Python3标准库：asyncio异步I/O、事件循环和并发工具(6)

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Python3标准库：asyncio异步I/O、事件循环和并发工具(6)

 asyncio
			 
			async def phase(i):

			print('in phase {}'.format(i))

			await asyncio.sleep(0.5 - (0.1 * i))

			print('done with phase {}'.format(i))

			return 'phase {} result'.format(i)

			async def main(num_phases):

			print('starting main')

			phases = [

			phase(i)

			for i in range(num_phases)

			]

			print('waiting for phases to complete')

			results = []

			for next_to_complete in asyncio.as_completed(phases):

			answer = await next_to_complete

			print('received answer {!r}'.format(answer))

			results.append(answer)

			print('results: {!r}'.format(results))

			return results

			event_loop = asyncio.get_event_loop()

			try:

			event_loop.run_until_complete(main(3))

			finally:

			event_loop.close()

这个例子启动了多个后台阶段，它们会按其启动顺序的逆序完成。消费生成器时，循环会使用await等待协程的结果。

1.7 同步原语

尽管asyncio应用通常作为单线程的进程运行，不过仍被构建为并发应用。由于I/0以及其他外部事件的延迟和中断，每个协程或任务可能按一种不可预知的顺序执行。为了支持安全的并发执行，asyncio包含了threading和multiprocessing模块中一些底层原语的实现。

1.7.1 锁

Lock可以用来保护对一个共享资源的访问。只有锁的持有者可以使用这个资源。如果有多个请求要得到这个锁，那么其将会阻塞，以保证一次只有一个持有者。

import asyncio
import functools
def unlock(lock):
print('callback releasing lock')
lock.release()
async def coro1(lock):
print('coro1 waiting for the lock')
async with lock:
print('coro1 acquired lock')
print('coro1 released lock')
async def coro2(lock):
print('coro2 waiting for the lock')
await lock.acquire()
try:
print('coro2 acquired lock')
finally:
print('coro2 released lock')
lock.release()
async def main(loop):
# Create and acquire a shared lock.
lock = asyncio.Lock()
print('acquiring the lock before starting coroutines')
await lock.acquire()
print('lock acquired: {}'.format(lock.locked()))
# Schedule a callback to unlock the lock.
loop.call_later(0.1, functools.partial(unlock, lock))
# Run the coroutines that want to use the 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()

锁可以直接调用，使用await来得到，并且使用结束时可以调用release()方法释放锁，如这个例子中的coro2()所示。还可以结合with await关键字使用锁作为异步上下文管理器，如coro1()中所示。

1.7.2 事件

asyncio.Event基于threading.Event。它允许多个消费者等待某个事件发生，而不必寻找一个特定值与通知关联。

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')

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