Tornado testing 4.0.1 WTF

hello.py

from tornado.web import RequestHandler


class HelloHandler(RequestHandler):
    def get(self):
        self.write('Hello world!')

test.py

from tornado.testing import AsyncHTTPTestCase, gen_test
from tornado.web import Application

from hello import HelloHandler


class HelloHandlerTest(AsyncHTTPTestCase):
    def get_app(self):
        return Application(
            [(r'/', HelloHandler)]
        )

    @gen_test
    def test_response(self):
        res = self.fetch('/')
        assert res.error is None, 'error: %s' % str(res.error)