Python Asyncio#

await task

# throw TimeoutError if timeout
await asyncio.wait_for(task, timeout)

# still throw TimeoutError if timeout, but task.cancelled()
# inside of try/catch asyncio.TimeoutError block will be ignored, a
# nd task continues to run.
await asyncio.wait_for(asyncio.shield(task), 1)

import asyncio

async def delay(seconds):
    print(f"start sleep {seconds}")
    await asyncio.sleep(seconds)
    print(f"end sleep")
    return seconds

async def main():
    delay_task = asyncio.create_task(delay(2))
    try:
        result = await asyncio.wait_for(asyncio.shield(delay_task), 1)
        print("return value:", result)
    except asyncio.TimeoutError:
        # shield() does not protect from timeout, so it throws TimeoutError
        print("timeout")
        # shield() does protect from being cancelled
        print("whether the task is cancelled:", delay_task.cancelled())
        # from where it throws TimeoutError, continue to run, and wait for it to finish
        result = await delay_task
        print("return value:", result)

asyncio.run(main())

"""
start sleep 2
timeout
whether the task is cancelled: False
end sleep
return value: 2
"""

import asyncio
import os

import aiohttp


async def download_img(session, url):
    file_name = os.path.basename(url)
    print(f"Downloading：{file_name}")
    response = await session.get(url, ssl=False)
    content = await response.content.read()
    with open(file_name, mode="wb") as file:
        file.write(content)
    print(f"Done：{file_name}")


async def main():
    urls = [
        "https://tenfei05.cfp.cn/creative/vcg/800/new/VCG41560336195.jpg",
        "https://tenfei03.cfp.cn/creative/vcg/800/new/VCG41688057449.jpg",
    ]
    async with aiohttp.ClientSession() as session:
        # download_img(session, url) returns a coroutine
        tasks = [asyncio.create_task(download_img(session, url)) for url in urls]
        await asyncio.wait(tasks)


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

# above commented 2 lines are low level API and could be replaced by
# below asyncio.run() introduced by python 3.7.
# asyncio.get_event_loop() creates new event loop if doesn't exist.
# asyncio.run() raises exception if already in a event loop.
# This function always creates a new event loop and closes it at the end.
# It should be used as a main entry point for asyncio programs, and should
# ideally only be called once.
asyncio.run(main())

import asyncio
import time

print(f"started at {time.strftime('%X')}")
await asyncio.sleep(2)
await asyncio.sleep(2)
print(f"started at {time.strftime('%X')}")

# output, duration 4s
started at 23:48:19
started at 23:48:23

import asyncio
import time

print(f"started at {time.strftime('%X')}")

# create_task() must be inside a running event loop,
# often created by asyncio.run()
task1 = asyncio.create_task(asyncio.sleep(2))
task2 = asyncio.create_task(asyncio.sleep(2))

await task1
await task2
# or: await asyncio.wait([task1, task2])

print(f"started at {time.strftime('%X')}")

# output, duration 2s
started at 23:49:08
started at 23:49:10

import asyncio
import time

async def main()
    start = time.time()
    tasks = [
        asyncio.create_task(asyncio.sleep(2)),
        asyncio.create_task(asyncio.sleep(2)),
    ]
    await asyncio.wait(tasks)
    print(time.time() - start)

asyncio.run(main())

# output
2.0010249614715576

import asyncio
import time


coroutines = [
    # Without asyncio.create_task(), the coroutines are not scheduled,
    # there's even no event loop.
    # If use asyncio.create_task(), the coroutines are scheduled,
    # and run concurrently right now, but must be run in a running event loop.
    asyncio.sleep(2),
    asyncio.sleep(2)
]

start = time.time()

# asyncio.run() creates an event loop,
# then asyncio.wait() wraps the coroutines into tasks, and start them.
asyncio.run(asyncio.wait(coroutines))

print(time.time() - start)

# output
2.0026962757110596

import asyncio
from asyncio import Future
from functools import partial


async def f1():
    await asyncio.sleep(2)
    return "f1"


def callback1(future: Future):
    print(future.result())
    print("this is callback1")


def callback2(t1, future: Future):
    print(t1)
    print(future.result())


async def main():

    task1 = asyncio.create_task(f1())

    # bind callback1 to task1
    task1.add_done_callback(callback1)

    # bind callback2 to task2 with param
    task1.add_done_callback(partial(callback2, "this is param t1"))

    # await task1
    tasks = [task1]
    await asyncio.wait(tasks)


asyncio.run(main())

def _run_until_complete_cb(fut):
    if not fut.cancelled():
        exc = fut.exception()
        if isinstance(exc, (SystemExit, KeyboardInterrupt)):
            # Issue #22429: run_forever() already finished, no need to
            # stop it.
            return
    futures._get_loop(fut).stop()

import asyncio
import time
from concurrent.futures import ThreadPoolExecutor


# non asyncable function, will be wrapped into async task by loop.run_in_executor()
def download_img(url):
    print(f"Downloading：{url}")
    time.sleep(1)
    print(f"Downloaded：{url}")


async def main():
    executor = ThreadPoolExecutor(2)

    loop = asyncio.get_running_loop()
    tasks = []
    for i in range(10):
        # ThreadPoolExecutor is also the default executor, set None to use it.
        # t = loop.run_in_executor(None, download_img, i)
        t = loop.run_in_executor(executor, download_img, i)
        tasks.append(t)

    await asyncio.wait(tasks)


asyncio.run(main())

# asyncio.base_events.py
def run_in_executor(self, executor, func, *args):
    self._check_closed()
    if self._debug:
        self._check_callback(func, 'run_in_executor')
    if executor is None:
        executor = self._default_executor
        # Only check when the default executor is being used
        self._check_default_executor()
        if executor is None:
            executor = concurrent.futures.ThreadPoolExecutor(
                thread_name_prefix='asyncio'
            )
            self._default_executor = executor
    return futures.wrap_future(
        executor.submit(func, *args), loop=self)