python | run_until_complete 和 run_forever

首先 run_forever 在 3.7 之后就废弃了，但是，它还是非常非常有用的。

另外，暂时，没发现 3.7 可以代替 run_forever 的方法。

本文，除了想探讨上面两个方法怎么用之外，还想探讨一个 event loop 的生命周期。

run_forever

import asyncio
import traceback

import aiohttp


class T:
    def __init__(self):
        self._session = None

async def get(self):
        try:
            if self._session is None:
                self._session = aiohttp.ClientSession()
            async with self._session.get("http://www.baidu.com") as r:
                t = await r.text()
                print(1)
        except Exception as e:
            print(traceback.format_exc())


def main():
    test = T()
    loop = asyncio.get_event_loop()
    for i in range(10):
        loop.create_task(test.get())
    loop.run_forever()


if __name__ == '__main__':
    main()

在程序中我们创建了一个 event loop ，然后向这个 loop 中不断的添加 task，由于 loop.run_forever() 可以让 loop 一直存在。

执行了 run_forever 之后，后面的代码就不能运行了。

run_until_complete

import aiohttp
import asyncio
from concurrent.futures import ThreadPoolExecutor, as_completed
import traceback
import time


class T:
    def __init__(self):
        self._session = None

async def get(self):
        try:
            if self._session is None:
                self._session = aiohttp.ClientSession()
            for i in range(100):
                async with self._session.get("http://www.baidu.com") as r:
                    t = await r.text()
                    print(1)
        except Exception as e:
            print(traceback.format_exc())
        finally:
            await self._session.close()


def main():
    test = T()
    loop = asyncio.get_event_loop()
    loop.run_until_complete(test.get())


if __name__ == '__main__':
    main()

run_until_complete() 需要传如一个 future 对象，我们传入一个协程对象之所以可以，是因为，run_until_complete 会智能的将其转换，想当于

loop.run_until_complete(asyncio.ensure_future(test.get()))

另外，run_until_complete 是阻塞的，只有运行完里面的 future 对象后，才能继续向下运行，另外，run_until_complete 执行完毕后，event loop 依然不会关闭，还将继续运行。

import aiohttp
import asyncio
from concurrent.futures import ThreadPoolExecutor, as_completed
import traceback
import time


class T:
    def __init__(self):
        self._session = None

async def get(self):
        try:
            if self._session is None or self._session.closed:
                self._session = aiohttp.ClientSession()
            for i in range(10):
                async with self._session.get("http://www.baidu.com") as r:
                    t = await r.text()
                    print(1)
        except Exception as e:
            print(traceback.format_exc())
        finally:
            await self._session.close()


def main():
    test = T()
    loop = asyncio.get_event_loop()
    loop.run_until_complete(asyncio.ensure_future(test.get()))
    print(2)
    loop.run_until_complete(asyncio.ensure_future(test.get()))


if __name__ == '__main__':
    main()

在 3.7 之后，run_until_complete 可以用 asyncio.run 代替。

import aiohttp
import asyncio
from concurrent.futures import ThreadPoolExecutor, as_completed
import traceback
import time


class T:
    def __init__(self):
        self._session = None

async def get(self):
        try:
            if self._session is None or self._session.closed:
                self._session = aiohttp.ClientSession()
            for i in range(10):
                async with self._session.get("http://www.baidu.com") as r:
                    t = await r.text()
                    print(1)
        except Exception as e:
            print(traceback.format_exc())
        finally:
            await self._session.close()


def main():
    test = T()
    asyncio.run(test.get())


if __name__ == '__main__':
    main()

while 和 event loop

我们印象中，while 在执行的过程中是不能跳出循环的，但是，借助于 event loop 是可以跳出的。

import asyncio


async def first_worker():
    while True:
        await asyncio.sleep(1)
        print("第一个任务完成")


async def second_worker():
    while True:
        await asyncio.sleep(1)
        print("第二个任务完成")


loop = asyncio.get_event_loop()
try:
    loop.create_task(first_worker())
    loop.create_task(second_worker())
    loop.run_forever()
except KeyboardInterrupt:
    pass
finally:
    print("关闭 Loop")
    loop.close()