Use a copy of `self.contents` in this list comprehension.

Fixes #10782.

diff --git a/lib/sqlalchemy/orm/clsregistry.py b/lib/sqlalchemy/orm/clsregistry.py
index 4f4dab895e416a5f9e8f484cffb8867f0012240a..b78f4bdeea615085865e020736d22e782b82b151 100644 (file)
--- a/lib/sqlalchemy/orm/clsregistry.py
+++ b/lib/sqlalchemy/orm/clsregistry.py
@@ -239,10 +239,10 @@ class _MultipleClassMarker(ClsRegistryToken):
     def add_item(self, item: Type[Any]) -> None:
         # protect against class registration race condition against
         # asynchronous garbage collection calling _remove_item,
-        # [ticket:3208]
+        # [ticket:3208] and [ticket:10782]
         modules = {
             cls.__module__
-            for cls in [ref() for ref in self.contents]
+            for cls in [ref() for ref in self.contents.copy()]
             if cls is not None
         }
         if item.__module__ in modules: