import asyncio
import random
 
 
@asyncio.coroutine
def main():
    pool = asyncio.Semaphore(3)
 
    @asyncio.coroutine
    def pooled(coro):
        with (yield from pool):
            print("+ task")
            try:
                return (yield from coro)
            finally:
                print("- task")
 
    tasks = [
        pooled(asyncio.sleep(random.randint(1, 2)))
        for i in range(10)
    ]
 
    yield from asyncio.wait(tasks)
 
 
loop = asyncio.get_event_loop()
loop.run_until_complete(main())

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