async def worker(n):  # dummy worker
    await aio.sleep(1 / n)
    return n


async def spawn_n_usage(todo=[range(1,51), range(51,101), range(101,200)]):
    futures = []
    async with AioPool(size=20) as pool:
        for tasks in todo:
            for i in tasks:  # too many tasks
                # Returns quickly for all tasks, does not wait for pool space.
                # Workers are not spawned, they wait for pool space in their
                # own background tasks.
                fut = pool.spawn_n(worker(i))
                futures.append(fut)
        # At this point not a single worker should start.

        # Context manager calls `join` at exit, so this will finish when all
        # workers return, crash or cancelled.

    assert sum(itertools.chain.from_iterable(todo)) == \
        sum(f.result() for f in futures)


async def spawn_usage(todo=range(1,4)):
    futures = []
    async with AioPool(size=2) as pool:
        for i in todo:  # 1, 2, 3
            # Returns quickly for 1 and 2, then waits for empty space for 3,
            # spawns 3 and returns. Can save some resources I guess.
            fut = await pool.spawn(worker(i))
            futures.append(fut)
        # At this point some of the workers already started.

        # Context manager calls `join` at exit, so this will finish when all
        # workers return, crash or cancelled.

    assert sum(todo) == sum(fut.result() for fut in futures)  # all done


async def map_usage(todo=range(100)):
    pool = AioPool(size=10)
    # Waits and collects results from all spawned workers,
    # returns them in same order as `todo`, if worker crashes or cancelled:
    # returns exception object as a result.
    # Basically, it wraps `spawn_usage` code into one call.
    results = await pool.map(worker, todo)

    # await pool.join()  # is not needed here, bcs no other tasks were spawned

    assert isinstance(results[0], ZeroDivisionError) \
        and sum(results[1:]) == sum(todo)


async def itermap_usage(todo=range(1,11)):
    result = 0
    async with AioPool(size=10) as pool:
        # Combines spawn_n and iterwait, which is a wrapper for asyncio.wait,
        # which yields results of finished workers according to `timeout` and
        # `yield_when` params passed to asyncio.wait (see it's docs for details)
        async for res in pool.itermap(worker, todo, timeout=0.5):
            result += res
        # technically, you can skip join call

    assert result == sum(todo)


async def callbacks_usage():

    async def wrk(n):  # custom dummy worker
        await aio.sleep(1 / n)
        return n

    async def cb(res, err, ctx):  # callback
        if err:  # error handling
            exc, tb = err
            assert tb  # the only purpose of this is logging
            return exc

        pool, n = ctx  # context can be anything you like
        await aio.sleep(1 / (n-1))
        return res + n

    todo = range(5)
    futures = []

    async with AioPool(size=2) as pool:
        for i in todo:
            fut = pool.spawn_n(wrk(i), cb, (pool, i))
            futures.append(fut)

    results = []
    for fut in futures:
        # there are helpers for result extraction. `flat` one will do
        # exactly what's written below
        #   from asyncio_pool import getres
        #   results.append(getres.flat(fut))
        try:
            results.append(fut.result())
        except Exception as e:
            results.append(e)

    # First error happens for n == 0 in wrk, exception of it is passed to
    # callback, callback returns it to us. Second one happens in callback itself
    # and is passed to us by pool.
    assert all(isinstance(e, ZeroDivisionError) for e in results[:2])

    # All n's in `todo` are passed through `wrk` and `cb` (cb adds wrk result
    # and # number, passed inside context), except for n == 0 and n == 1.
    assert sum(results[2:]) == 2 * (sum(todo) - 0 - 1)


async def exec_usage(todo=range(1,11)):
    async with AioPool(size=4) as pool:
        futures = pool.map_n(worker, todo)

        # While other workers are waiting or active, you can "synchronously"
        # execute one task. It does not interrupt  others, just waits for pool
        # space, then waits for task to finish and then returns it's result.
        important_res = await pool.exec(worker(2))
        assert 2 == important_res

        # You can continue working as usual:
        moar = await pool.spawn(worker(10))

    assert sum(todo) == sum(f.result() for f in futures)


async def cancel_usage():

    async def wrk(*arg, **kw):
        await aio.sleep(0.5)
        return 1

    pool = AioPool(size=2)

    f_quick = pool.spawn_n(aio.sleep(0.1))
    f12 = await pool.spawn(wrk()), pool.spawn_n(wrk())
    f35 = pool.map_n(wrk, range(3))

    # At this point, if you cancel futures, returned by pool methods,
    # you just won't be able to retrieve spawned task results, task
    # themselves will continue working. Don't do this:
    #   f_quick.cancel()
    # use `pool.cancel` instead:

    # cancel some
    await aio.sleep(0.1)
    cancelled, results = await pool.cancel(f12[0], f35[2])  # running and waiting
    assert 2 == cancelled  # none of them had time to finish
    assert 2 == len(results) and \
        all(isinstance(res, aio.CancelledError) for res in results)

    # cancel all others
    await aio.sleep(0.1)

    # not interrupted and finished successfully
    assert f_quick.done() and f_quick.result() is None

    cancelled, results = await pool.cancel()  # all
    assert 3 == cancelled
    assert len(results) == 3 and \
        all(isinstance(res, aio.CancelledError) for res in results)

    assert await pool.join()  # joins successfully


async def details(todo=range(1,11)):
    pool = AioPool(size=5)

    # This code:
    f1 = []
    for i in todo:
        f1.append(pool.spawn_n(worker(i)))
    # is equivalent to one call of `map_n`:
    f2 = pool.map_n(worker, todo)

    # Afterwards you can await for any given future:
    try:
        assert 3 == await f1[2]  # result of spawn_n(worker(3))
    except Exception as e:
        # exception happened in worker (or CancelledError) will be re-raised
        pass

    # Or use `asyncio.wait` to handle results in batches (see `iterwait` also):
    important_res = 0
    more_important = [f1[1], f2[1], f2[2]]
    while more_important:
        done, more_important = await aio.wait(more_important, timeout=0.5)
        # handle result, note it will re-raise exceptions
        important_res += sum(f.result() for f in done)

    assert important_res == 2 + 2 + 3

    # But you need to join, to allow all spawned workers to finish
    # (of course you can `asyncio.wait` all of the futures if you want to)
    await pool.join()

    assert all(f.done() for f in itertools.chain(f1,f2))  # this is guaranteed
    assert 2 * sum(todo) == sum(f.result() for f in itertools.chain(f1,f2))