def test_constructor(self):
        """
        Given C{exception}, C{roots}, and C{traceback} arguments, the
        L{FlattenerError} constructor assigns the roots to the C{_roots}
        attribute.
        """
        e = self.makeFlattenerError(roots=['a', 'b'])
        self.assertEqual(e._roots, ['a', 'b'])

        Given C{exception}, C{roots}, and C{traceback} arguments, the
        L{FlattenerError} constructor assigns the roots to the C{_roots}
        attribute.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/web/test/test_error.py