What is an 'async_generator' & how is it different from a 'coroutine' in Python 3.6?

Question

I suspect this has something to do with the differences b/w yield from & await. However, & aside from the new object's designation as an async_generator, I'm unclear about consequences regarding the differences between it and a coroutine. (I'm not sure how else to ask the question other than the one I put in the title...)

import asyncio

async def async_generator_spits_out_letters():
    yield 'a'
    yield 'b'
    yield 'c'
    yield 'd'
    await asyncio.sleep(0)

async def coroutine_prints_messages():
    while True:
        print('hi')
        await asyncio.sleep(2)

def test_it():
    print(type(async_generator_spits_out_letters))
    print(type(coroutine_prints_messages))
    # This is how I choose to do newlines....it's easier for me to read. :[
    print(); print()

    print(type(async_generator_spits_out_letters()))
    print(type(coroutine_prints_messages()))

This gives:

<class 'async_generator'>
<class 'coroutine'>


<class 'function'>
<class 'function'>

I can't make heads or tails of this...

A generator yields, a coroutine does not.

deceze
– deceze ♦

2017-08-16 19:45:41 +00:00
Commented Aug 16, 2017 at 19:45 — deceze
– deceze ♦, Commented Aug 16, 2017 at 19:45
Is that the only difference b/w the two?

Rob Truxal
– Rob Truxal

2017-08-16 20:58:31 +00:00
Commented Aug 16, 2017 at 20:58 — Rob Truxal
– Rob Truxal, Commented Aug 16, 2017 at 20:58

Rob Truxal · Accepted Answer · 2017-08-18 20:14:41Z

In order for an `async_generator`-producing function to run in an eventloop, its output must be wrapped in a `coroutine.`

This is to prevent the async_generator from yielding values directly INTO the event-loop.

import asyncio

# This produces an async_generator
async def xrange(numbers):
    for i in range(numbers):
        yield i
        await asyncio.sleep(0)

# This prevents an async_generator from yielding into the loop.
async def coroutine_wrapper(async_gen, args):
    try:
        print(tuple([i async for i in async_gen(args)]))
    except ValueError:
        print(tuple([(i, j) async for i, j in async_gen(args)]))

Loops only like tasks & futures.

If a loop receives an integer or string or..anything not derived from a future from one of its tasks, it will break.

Therefore coroutines must either:

produce futures (or subclasses of future,)
OR NOT pass any values back into the loop.

Here's main():

def main():
    print('BEGIN LOOP:')
    print()
    loop = asyncio.get_event_loop()
    xrange_iterator_task = loop.create_task(coroutine_wrapper(xrange, 20))
    try:
        loop.run_until_complete(xrange_iterator_task)
    except KeyboardInterrupt:
        loop.stop()
    finally:
        loop.close()
    print()
    print('END LOOP')
    print(); print()
    print('type(xrange) == {}'.format(type(xrange)))
    print('type(xrange(20) == {}'.format(type(xrange(20))))
    print()
    print('type(coroutine_wrapper) == {}'.format(type(coroutine_wrapper)))
    print('type(coroutine_wrapper(xrange,20)) == {}'.format(type(coroutine_wrapper(xrange, 20))))
if __name__ == '__main__':
    main()

Here's the output:

BEGIN LOOP:

(0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19)

END LOOP


type(xrange) == <class 'function'>
type(xrange(20)) == <class 'async_generator'>

type(coroutine_wrapper) == <class 'function'>
type(coroutine_wrapper(xrange,20)) == <class 'coroutine'>

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What is an 'async_generator' & how is it different from a 'coroutine' in Python 3.6?

1 Answer 1

In order for an `async_generator`-producing function to run in an eventloop, its output must be wrapped in a `coroutine.`

Loops only like tasks & futures.

Here's main():

Here's the output:

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Collectives™ on Stack Overflow

1 Answer 1

In order for an async_generator-producing function to run in an eventloop, its output must be wrapped in a coroutine.

Loops only like tasks & futures.

Here's main():

Here's the output:

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In order for an `async_generator`-producing function to run in an eventloop, its output must be wrapped in a `coroutine.`