1 files changed, 4 insertions, 6 deletions
diff --git a/lib/sqlalchemy/event.py b/lib/sqlalchemy/event.py
index f28f19ee9..de054b0fd 100644
--- a/lib/sqlalchemy/event.py
+++ b/lib/sqlalchemy/event.py
@@ -201,11 +201,9 @@ def _remove_dispatcher(cls):
                 del _registrars[k]
 
 
-class Events(object):
+class Events(object, metaclass=_EventMeta):
     """Define event listening functions for a particular target type."""
 
-    __metaclass__ = _EventMeta
-
     @classmethod
     def _accept_with(cls, target):
         # Mapper, ClassManager, Session override this to
@@ -302,7 +300,7 @@ class _DispatchDescriptor(object):
     def clear(self):
         """Clear all class level listeners"""
 
-        for dispatcher in self._clslevel.values():
+        for dispatcher in list(self._clslevel.values()):
             dispatcher[:] = []
 
     def for_modify(self, obj):
@@ -377,7 +375,7 @@ class _EmptyListener(object):
     def __iter__(self):
         return iter(self.parent_listeners)
 
-    def __nonzero__(self):
+    def __bool__(self):
         return bool(self.parent_listeners)
 
 
@@ -414,7 +412,7 @@ class _CompoundListener(object):
     def __iter__(self):
         return chain(self.parent_listeners, self.listeners)
 
-    def __nonzero__(self):
+    def __bool__(self):
         return bool(self.listeners or self.parent_listeners)