Comment 1 for bug 654474

What unittest does in Python 2.7:

    except _UnexpectedSuccess:
        addUnexpectedSuccess = getattr(result, 'addUnexpectedSuccess', None)
        if addUnexpectedSuccess is not None:
            addUnexpectedSuccess(self)
        else:
            warnings.warn("TestResult has no addUnexpectedSuccess method, reporting as failures",
                          RuntimeWarning)
            result.addFailure(self, sys.exc_info())