Python模拟：方法调用的结果是_getitem而不是实际值

我有一个单元测试和模拟一个外部调用。下面是我试图在service.py文件中测试的函数的缩写代码

def post_data()
  req = request.Request()
  response = req.post(payload, url, json.dumps({"data": kwargs['data']}))

  if response['request']['status'] == 'SUCCESS' and response['data']:
      run_id = response.json()['data']['run_id']
      response = track_run_to_completion(run_id, **kwargs)

  return response

这是我的单元测试方法

@patch('service.request.Request.post')
    def test_post_data(self, mock_post):
        kwargs = {'a':'abc'}
        expected = json.dumps({'request':{'status':'ERROR'},'data':{}})
        mock_post.return_value = MagicMock(status_code=200, response=expected)
mock_post.assert_called_once_with({'action': 'trigger'}, 'a/abc', '{"data": {}}') # SUCCESS!
            result = service.post_data(**kwargs)
            print result 

当我打印结果时，我希望看到json，但是得到了

。我错过了什么？我是Python新手，开始为现有应用程序编写单元测试