问题与解答¶

• 为什么在例子中调用 time.sleep() 不会并发执行?
• 我的代码是异步的, 但它不能在两个浏览器标签页上并行运行.

为什么在例子中调用 time.sleep() 不会并发执行?¶

Many people’s first foray into Tornado’s concurrency looks something likethis:

class BadExampleHandler(RequestHandler):
    def get(self):
        for i in range(5):
            print(i)
            time.sleep(1)

Fetch this handler twice at the same time and you’ll see that the secondfive-second countdown doesn’t start until the first one has completelyfinished. The reason for this is that time.sleep is a blockingfunction: it doesn’t allow control to return to the IOLoop so that otherhandlers can be run.

Of course, time.sleep is really just a placeholder in these examples,the point is to show what happens when something in a handler gets slow.No matter what the real code is doing, to achieve concurrency blockingcode must be replaced with non-blocking equivalents. This means one of three things:

• Find a coroutine-friendly equivalent. For time.sleep, usetornado.gen.sleep instead:

class CoroutineSleepHandler(RequestHandler):
@gen.coroutine
def get(self):
for i in range(5):
print(i)
yield gen.sleep(1)

When this option is available, it is usually the best approach.See the Tornado wikifor links to asynchronous libraries that may be useful.

• Find a callback-based equivalent. Similar to the first option,callback-based libraries are available for many tasks, although theyare slightly more complicated to use than a library designed forcoroutines. These are typically used with tornado.gen.Task as anadapter:

class CoroutineTimeoutHandler(RequestHandler):
@gen.coroutine
def get(self):
io_loop = IOLoop.current()
for i in range(5):
print(i)
yield gen.Task(io_loop.add_timeout, io_loop.time() + 1)

Again, theTornado wikican be useful to find suitable libraries.

• Run the blocking code on another thread. When asynchronous librariesare not available, concurrent.futures.ThreadPoolExecutor can be usedto run any blocking code on another thread. This is a universal solutionthat can be used for any blocking function whether an asynchronouscounterpart exists or not:

executor = concurrent.futures.ThreadPoolExecutor(8)

class ThreadPoolHandler(RequestHandler):
@gen.coroutine
def get(self):
for i in range(5):
print(i)
yield executor.submit(time.sleep, 1)

我的代码是异步的, 但它不能在两个浏览器标签页上并行运行.¶

即使你是用了异步和非阻塞的控制器, 你会发现在测试过程中可能也会出现问题.流量器将会发现你试图在两个不同的标签页加载同一个页面,这时浏览器会延迟加载第二个页面一直等到第一个页面加载完成. 如果你想要看到异步的效果,请尝试以下两种方法中的任意一个:

• 在url上添加一些参数让请求变得不同. 之前是将http://localhost:8888 在两个标签页中打开, 现在可以尝试在一个标签页中打开http://localhost:8888/?x=1 而后在另一个标签页打开 http://localhost:8888/?x=2 .
• 使用两个不同的浏览器. 例如, Firefox 和 Chrome 将会同时加载同样的url而不会等待对方.

原文:

https://tornado-zh-cn.readthedocs.io/zh_CN/latest/faq.html