def test_downloadHeaders(self):
        """
        After L{client.HTTPDownloader.deferred} fires, the
        L{client.HTTPDownloader} instance's C{status} and C{response_headers}
        attributes are populated with the values from the response.
        """
        def checkHeaders(factory):
            self.assertEqual(factory.status, b'200')
            self.assertEqual(factory.response_headers[b'content-type'][0], b'text/html')
            self.assertEqual(factory.response_headers[b'content-length'][0], b'10')
            os.unlink(factory.fileName)
        factory = client._makeGetterFactory(
            self.getURL('file'),
            client.HTTPDownloader,
            fileOrName=self.mktemp())
        return factory.deferred.addCallback(lambda _: checkHeaders(factory))

        After L{client.HTTPDownloader.deferred} fires, the
        L{client.HTTPDownloader} instance's C{status} and C{response_headers}
        attributes are populated with the values from the response.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/web/test/test_webclient.py