644
--- a/Lib/weakref.py
+++ b/Lib/weakref.py
@@ -78,7 +78,7 @@ class WeakValueDictionary(collections.MutableMapping):
         del self.data[key]
 
     def __len__(self):
-        return sum(wr() is not None for wr in self.data.values())
+        return len(self.data) - len(self._pending_removals)
 
     def __contains__(self, key):
         try:
@@ -290,7 +290,7 @@ class WeakKeyDictionary(collections.MutableMapping):
         return self.data[ref(key)]
 
     def __len__(self):
-        return len(self.data)
+        return len(self.data) - len(self._pending_removals)
 
     def __repr__(self):
         return "<WeakKeyDictionary at %s>" % id(self)