如何调试龙卷风异步操作

Question

I am new to Tornado framework, and according to the link Asynchronous and non-Blocking I/O , I wrote some demo code as below. Unfortunately, the sync http client works, but async http client not. It looks like, the callback function that I passed to AsyncHTTPClient.fetch never has the chance to run. So my question is:

• Why tornado's async API not work for me?
• How should I debug this kind of problem? Set a break-point to my callback function is useless because it never has chance to run.

Any help is great appreciated. Below is my demo code:

from tornado.httpclient import AsyncHTTPClient
from tornado.httpclient import HTTPClient
import time

myUrl = 'a http url serving RESTful service'


def async_fetch(url, callback):
    http_client = AsyncHTTPClient()
    def handle_test(response):
        callback(response.body)
    http_client.fetch(url, handle_test)


def sync_fetch(url):
    http_client = HTTPClient()
    response = http_client.fetch(url)
    return response.body


def printResponse(data):
    print("response is:" + data)


def main():
    sync_fetch(myUrl)   #this works
    async_fetch(myUrl, printResponse)  #this not work


if __name__ == '__main__':
    main()
    print("begin of sleep!")
    time.sleep(2)
    print("end of sleep!")

Answer 1

You need to start the IOLoop, otherwise your asynchronous task never makes progress:

from tornado.ioloop import IOLoop

def printResponse(data):
    print("response is:" + data)
    IOLoop.current().stop()


def main():
    sync_fetch(myUrl)   #this works
    async_fetch(myUrl, printResponse)
    IOLoop.current().start()

In this example I stop the loop at the bottom of printResponse. In a real web server application you might never explicitly stop the loop.