Asyncio.gather vs asyncio.wait

Question

asyncio.gather and asyncio.wait seem to have similar uses: I have a bunch of async things that I want to execute/wait for (not necessarily waiting for one to finish before the next one starts). They use a different syntax, and differ in some details, but it seems very un-pythonic to me to have 2 functions that have such a huge overlap in functionality. What am I missing?

Answer 1

asyncio.wait is more low level than asyncio.gather.

As the name suggests, asyncio.gather mainly focuses on gathering the results. it waits on a bunch of futures and return their results in a given order.

asyncio.wait just waits on the futures. and instead of giving you the results directly, it gives done and pending tasks. you have to mannually collect the values.

Moreover, you could specify to wait for all futures to finish or the just the first one with wait.

Answer 2

Although similar in general cases ("run and get results for many tasks"), each function has some specific functionality for other cases:

asyncio.gather()

Returns a Future instance, allowing high level grouping of tasks:

import asyncio
from pprint import pprint

import random


async def coro(tag):
    print(">", tag)
    await asyncio.sleep(random.uniform(1, 3))
    print("<", tag)
    return tag


loop = asyncio.get_event_loop()

group1 = asyncio.gather(*[coro("group 1.{}".format(i)) for i in range(1, 6)])
group2 = asyncio.gather(*[coro("group 2.{}".format(i)) for i in range(1, 4)])
group3 = asyncio.gather(*[coro("group 3.{}".format(i)) for i in range(1, 10)])

all_groups = asyncio.gather(group1, group2, group3)

results = loop.run_until_complete(all_groups)

loop.close()

pprint(results)

All tasks in a group can be cancelled by calling group2.cancel() or even all_groups.cancel(). See also .gather(..., return_exceptions=True),

asyncio.wait()

Supports waiting to be stopped after the first task is done, or after a specified timeout, allowing lower level precision of operations:

import asyncio
import random


async def coro(tag):
    print(">", tag)
    await asyncio.sleep(random.uniform(0.5, 5))
    print("<", tag)
    return tag


loop = asyncio.get_event_loop()

tasks = [coro(i) for i in range(1, 11)]

print("Get first result:")
finished, unfinished = loop.run_until_complete(
    asyncio.wait(tasks, return_when=asyncio.FIRST_COMPLETED))

for task in finished:
    print(task.result())
print("unfinished:", len(unfinished))

print("Get more results in 2 seconds:")
finished2, unfinished2 = loop.run_until_complete(
    asyncio.wait(unfinished, timeout=2))

for task in finished2:
    print(task.result())
print("unfinished2:", len(unfinished2))

print("Get all other results:")
finished3, unfinished3 = loop.run_until_complete(asyncio.wait(unfinished2))

for task in finished3:
    print(task.result())

loop.close()

Answer 3

I also noticed that you can provide a group of coroutines in wait() by simply specifying the list:

result=loop.run_until_complete(asyncio.wait([
        say('first hello', 2),
        say('second hello', 1),
        say('third hello', 4)
    ]))

Whereas grouping in gather() is done by just specifying multiple coroutines:

result=loop.run_until_complete(asyncio.gather(
        say('first hello', 2),
        say('second hello', 1),
        say('third hello', 4)
    ))