Python3.5新增async和await关键字，这里提供用法和例子

根据PEP 492，Python3.5引入关键字async、await。这里给出两个关键字的使用方法。

关键字async、await

async-await的示例。对比在3.5前没有这两个关键字前的编程方式。

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import asyncio

async def printer():
    print("A")
    await asyncio.sleep(1)
    print("C")

async def printer2():
    print("B")
    await asyncio.sleep(1)
    print("D")

async def main():
    await asyncio.wait([printer2(), printer()])

loop = asyncio.get_event_loop()
loop.run_until_complete(main())

  • 在没有async、await前的协程编程
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import asyncio

@asyncio.coroutine
def task(n):
    yield from asyncio.sleep(1)
    print("task {}".format(n))

@asyncio.coroutine
def main():
    yield from asyncio.wait([
        task(1),
        task(2),
        task(3)
        ])

loop = asyncio.get_event_loop()
loop.run_until_complete(main())
  • 使用async、await时
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import asyncio

async def task(n):
    await asyncio.sleep(1)
    print("task {}".format(n))

async def main():
    await asyncio.wait([
        task(1),
        task(2),
        task(3)
        ])

loop = asyncio.get_event_loop()
loop.run_until_complete(main())

对比可以发现async化简了asyncio.coroutine，await化简了yield from

async for & async with

  • async for 是异步迭代器的语法
  • async with 是with语句（上下文管理器语法）的异步化

async for instance:

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import random
import asyncio

class AsyncIterable:

    def __init__(self):
        self.count = 0
        self._container = list(range(1, 101))

    async def __aiter__(self):
        return self

    async def __anext__(self):
        if self.count >= 5:
            raise StopAsyncIteration
        data = await self.fetch_data()
        self.count += 1
        return data

    async def fetch_data(self):
        return random.choice(self._container)

async def main():
    async for data in AsyncIterAble():
        print(data)

loop = asyncio.get_event_loop()
loop.run_util_complete(main())

async with instance:

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import asyncio
import random

async def task(n):
    print("task {}".format(n))

class AsyncContextManager:
    async def __aenter__(self):
        await log("entering context")

    async def __aexit__(self, exc_type, exc, tb):
        await log("exiting context")

async def coroutine():
    async with AsyncContextManager():
        print("inner body")

c = coroutine()

try:
    c.send(None)
except StopIteration:
    print("end")

Future

Future的例子：

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import asyncio

async def future_demo(future):
    await asyncio.sleep(1)
    future.set_result('future is done!')

loop = asyncio.get_event_loop()
future = asyncio.Future()
asyncio.ensure_future(future_demo(future))
loop.run_until_complete(future)

print(future.result())

例子二：

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import asyncio

async def future_callback(future):
    await asyncio.sleep(1)
    future.set_result("future is done!")

def got_result(future):
    print(future.result())
    loop.stop()

loop = asyncio.get_event_loop()
future = asyncio.Future()
asyncio.ensure_future(future_callback(future))
future.add_done_callback(got_result)

try:
    loop.run_forever()
finally:
    loop.close()

转载请包括本文地址：https://allenwind.github.io/blog/6363
更多文章请参考：https://allenwind.github.io/blog/archives/