try..finally for tearing down test

IPython/utils/tests/test_traitlets.py

@@ -678,6 +678,7 b' class TraitTestBase(TestCase):'
         None in self._bad_values):
             trait=self.obj.traits()['value']
             if isinstance(trait, AllowNone) and not trait._allow_none:
+                try:
                 trait._allow_none = True
                 self._bad_values.remove(None)
                 #skip coerce. Allow None casts None to None.
@@ -685,6 +686,7 b' class TraitTestBase(TestCase):'
                 self.assertEqual(self.obj.value,None)
                 self.test_good_values()
                 self.test_bad_values()
+                finally:
                 #tear down
                 trait._allow_none = False
                 self._bad_values.append(None)