关于asyncio知识(四)

一、使用 asyncio 总结

最近在公司的一些项目中开始慢慢使用python 的asyncio, 使用的过程中也是各种踩坑,遇到的问题也不少,其中有一次是内存的问题,自己也整理了遇到的问题以及解决方法详细内容看:https://www.syncd.cn/article/memory_trouble

在前面整理的三篇asyncio文章中,也都是使用asyncio的一些方法,但是在实际项目中使用还是避免不了碰到问题, 在这周的工作中遇到之前碰见过的问题,一个初学asyncio写代码中经常会碰到的问题,我的业务代码在运行一段时间后提示如下错误提示:

Task was destroyed but it is pending!task: <Task pending coro=<HandleMsg.get_msg() done, defined at ex10.py:17> wait_for=<Future cancelled>>

这个错误我在前面几篇关于asyncio的系列文章中也反复说过这个问题,我也认为自己不会在出现这种问题,但是意外的是,我的程序还是出现了这个错误。

我将我的业务代码通过一个demo代码进行模拟复现以及解决这个问题,下面整理的就是这个过程

二、“Task was destroyed but it is pending!”

我通过下面这张图先描述一下demo程序的逻辑:

 

 

import asyncio
from asyncio import Queue
import uuid
from asyncio import Lock
from asyncio import CancelledError
queue = Queue()
class HandleMsg(object):
    def __init__(self, unid, coroutine_queue, handle_manager):
        self.unid = unid
        self.coroutine_queue = coroutine_queue
        self.handle_manager = handle_manager
    async def get_msg(self):
        while True:
            coroutine_msg = await self.coroutine_queue.get()
            msg_type = coroutine_msg.get("msg")
            if msg_type == "start":
                print("recv unid [%s] is start" % self.unid)
            else:
                print("recv unid [%s] is end" % self.unid)
                # 每个当一个unid收到end消息为结束
                await self.handle_manager.del_unid(self.unid)
class HandleManager(object):
    """
    用于unid和queue的关系的处理
    """
    def __init__(self):
        self.loop = asyncio.get_event_loop()
        self.lock = Lock(loop=self.loop)
        self.handle_dict = dict()
    async def unid_bind(self, unid, coroutine_queue):
        async with self.lock:
            self.handle_dict[unid] = coroutine_queue
    async def get_queue(self, unid):
        async with self.lock:
            if unid in self.handle_dict:
                return self.handle_dict[unid]
    async def del_unid(self, unid):
        async with self.lock:
            if unid in self.handle_dict:
                self.handle_dict.pop(unid)
def make_uniqueid():
    """
    生成unid
    """
    uniqueid = str(uuid.uuid1())
    uniqueid = uniqueid.split("-")
    uniqueid.reverse()
    uniqueid = "".join(uniqueid)
    return uniqueid
async def product_msg():
    """
    生产者
    """
    while True:
        unid = make_uniqueid()
        msg_start = {"unid": unid, "msg": "start"}
        await queue.put(msg_start)
        msg_end = {"unid": unid, "msg": "end"}
        await queue.put(msg_end)
        loop = asyncio.get_event_loop()
        await asyncio.sleep(0.2, loop=loop)
async def consumer_from_queue(handle_manager):
    """
    消费者
    """
    while True:
        msg = await queue.get()
        print("consumer recv %s" % msg)
        msg_type = msg.get("msg")
        unid = msg.get("unid")
        if msg_type == "start":
            coroutine_queue = Queue()  # 用于和handle_msg协程进行数据传递
            handle_msg = HandleMsg(unid, coroutine_queue, handle_manager)
            await handle_manager.unid_bind(unid, coroutine_queue)
            await coroutine_queue.put(msg)
            loop = asyncio.get_event_loop()
            # 每次的start消息创建一个task 去处理消息
            loop.create_task(handle_msg.get_msg())
        else:
            coroutine_queue = await handle_manager.get_queue(unid)
            await coroutine_queue.put(msg)
if __name__ == "__main__":
    loop = asyncio.get_event_loop()
    handle_manager = HandleManager()
    #  在最开始创建了两个task 分别是生产者和消费者
    loop.create_task(product_msg())
    loop.create_task(consumer_from_queue(handle_manager))
    loop.run_forever()

