asyncio 学习笔记:同步原语

本文是 https://pymotw.com/3/asyncio/synchronization.html 的学习笔记,强烈推荐直接阅读原文。

虽然 asyncio 应用一般都是单线程程序,但是它们仍然是并发程序。 为了支持安全的并发,asyncio 实现了一些 threadingmultiprocessing 模块中比较常用的类似的同步原语。

Lock

Lock 可以用来安全的访问共享资源。

# asyncio_lock.py
import asyncio
import functools


def unlock(lock):
    print('callback releasing lock')
    lock.release()


async def coro1(lock):
    print('coro1 wating for the lock')
    with await lock:
        print('coro1 acquired lock')
    print('coro1 released lock')


async def coro2(lock):
    print('coro2 wating for the lock')
    await lock
    try:
        print('coro2 acquired lock')
    finally:
        print('coro2 released lock')
        lock.release()


async def main(loop):
    lock = asyncio.Lock()
    print('acquiring the lock before starting coroutines')
    await lock.acquire()
    print('lock acquired: {}'.format(lock.locked()))

    loop.call_later(0.1, functools.partial(unlock, lock))

    print('waiting for coroutines')
    await asyncio.wait([coro1(lock), coro2(lock)])


event_loop = asyncio.get_event_loop()
try:
    event_loop.run_until_complete(main(event_loop))
finally:
    event_loop.close()

可以使用 await 来获取锁,使用 release() 方法来释放锁。也可以用 with await 上下文语句来获取和释放锁。

$ python3.6 asyncio_lock.py
acquiring the lock before starting coroutines
lock acquired: True
waiting for coroutines
coro1 wating for the lock
coro2 wating for the lock
callback releasing lock
coro1 acquired lock
coro1 released lock
coro2 acquired lock
coro2 released lock

Event

asyncio.Event 类似 threading.Event 用来允许多个消费者等待某个事情发生,不用通过监听一个特殊的值的来说实现类似通知的功能。

# asyncio_event.py
import asyncio
import functools


def set_event(event):
    print('setting event in callback')
    event.set()


async def coro1(event):
    print('coro1 waiting for event')
    await event.wait()
    print('coro1 triggered')


async def coro2(event):
    print('coro2 waiting for event')
    await event.wait()
    print('coro2 triggered')


async def main(loop):
    event = asyncio.Event()
    print('event start state: {}'.format(event.is_set()))
    loop.call_later(
        0.1, functools.partial(set_event, event)
    )
    await asyncio.wait([coro1(event), coro2(event)])
    print('event end state: {}'.format(event.is_set()))


event_loop = asyncio.get_event_loop()
try:
    event_loop.run_until_complete(main(event_loop))
finally:
    event_loop.close()
Lock 一样,coro1()coro2() 都在等待 event 被设置。
不同的是它们都在 event 状态一发生变化的时候就启动了,它们不需要对 event 对象获取一个唯一的所有权。
$ python3.6 asyncio_event.py
event start state: False
coro1 waiting for event
coro2 waiting for event
setting event in callback
coro1 triggered
coro2 triggered
event end state: True

Condition

Condition 的效果类似 Event,不同的是它不是唤醒所有等待中的 coroutine, 而是通过 notify() 唤醒指定数量的待唤醒 coroutine。

# asyncio_condition.py
import asyncio


async def consumer(condition, n):
    with await condition:
        print('consumer {} is waiting'.format(n))
        await condition.wait()
        print('consumer {} triggered'.format(n))
    print('ending consumer {}'.format(n))


async def manipulate_condition(condition):
    print('starting manipulate_condition')

    await asyncio.sleep(0.1)

    for i in range(1, 3):
        with await condition:
            print('notifying {} consumers'.format(i))
            condition.notify(n=i)
        await asyncio.sleep(0.1)

    with await condition:
        print('notifying remaining consumers')
        condition.notify_all()

    print('ending manipulate_condition')


async def main(loop):
    condition = asyncio.Condition()

    consumers = [
        consumer(condition, i)
        for i in range(5)
    ]

    loop.create_task(manipulate_condition(condition))

    await asyncio.wait(consumers)


event_loop = asyncio.get_event_loop()
try:
    result = event_loop.run_until_complete(main(event_loop))
finally:
    event_loop.close()

