- Fixed an unlikely race condition observed in some exotic end-user

setups, where the attempt to check for "duplicate class name" in
declarative would hit upon a not-totally-cleaned-up weak reference
related to some other class being removed; the check here now ensures
the weakref still references an object before calling upon it further.
fixes #3208

1 files changed, 6 insertions, 1 deletions

diff --git a/lib/sqlalchemy/ext/declarative/clsregistry.py b/lib/sqlalchemy/ext/declarative/clsregistry.py
index 4595b857a..3ef63a5ae 100644
--- a/lib/sqlalchemy/ext/declarative/clsregistry.py
+++ b/lib/sqlalchemy/ext/declarative/clsregistry.py
@@ -103,7 +103,12 @@ class _MultipleClassMarker(object):
                 self.on_remove()
 
     def add_item(self, item):
-        modules = set([cls().__module__ for cls in self.contents])
+        # protect against class registration race condition against
+        # asynchronous garbage collection calling _remove_item,
+        # [ticket:3208]
+        modules = set([
+            cls.__module__ for cls in
+            [ref() for ref in self.contents] if cls is not None])
         if item.__module__ in modules:
             util.warn(
                 "This declarative base already contains a class with the "