上面的代码表面上看没啥问题,我们先看看运行效果:

consumer recv {'unid': '784f436cfaf388f611e94ca974e1ffbe', 'msg': 'start'}
consumer recv {'unid': '784f436cfaf388f611e94ca974e1ffbe', 'msg': 'end'}
Task was destroyed but it is pending!
task: <Task pending coro=<HandleMsg.get_msg() done, defined at demo.py:17> wait_for=<Future cancelled>>
Task was destroyed but it is pending!
task: <Task pending coro=<HandleMsg.get_msg() done, defined at demo.py:17> wait_for=<Future cancelled>>
Task was destroyed but it is pending!
task: <Task pending coro=<HandleMsg.get_msg() done, defined at demo.py:17> wait_for=<Future cancelled>>
Task was destroyed but it is pending!
task: <Task pending coro=<HandleMsg.get_msg() done, defined at demo.py:17> wait_for=<Future cancelled>>
Task was destroyed but it is pending!
task: <Task pending coro=<HandleMsg.get_msg() done, defined at demo.py:17> wait_for=<Future cancelled>>
Task was destroyed but it is pending!
task: <Task pending coro=<HandleMsg.get_msg() done, defined at demo.py:17> wait_for=<Future cancelled>>
..........

程序没运行一段时间都会出现上面显示的错误提示,我先看看错误提示的信息:

 

  1. Task was destroyed but it is pending!
  2. task: <Task pending coro=<HandleMsg.get_msg() done, defined at demo.py:17> wait_for=<Future cancelled>>

 

上面提示的其实就是我的task 是在pendding状态的时候被destroyed了,代码行数以及调用方法都告诉我们了是在:HandleMsg.get_msg() done, defined at demo.py:17

其实问题也比较好找,我们为每个unid创建了一个task来处理消息,但是当我们收到每个unid消息的end消息之后其实这个task任务对于我们来说就已经完成了,同时我们删除了我的unid和queue的绑定,但是我们并没有手动去取消这个task。


注意:这里我其实也有一个不理解的地方:关于这个task为什么会会destroyed,这个协程里是一个死循环一直在收消息,当queue里面没有消息协程也应该一直在await 地方在等待才对,但是如果我们把收到end消息的那个地方的删除unid和queue的绑定关系不删除,那么这个任务是不会被descroyed。所以没有完全明白这里的机制,如果明白的同学欢迎留言讨论

但是即使上面的机制我们有点不是特别明白,我们其实也应该把这个task手动进行cancel的,我们们将上面的代码稍微进行改动如下:

async def get_msg(self):
        try:
            while True:
                coroutine_msg = await self.coroutine_queue.get()
                msg_type = coroutine_msg.get("msg")
                if msg_type == "start":
                    print("recv unid [%s] is start" % self.unid)
                else:
                    print("recv unid [%s] is end" % self.unid)
                    # 每个当一个unid收到end消息为结束
                    await self.handle_manager.del_unid(self.unid)
                    current_task = asyncio.Task.current_task()
                    current_task.cancel()   # 手动cancel 当前的当前的task
        except CancelledError as e:
            print("unid [%s] cancelled success" %self.unid)

这里有个问题需要注意就是当我们对task进行cancel的时候会抛出cancelledError异常,我们需要对异常进行处理。官网也对此进行专门说明:
https://docs.python.org/3.6/library/asyncio-task.html#coroutine

内容如下:

cancel()
Request that this task cancel itself.
This arranges for a CancelledError to be thrown into the wrapped coroutine on the next cycle through the event loop. The coroutine then has a chance to clean up or even deny the request using try/except/finally.
Unlike Future.cancel(), this does not guarantee that the task will be cancelled: the exception might be caught and acted upon, delaying cancellation of the task or preventing cancellation completely. The task may also return a value or raise a different exception.
Immediately after this method is called, cancelled() will not return True (unless the task was already cancelled). A task will be marked as cancelled when the wrapped coroutine terminates with a CancelledError exception (even if cancel() was not called).

三、小结

虽然还有一些地方不太明白,但是随着用的越多,碰到的问题越多,一个一个解决,可能现在对某些知识还有点模糊,但是至少比刚开始使用asyncio的时候清晰了好多,之前整理的三篇文章的连接如下:
https://www.syncd.cn/article/asyncio_article_01
https://www.syncd.cn/article/asyncio_article_02
https://www.syncd.cn/article/asyncio_article_03

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posted @ 2019-03-22 23:02  syncd  阅读(1970)  评论(0编辑  收藏