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手动指出， A task is automatically scheduled for execution when it is created. The event loop stops when all tasks are done. 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yield from这个协程中产生而不是使用asyncio.async运行它时，我可以看到该行。 然而，真正奇怪的是，当我使用asyncio.async运行这个asyncio.async ，测试条目实际上存在于MongoDB中，所以尽管它看起来有效，但我不明白为什么我看不到print语句表明查询已经执行。 尽管我使用run_until_completed运行事件循环，但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。 并且run_until_complete()不会等待它。 这是使用asyncio.sleep() coroutine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))在不到2毫秒而不是2秒内返回。 并且run_until_complete(sleep(n))按原样工作：它在2秒内返回。 double_sleep()表明，定协程async.async()是在运行yield from （两个并发睡在平行），即，它睡2秒，没有4.如果你添加一个延迟（不使事件循环运行）在从那时起第一次yield from之前，你会看到yield from f不会很快返回，即asyncio.async不会运行协同程序; 它只安排它们运行。