assert there are no reference cycles at all

Author: graingert

Lib/test/test_concurrent_futures/executor.py

@@ -69,14 +69,7 @@ def test_map_exception(self):
 
         # a failed future should not be captured in its
         # future._exception.__traceback__ to avoid a reference cycle
-        self.assertFalse(
-            [
-                referrer
-                for referrer in gc.get_referrers(error)
-                if isinstance(referrer, futures.Future)
-            ],
-            msg="none of the referrers should be a Future",
-        )
+        self.assertListEqual(gc.get_referrers(error), [])
 
     @support.requires_resource('walltime')
     def test_map_timeout(self):