在python裡使用await關鍵字來等另外一個協程的實例

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循環裡，而不需要明確地添加協程到這個事件循環裡。
補充知識：python裡使用Condition對象來喚醒指定數量的協程
在asyncio庫裡，定義Condition對象，它的行為與事件Event有點像，區別是事件是通知所有對象，Condition對象可以指定一定數量的協程被通知，它是通過函數notify()來實現的，如果參數裡放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