在python里使用await关键字来等另外一个协程的实例

一个协程里可以启动另外一个协程，并等待它完成返回结果，采用await关键字，

例子如下：

import asyncio async def outer(): print(’in outer’) print(’waiting for result1’) result1 = await phase1() print(’waiting for result2’) result2 = await phase2(result1) return (result1, result2) async def phase1(): print(’in phase1’) return ’result1’ async def phase2(arg): print(’in phase2’) return ’result2 derived from {}’.format(arg) event_loop = asyncio.get_event_loop()try: return_value = event_loop.run_until_complete(outer()) print(’return value: {!r}’.format(return_value))finally: event_loop.close()输出结果如下：in outerwaiting for result1in phase1waiting for result2in phase2return value: (’result1’, ’result2 derived from result1’)

await关键字添加了一个新的协程到循环里，而不需要明确地添加协程到这个事件循环里。

补充知识：python里使用Condition对象来唤醒指定数量的协程

在asyncio库里，定义Condition对象，它的行为与事件Event有点像，区别是事件是通知所有对象，Condition对象可以指定一定数量的协程被通知，它是通过函数notify()来实现的，如果参数里放2，就是通知两个协程，例子如下：

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’) # pause to let consumers start 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): # Create a condition condition = asyncio.Condition() # Set up tasks watching the condition consumers = [ consumer(condition, i) for i in range(5) ] # Schedule a task to manipulate the condition variable loop.create_task(manipulate_condition(condition)) # Wait for the consumers to be done await asyncio.wait(consumers) event_loop = asyncio.get_event_loop()try: result = event_loop.run_until_complete(main(event_loop))finally: event_loop.close()

结果输出如下：

starting manipulate_conditionconsumer 2 is waitingconsumer 3 is waitingconsumer 4 is waitingconsumer 1 is waitingconsumer 0 is waitingnotifying 1 consumersconsumer 2 triggeredending consumer 2notifying 2 consumersconsumer 3 triggeredending consumer 3consumer 4 triggeredending consumer 4notifying remaining consumersending manipulate_conditionconsumer 1 triggeredending consumer 1consumer 0 triggeredending consumer 0

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