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Reputation: 3317

Python asyncio non blocking for loop

(I used Python 3.7)

import asyncio


result = 0


async def sub1():
    global result
    print('[*] sub1() start')
    for i in range(1, 10000000):
        result += i
    print('[*] sub1() end')

async def sub2():
    global result
    print('[*] sub2() start')
    for i in range(10000000, 20000000):
        result += i
    print('[*] sub2() end')

async def main():
    await asyncio.gather(
        asyncio.create_task(sub1()),
        asyncio.create_task(sub2()),
    )


asyncio.run(main())

When I execute above code, it's result is

[*] sub1() start
[*] sub1() end
[*] sub2() start
[*] sub2() end
[Finished in 1.7s]

You know that, sub2() is started after sub1() finished.

It seems doesn't work in parallel.

I want to give control over to sub2() until sub1() finished.

Maybe I have to insert await to somewhere, but I don't know where is the point.

Question

How can I execute both functions like parallel using for loop?

Thanks.

Upvotes: 2

Views: 6590

Answers (1)

felipe
felipe

Reputation: 8055

As @KlausD pointed out in his comment simply adding the async tag does not effectively make your code truly asynchronous. asyncio requires some sort of break-point within the for-loops to effectively know when to switch between functions. Here is a possible solution utilizing synchronous functions and running them inside the loop's executor (documentation here):

import asyncio

result = 0

def sub1():
    global result
    print('[*] sub1() start')
    for i in range(1, 10000000):
        result += i
    print('[*] sub1() end')

def sub2():
    global result
    print('[*] sub2() start')
    for i in range(10000000, 20000000):
        result += i
    print('[*] sub2() end')

async def main(loop):
    res = await asyncio.gather(
        loop.run_in_executor(None, sub1),
        loop.run_in_executor(None, sub2)
    )


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

It is worth noting that to write asynchronous code one must remember to engage in practices that ensure the code actually runs asynchronously. A native for-loop in Python is in fact a synchronous piece of code, and therefore when your functions are called, the for-loop is run until completion before making space in the thread for the next operation.

Another thing is the fact that since your code is simple additions, you aren't actually saving any time by running it asynchronously. Hell, you might be losing time due to asyncio operations having to switch between functions. Async functionalities are good for large processing that requires some sort of waiting between operations (i.e. doing various server calls, where one has to wait for servers to respond; reading in multiple files, where one has to be dependent on the hard drive reading speed, etc.) In these cases, async becomes useful since one can switch operations within the thread and ensure that all the calls (pull from server; get file from harddrive) are run before bothering moving on to processing the requests or the files.

Upvotes: 4

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