Testing Tornado applications with py.test (and requests)

The following is a proof of concept for testing web applications written in Tornado with py.test.

The problem

Testing applications written in Tornado is a piece of cake. The package provides a bunch of handy tools for that, and in most cases that's all you need.

However, sometimes out-of-the-box tools are not enough. For instance, if you want to test a case where your handler responds with 4xx status code. Or if the aplication you're testing is using cookies. It is particularly uncomfortable to test such cases with tornado.testing.

Don't you think it would be great to accomplish this with a clean and elegant interface like requests?

The solution

Good news: it is possible with py.test and xprocess plugin. Just have a look at the code below:

@pytest.fixture(scope='session')
def test_server(xprocess, request):
  def preparefunc(cwd):
    def check_port():
      sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
      for i in xrange(10):
        result = sock.connect_ex(('127.0.0.1', TEST_APP_PORT))
        if result == 0:
          return True
        time.sleep(1)
      return False
      
    os.environ['APP_PORT'] = str(TEST_APP_PORT)
    server_path = os.path.join(sys.prefix, 'bin', 'tornado_pytest_app.py')
    return (check_port, [sys.executable, server_path])
    
  pid, log = xprocess.ensure('tornado_pytest_app', preparefunc, restart=True)
  
  def fin():
    os.kill(pid, signal.SIGKILL)
  request.addfinalizer(fin)

  def get_url(url):
    return 'http://localhost:%s%s' % (TEST_APP_PORT, url)

  return get_url

The fixture above does a few simple things:

1. Runs your application in a subprocess.
2. Waits until the application starts (the check_port function).
3. Kills the application once the fixture is finalized.
4. Returns get_url function, which you can use to get a URL for the test server.

Now you can write readable and elegant tests like the one below:

def test(test_server):
  response = requests.get(test_server('/'))
  assert response.status_code == 200

  json = response.json()
  assert 'status' in json
  assert json['status'] == 'ok'

The result

The approach I'm proposing is not a replacement for unit tests. But it's probably the easiest way to write functional tests for your Tornado-based application. Have fun!