python中的协程的理解

yield from x
对x做的第一件事情就是调用iter(x)，从中获取一个迭代器(iterator)，这意味着x可以是任何可迭代(iterable)的对象。

1. yield from可用于简化for循环中的yield表达式

>>> def gen():
...    for c in 'AB':
...        yield c
...     for i in range(1, 3):
...        yield i
...
>>> list(gen())
['A', 'B', 1, 2]

可简化为:

>>> def gen():
...     yield from 'AB'
...     yield from range(1, 3)
...
>>> list(gen())
['A', 'B', 1, 2]

另外一个例子:

>>> def chain(*iterables):
...     for it in iterables:
...         yield from it
...
>>> s = 'ABC'
>>> t = tuple(range(3))
>>> list(chain(s, t))
['A', 'B', 'C', 0, 1, 2]

Chaining Coroutines

1. Coroutines(async def 和await)

python 3.5引入了async def表示一个函数是一个coroutine对象，和await结合使用。
一个coroutine能启动另一个coroutine、并等待它的结果。一个应用场景是：将一个task拆分为几个可用的组件、并按顺序执行。
例子：

#!/usr/bin/env python3
# asyncio_coroutine_chain.py
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)


if __name__ == '__main__':
    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:
in outer
waiting for result1
in phase1
waiting for result2
in phase2
return value: ('result1', 'result2 derived from result1')

2. generator functions(@asyncio.coroutine和yield from)

python3.5以前的版本使用generator functions、并加上asyncio.coroutine() decorator、和yield from一起可以实现同样的效果。

#!/usr/bin/env python3
# asyncio_generator_chain.py

import asyncio


@asyncio.coroutine
def outer():
    print('in outer')
    print('waiting for result1')
    result1 = yield from phase1()
    print('waiting for result2')
    result2 = yield from phase2(result1)
    return result1, result2


@asyncio.coroutine
def phase1():
    print('in phase1')
    return 'result1'


@asyncio.coroutine
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：
in outer
waiting for result1
in phase1
waiting for result2
in phase2
return value: ('result1', 'result2 derived from result1')