在python裡使用await關鍵字來等另外一個協程的例項
阿新 • • 發佈:2020-05-07
一個協程裡可以啟動另外一個協程,並等待它完成返回結果,採用await關鍵字,
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() 輸出結果如下: in outer waiting for result1 in phase1 waiting for result2 in phase2 return value: ('result1','result2 derived from result1')
await關鍵字添加了一個新的協程到迴圈裡,而不需要明確地新增協程到這個事件迴圈裡。
補充知識:python裡使用Condition物件來喚醒指定數量的協程
在asyncio庫裡,定義Condition物件,它的行為與事件Event有點像,區別是事件是通知所有物件,Condition物件可以指定一定數量的協程被通知,它是通過函式notify()來實現的,如果引數裡放2,就是通知兩個協程,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()
結果輸出如下:
starting manipulate_condition consumer 2 is waiting consumer 3 is waiting consumer 4 is waiting consumer 1 is waiting consumer 0 is waiting notifying 1 consumers consumer 2 triggered ending consumer 2 notifying 2 consumers consumer 3 triggered ending consumer 3 consumer 4 triggered ending consumer 4 notifying remaining consumers ending manipulate_condition consumer 1 triggered ending consumer 1 consumer 0 triggered ending consumer 0
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