Answer of www.zhihu.com/question/51772677

semaphore-demo.py

from tornado import httpclient, gen, ioloop, locks

def fetch(urlList, minRequire):
    l = locks.Semaphore(len(urlList))
    resultList = []

    @gen.coroutine
    def _fetch():
        @gen.coroutine
        def __fetch(url):
            r = yield httpclient.AsyncHTTPClient().fetch(url)
            l.release()
            resultList.append(r.body)
            print('Fetched: %s' % url)

        for i in range(len(urlList)): l.acquire()
        for url in urlList: __fetch(url)
        for i in range(minRequire): yield l.acquire()
    ioloop.IOLoop.current().run_sync(_fetch)
    return resultList

if __name__ == '__main__':
    urlList = ['http://127.0.0.1:5000/%s' % i for i in range(5)]
    r = fetch(urlList, 3)
    print(r)