def test_headersShouldEachHaveKeyAndValue(self):
        """
        Each header passed to the I{start_response} callable MUST hold a key
        and a value, and ONLY a key and a value.
        """
        def application(environ, startResponse):
            startResponse("200 OK", [("too", "many", "cooks")])
            return iter(())

        request, result = self.prepareRequest(application)
        request.requestReceived()

        def checkMessage(error):
            self.assertEqual(
                "header must be a (str, str) tuple, not "
                "('too', 'many', 'cooks')", str(error))

        return self.assertFailure(result, TypeError).addCallback(checkMessage)

        Each header passed to the I{start_response} callable MUST hold a key
        and a value, and ONLY a key and a value.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/web/test/test_wsgi.py