asyncio 学习笔记：产生异步结果

本文是 https://pymotw.com/3/asyncio/futures.html 的学习笔记，强烈推荐直接阅读原文。

Future 对象表示一个还未完成的工作，事件循环可以监视 Future 对象的状态直至它变成 done，这将运行程序的一部分等待另一部分完成一些工作。

等待一个 Future 对象

示例:

# asyncio_future_event_loop.py
import asyncio

def mark_done(future, result):
    print('setting future result to {!r}'.format(result))
    future.set_result(result)


event_loop = asyncio.get_event_loop()
all_done = asyncio.Future()
try:
    print('scheduling make_done')
    event_loop.call_soon(mark_done, all_done, 'the result')
    print('entering event loop')
    result = event_loop.run_until_complete(all_done)
    print('returned result: {!r}'.format(result))
finally:
    print('closing event loop')
    event_loop.close()

print('future result: {!r}'.format(all_done.result()))

当调用 set_result 方法后，Future 对象的状态会被修改为 done, 同时 `Future`` 实例也会保存设置的结果值，供随后使用:

$ python3.6 asyncio_future_event_loop.py
scheduling make_done
entering event loop
setting future result to 'the result'
returned result: 'the result'
closing event loop
future result: 'the result'

Future 对象也可以同 await 关键字一起使用:

# asyncio_future_await.py
import asyncio


def mark_done(future, result):
    print('setting future result to {!r}'.format(result))
    future.set_result(result)


async def main(loop):
    all_done = asyncio.Future()
    print('scheduling mark_done')
    loop.call_soon(mark_done, all_done, 'the result')
    result = await all_done
    print('returned result: {!r}'.format(result))


event_loop = asyncio.get_event_loop()
try:
    event_loop.run_until_complete(main(event_loop))
finally:
    event_loop.close()

await 会返回 Future 的结果：

$ python3.6 asyncio_future_await.py
scheduling mark_done
setting future result to 'the result'
returned result: 'the result'

Future 回调函数

Future 在完成的时候可以执行一些回调函数，回调函数按注册时的顺序进行调用:

# asyncio_future_callback.py
import asyncio
import functools


def callback(future, n):
    print('{}: future done: {}'.format(n, future.result()))


async def register_callbacks(all_done):
    print('registering callbacks on future')
    all_done.add_done_callback(functools.partial(callback, n=1))
    all_done.add_done_callback(functools.partial(callback, n=2))


async def main(all_done):
    await register_callbacks(all_done)
    print('setting result of future')
    all_done.set_result('the result')


event_loop = asyncio.get_event_loop()
try:
    all_done = asyncio.Future()
    event_loop.run_until_complete(main(all_done))
finally:
    event_loop.close()

回调函数的第一个参数是 Future 实例，要传递其他参数可以使用 functools.partial() 来实现。

$ python3.6 asyncio_future_callback.py
registering callbacks on future
setting result of future
1: future done: the result
2: future done: the result

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