[FIXED] Set max concurrency with asyncio.create_subprocess_exec

Issue

I need to run a program about 500 times with different inputs. I'd like to use asyncio.create_subprocess_exec and want to limit the number of processes running at the same time so as not to clog up the machine. Is there a way to set the concurrency level? For example, I'd expect something like AbstractEventLoop.set_max_tasks.

Solution

As suggested by @AndrewSvetlov, you can use an asyncio.Semaphore to enforce the limit:

async def run_program(input):
    p = await asyncio.create_subprocess_exec(...)
    # ... communicate with the process ...
    p.terminate()
    return something_useful

async def run_throttled(input, sem):
    async with sem:
        result = await run_program(input)
    return result

LIMIT = 10

async def many_programs(inputs):
    sem = asyncio.Semaphore(LIMIT)
    results = await asyncio.gather(
        *[run_throttled(input, sem) for input in inputs])
    # ...

Answered By - user4815162342