Python 3.4 asyncio任务没有完全执行

Question

我正在试验Python 3.4的asyncio模块.由于没有使用asyncio的MongoDB生产就绪包,我编写了一个小包装类,它在执行程序中执行所有mongo查询.这是包装器:

import asyncio
from functools import wraps
from pymongo import MongoClient


class AsyncCollection(object):
    def __init__(self, client):
        self._client = client
        self._loop = asyncio.get_event_loop()

    def _async_deco(self, name):
        method = getattr(self._client, name)

        @wraps(method)
        @asyncio.coroutine
        def wrapper(*args, **kwargs):
            print('starting', name, self._client)
            r = yield from self._loop.run_in_executor(None, method, *args, **kwargs)
            print('done', name, self._client, r)
            return r

        return wrapper

    def __getattr__(self, name):
        return self._async_deco(name)


class AsyncDatabase(object):
    def __init__(self, client):
        self._client = client
        self._collections = {}


    def __getitem__(self, col):
        return self._collections.setdefault(col, AsyncCollection(self._client[col]))


class AsyncMongoClient(object):
    def __init__(self, host, port):
        self._client = MongoClient(host, port)
        self._loop = asyncio.get_event_loop()
        self._databases = {}

    def __getitem__(self, db):
        return self._databases.setdefault(db, AsyncDatabase(self._client[db]))

我想异步执行插入,这意味着执行它们的协同程序不希望等待执行完成.asyncio手册指出A task is automatically scheduled for execution when it is created. The event loop stops when all tasks are done.,所以我构建了这个测试脚本:

from asyncdb import AsyncMongoClient
import asyncio

@asyncio.coroutine
def main():
    print("Started")
    mongo = AsyncMongoClient("host", 27017)
    asyncio.async(mongo['test']['test'].insert({'_id' : 'test'}))
    print("Done")

loop = asyncio.get_event_loop()
loop.run_until_complete(main())

当我运行脚本时,我得到以下结果:

Started
Done
starting insert Collection(Database(MongoClient('host', 27017), 'test'), 'test')

应该有一行表示mongo查询已完成.当我使用yield from此协程而不是使用它时,我可以看到该行asyncio.async.然而,真正奇怪的是当我运行这个corouting时,测试条目实际上存在于MongoDB中asyncio.async,所以尽管它似乎工作,但我不明白为什么我不能看到指示查询具有的print语句已经预成型.尽管我使用了事件循环run_until_completed,它应该等待插入任务完成,即使主协程已经完成.

Answer 1

asyncio.async(mongo...))只是安排 mongo查询.而run_until_complete()不是等待它.这是使用asyncio.sleep()协同程序显示它的代码示例:

#!/usr/bin/env python3
import asyncio
from contextlib import closing
from timeit import default_timer as timer

@asyncio.coroutine
def sleep_BROKEN(n):
    # schedule coroutine; it runs on the next yield
    asyncio.async(asyncio.sleep(n))

@asyncio.coroutine
def sleep(n):
    yield from asyncio.sleep(n)

@asyncio.coroutine
def double_sleep(n):
    f = asyncio.async(asyncio.sleep(n))
    yield from asyncio.sleep(n) # the first sleep is also started
    yield from f

n = 2
with closing(asyncio.get_event_loop()) as loop:
    start = timer()
    loop.run_until_complete(sleep_BROKEN(n))
    print(timer() - start)
    loop.run_until_complete(sleep(n))
    print(timer() - start)
    loop.run_until_complete(double_sleep(n))
    print(timer() - start)

产量

0.0001221800921484828
2.002586881048046
4.005100341048092

输出显示run_until_complete(sleep_BROKEN(n))返回时间小于2毫秒而不是2秒.run_until_complete(sleep(n))并按预期工作:它在2秒后返回.double_sleep()显示调度的协同程序async.async()运行yield from(两个并发睡眠是并行的),即它睡眠2秒,而不是4.如果你在第一个之前添加一个延迟(不允许事件循环运行),yield from那么你看到yield from f没有返回更快,即asyncio.async不运行协同程序; 它只安排它们运行.