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Asyncio tips

2018-05-01 python asyncio

1. How to start async function without await, functions:

asyncio.ensure_future(func())  # returns Task
loop.create_task(func())  # returns Task

Example:

import asyncio

async def nowait():
    print('nowait start')
    await asyncio.sleep(1)
    print('nowait finished')

async def main():
    print('main: start')
    asyncio.ensure_future(nowait())
    await asyncio.sleep(0.6)
    print('main: sleep')
    await asyncio.sleep(0.6)
    print('main: finished')

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

2. How to start a usual function asynchronously, you can use:

loop.call_soon(func)          # start on next tick
loop.call_later(delay, func)  # wait some time before start
loop.call_at(when, func)      # start at a time

3. To await a usual function you need to return Future object:

import asyncio

def basic():
    fut = asyncio.Future()

    def called_after_second(_):
        fut.set_result(555)

    asyncio.ensure_future(asyncio.sleep(1)).add_done_callback(called_after_second)

    return fut

async def main():
    print('main: start')

    result = await basic()
    print(result)

    print('main: finished')

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

4. How to wait async func from a usual function

import asyncio

async def asyncfunc():
    await asyncio.sleep(1)

def basic():
    def on_complete(task):
        print('sleep finished')

    task = asyncio.ensure_future(asyncfunc())
    task.add_done_callback(on_complete)

async def main():
    print('main: start')

    basic()

    await asyncio.sleep(1.1)
    print('main: finished')

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

5. Wait all tasks finish or wait any task finish

asyncio.wait(tasks, timeout, return_when=ALL_COMPLETED)
asyncio.wait(tasks, timeout, return_when=FIRST_COMPLETED)

6. Other popular function

loop.run_until_complete(<coro/future/task>)  # blocks loop until future is done
loop.run_forever()  # block until loop is stopped

asyncio.wait_for(<coro>, timeout)  # set timeout for a coro
asyncio.gather(<list of coro>)  # returns coro, result = values in right order

Note: You don't have to create Task object, it's "system" object which is used as result for async functions, use Future instead

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