def test_simpleFailure(self):
        """
        L{renderElement} handles failures by writing a minimal
        error message to the request and finishing it.
        """
        element = FailingElement()

        d = self.request.notifyFinish()

        def check(_):
            flushed = self.flushLoggedErrors(FlattenerError)
            self.assertEqual(len(flushed), 1)
            self.assertEqual(
                b"".join(self.request.written),
                (b'<!DOCTYPE html>\n'
                 b'<div style="font-size:800%;'
                 b'background-color:#FFF;'
                 b'color:#F00'
                 b'">An error occurred while rendering the response.</div>'))
            self.assertTrue(self.request.finished)

        d.addCallback(check)

        self.assertIdentical(NOT_DONE_YET, renderElement(self.request, element))

        return d

        L{renderElement} handles failures by writing a minimal
        error message to the request and finishing it.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/web/test/test_template.py