[issue30782] Allow limiting the number of concurrent tasks in asyncio.as_completed

[Andrey Paramonov]

Andrey Paramonov <paramon at acdlabs.ru> added the comment:

Might as_completed() be considered a low-level API, but as of Python 3.7 there are seemingly no ready alternatives to achieve proposed behavior. All of asyncio.gather(), asyncio.wait(), asyncio.as_completed() expect awaitables list of limited size; doing something like https://www.artificialworlds.net/blog/2017/06/12/making-100-million-requests-with-python-aiohttp is not straightforward.

A function that takes iterator/async iterator of tasks and is itself generator/async generator is very much wanted, something in the spirit of (but more efficient?)
----
async def igather(tasks, limit=None):
    pending = set()
    while True:
        for task in islice(tasks, limit - len(pending) if limit else None):
            pending.add(task)
        if pending:
            done, pending = await asyncio.wait(pending, return_when=asyncio.FIRST_COMPLETED)
            for task in done:
                yield task
        else:
            break
----

It is an open question whether such function should yield results in the task submission order. Albeit useful, it's a bit harder to implement and (most importantly) has terrible worst-case memory behavior.

See also:
https://bugs.python.org/issue33533
https://github.com/h2non/paco/issues/38

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nosy: +aparamon

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<https://bugs.python.org/issue30782>
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