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 outer
waiting for result1
in phase1
waiting for result2
in phase2
return 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_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