在这个例子中,我们启动了五个 Condition 的消费者,每个都使用 wait() 方法来等待它们可以继续处理的通知。manipulate_condition() 首先通知了一个消费者,然后有通知了两个消费者,最后通知剩下的所有消费者。

$ python3.6 asyncio_condition.py
starting manipulate_condition
consumer 0 is waiting
consumer 4 is waiting
consumer 1 is waiting
consumer 2 is waiting
consumer 3 is waiting
notifying 1 consumers
consumer 0 triggered
ending consumer 0
notifying 2 consumers
consumer 4 triggered
ending consumer 4
consumer 1 triggered
ending consumer 1
notifying remaining consumers
ending manipulate_condition
consumer 2 triggered
ending consumer 2
consumer 3 triggered
ending consumer 3

Queue

asyncio.Queue 为 coroutines 实现了一个先进先出的数据结构,类似多线程中的 queue.Queue ,多进程中的 multiprocessing.Queue

# asyncio_queue.py
import asyncio


async def consumer(n, q):
    print('consumer {}: waiting for item'.format(n))
    while True:
        print('consumer {}: waiting for item'.format(n))
        item = await q.get()
        print('consumer {}: has item {}'.format(n, item))
        # 在这个程序中 None 是个特殊的值,表示终止信号
        if item is None:
            q.task_done()
            break
        else:
            await asyncio.sleep(0.01 * item)
            q.task_done()

    print('consumer {}: ending'.format(n))


async def producer(q, num_workers):
    print('producer: starting')
    # 向队列中添加一些数据
    for i in range(num_workers * 3):
        await q.put(i)
        print('producer: added task {} to the queue'.format(i))

    # 通过 None 这个特殊值来通知消费者退出
    print('producer: adding stop signals to the queue')
    for i in range(num_workers):
        await q.put(None)
    print('producer: waiting for queue to empty')
    await q.join()
    print('producer: ending')


async def main(loop, num_consumers):
    # 创建指定大小的队列,这样的话生产者将会阻塞
    # 直到有消费者获取数据
    q = asyncio.Queue(maxsize=num_consumers)

    # 调度消费者
    consumers = [
        loop.create_task(consumer(i, q))
        for i in range(num_consumers)
    ]

    # 调度生产者
    prod = loop.create_task(producer(q, num_consumers))

    # 等待所有 coroutines 都完成
    await asyncio.wait(consumers + [prod])


event_loop = asyncio.get_event_loop()
try:
    event_loop.run_until_complete(main(event_loop, 2))
finally:
    event_loop.close()

通过 put() 添加项或者通过 get() 移除项都是异步操作,同时有可能队列大小达到指定大小(阻塞添加操作)或者队列变空(阻塞所有获取项的调用)。

$ python3.6 asyncio_queue.py
consumer 0: waiting for item
consumer 0: waiting for item
consumer 1: waiting for item
consumer 1: waiting for item
producer: starting
producer: added task 0 to the queue
producer: added task 1 to the queue
consumer 0: has item 0
consumer 1: has item 1
producer: added task 2 to the queue
producer: added task 3 to the queue
consumer 0: waiting for item
consumer 0: has item 2
producer: added task 4 to the queue
consumer 1: waiting for item
consumer 1: has item 3
producer: added task 5 to the queue
producer: adding stop signals to the queue
consumer 0: waiting for item
consumer 0: has item 4
consumer 1: waiting for item
consumer 1: has item 5
producer: waiting for queue to empty
consumer 0: waiting for item
consumer 0: has item None
consumer 0: ending
consumer 1: waiting for item
consumer 1: has item None
consumer 1: ending
producer: ending

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