How to limit concurrency with Python asyncio?
I used Mikhails answer and ended up with this little gem
async def gather_with_concurrency(n, *tasks):
semaphore = asyncio.Semaphore(n)
async def sem_task(task):
async with semaphore:
return await task
return await asyncio.gather(*(sem_task(task) for task in tasks))
Which you would run instead of normal gather
await gather_with_concurrency(100, *tasks)
Before reading the rest of this answer, please note that the idiomatic way of limiting the number of parallel tasks this with asyncio is using asyncio.Semaphore
, as shown in Mikhail's answer and elegantly abstracted in Andrei's answer. This answer contains working, but a bit more complicated ways of achieving the same. I am leaving the answer because in some cases this approach can have advantages over a semaphore, specifically when the work to be done is very large or unbounded, and you cannot create all the coroutines in advance. In that case the second (queue-based) solution is this answer is what you want. But in most regular situations, such as parallel download through aiohttp, you should use a semaphore instead.
You basically need a fixed-size pool of download tasks. asyncio
doesn't come with a pre-made task pool, but it is easy to create one: simply keep a set of tasks and don't allow it to grow past the limit. Although the question states your reluctance to go down that route, the code ends up much more elegant:
import asyncio, random
async def download(code):
wait_time = random.randint(1, 3)
print('downloading {} will take {} second(s)'.format(code, wait_time))
await asyncio.sleep(wait_time) # I/O, context will switch to main function
print('downloaded {}'.format(code))
async def main(loop):
no_concurrent = 3
dltasks = set()
i = 0
while i < 9:
if len(dltasks) >= no_concurrent:
# Wait for some download to finish before adding a new one
_done, dltasks = await asyncio.wait(
dltasks, return_when=asyncio.FIRST_COMPLETED)
dltasks.add(loop.create_task(download(i)))
i += 1
# Wait for the remaining downloads to finish
await asyncio.wait(dltasks)
An alternative is to create a fixed number of coroutines doing the downloading, much like a fixed-size thread pool, and feed them work using an asyncio.Queue
. This removes the need to manually limit the number of downloads, which will be automatically limited by the number of coroutines invoking download()
:
# download() defined as above
async def download_worker(q):
while True:
code = await q.get()
await download(code)
q.task_done()
async def main(loop):
q = asyncio.Queue()
workers = [loop.create_task(download_worker(q)) for _ in range(3)]
i = 0
while i < 9:
await q.put(i)
i += 1
await q.join() # wait for all tasks to be processed
for worker in workers:
worker.cancel()
await asyncio.gather(*workers, return_exceptions=True)
As for your other question, the obvious choice would be aiohttp
.
If I'm not mistaken you're searching for asyncio.Semaphore. Example of usage:
import asyncio
from random import randint
async def download(code):
wait_time = randint(1, 3)
print('downloading {} will take {} second(s)'.format(code, wait_time))
await asyncio.sleep(wait_time) # I/O, context will switch to main function
print('downloaded {}'.format(code))
sem = asyncio.Semaphore(3)
async def safe_download(i):
async with sem: # semaphore limits num of simultaneous downloads
return await download(i)
async def main():
tasks = [
asyncio.ensure_future(safe_download(i)) # creating task starts coroutine
for i
in range(9)
]
await asyncio.gather(*tasks) # await moment all downloads done
if __name__ == '__main__':
loop = asyncio.get_event_loop()
try:
loop.run_until_complete(main())
finally:
loop.run_until_complete(loop.shutdown_asyncgens())
loop.close()
Output:
downloading 0 will take 3 second(s)
downloading 1 will take 3 second(s)
downloading 2 will take 1 second(s)
downloaded 2
downloading 3 will take 3 second(s)
downloaded 1
downloaded 0
downloading 4 will take 2 second(s)
downloading 5 will take 1 second(s)
downloaded 5
downloaded 3
downloading 6 will take 3 second(s)
downloading 7 will take 1 second(s)
downloaded 4
downloading 8 will take 2 second(s)
downloaded 7
downloaded 8
downloaded 6
An example of async downloading with aiohttp
can be found here. Note that aiohttp
has a Semaphore equivalent built in, which you can see an example of here. It has a default limit of 100 connections.
The asyncio-pool library does exactly what you need.
https://pypi.org/project/asyncio-pool/
LIST_OF_URLS = ("http://www.google.com", "......")
pool = AioPool(size=3)
await pool.map(your_download_coroutine, LIST_OF_URLS)