guard against unexpected weakref cleanups

Added some guards against ``KeyError`` in the event system to accommodate
the case that the interpreter is shutting down at the same time
:meth:`_engine.Engine.dispose` is being called, which would cause stack
trace warnings.

Fixes: #6740
Change-Id: I2c42e9edac2371e68b39d8360494c5fddfd6cd8c

1 files changed, 12 insertions, 2 deletions

diff --git a/lib/sqlalchemy/event/registry.py b/lib/sqlalchemy/event/registry.py
index 1f7cc43f1..ca85f3368 100644
--- a/lib/sqlalchemy/event/registry.py
+++ b/lib/sqlalchemy/event/registry.py
@@ -112,8 +112,18 @@ def _stored_in_collection_multi(newowner, oldowner, elements):
 
     for listen_fn in elements:
         listen_ref = weakref.ref(listen_fn)
-        key = old_listener_to_key[listen_ref]
-        dispatch_reg = _key_to_collection[key]
+        try:
+            key = old_listener_to_key[listen_ref]
+        except KeyError:
+            # can occur during interpreter shutdown.
+            # see #6740
+            continue
+
+        try:
+            dispatch_reg = _key_to_collection[key]
+        except KeyError:
+            continue
+
         if newowner in dispatch_reg:
             assert dispatch_reg[newowner] == listen_ref
         else: