1 files changed, 10 insertions, 3 deletions
diff --git a/lib/sqlalchemy/util/_collections.py b/lib/sqlalchemy/util/_collections.py
index b2e5c6250..a43115203 100644
--- a/lib/sqlalchemy/util/_collections.py
+++ b/lib/sqlalchemy/util/_collections.py
@@ -649,16 +649,23 @@ class IdentitySet(object):
 
 
 class WeakSequence(object):
-    def __init__(self, elements):
+    def __init__(self, __elements=()):
         self._storage = [
-            weakref.ref(element) for element in elements
+            weakref.ref(element, self._remove) for element in __elements
         ]
 
+    def append(self, item):
+        self._storage.append(weakref.ref(item, self._remove))
+
     def _remove(self, ref):
         self._storage.remove(ref)
 
+    def __len__(self):
+        return len(self._storage)
+
     def __iter__(self):
-        return (obj for obj in (ref() for ref in self._storage) if obj is not None)
+        return (obj for obj in
+                    (ref() for ref in self._storage) if obj is not None)
 
     def __getitem__(self, index):
         try: