i am a little new to asyncio in python. I was trying to run this simple code but i don't know why i am getting this unexpected output.
What i did is that, in outer
function, i created async tasks and stored it in an array tasks
. Before awaiting on these tasks i wrote a print statement print("outer")
that should run in every iteration. And inside the task i wrote another print statement print("inner")
in inner
function. But some how i am getting some unexpected output.
Here's the code -
import asyncio
def main():
loop = asyncio.get_event_loop()
loop.run_until_complete(outer(loop))
loop.close()
async def outer(loop):
tasks = []
for i in range(0, 5):
tasks.append(loop.create_task(inner()))
for task in tasks:
print("outer")
await task
async def inner():
print("inner")
await asyncio.sleep(0.5)
if __name__ == '__main__':
main()
Here's the output -
outer
inner
inner
inner
inner
inner
outer
outer
outer
outer
My Expected output was -
outer
inner
outer
inner
outer
inner
outer
inner
outer
inner
Why all the inner
are printing before outer
. What is the correct execution flow of asyncio. Thanks in advance.
In async def outer(loop)
for i in range(0, 5):
tasks.append(loop.create_task(inner()))
inner
tasks are created, scheduled and start running. If you add a little bit more to to inner
and outer
it shows this process better:
async def outer(loop):
tasks = []
for i in range(0, 5):
tasks.append(loop.create_task(inner(i)))
await asyncio.sleep(3)
for task in tasks:
print('outer')
await task
async def inner(n):
print(f"inner {n} start")
await asyncio.sleep(0.5)
print(f'inner {n} end')
while outer
is sleeping
You can see that the five tasks execute and finish before the second for loop even starts.
await task
has nothing to wait for and outer
is printed five times in succession. I'm a little hazy on the event loop controlling everything - I haven't found any explicit documentation for that - probably alluded to in the create_task
docs: Wrap the coro coroutine into a Task and schedule its execution.
When you create the task it gets scheduled. I have seen videos on pyvideo.org that shows that process, unfortunately I couldn't quickly find the one I wanted to link to.
The loop.create_task(inner())
immediately queues all 5 inner
tasks for running, not await task
. The await task
suspends outer
until the first task is completely done, ie at least 0.5 seconds. During this, all inner
tasks have been run once up await
, including their print
.
async def outer(loop):
tasks = []
for i in range(0, 5):
# queues task immediately
tasks.append(loop.create_task(inner()))
for task in tasks:
print("outer")
# suspends for at least 0.5 seconds
await task
async def inner():
# runs immediately
print("inner")
await asyncio.sleep(0.5)
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