I'm trying to asynchronously download a file in Python, using wget in a subprocess. My code looks like this:
async def download(url, filename):
wget = await asyncio.create_subprocess_exec(
'wget', url,
'O', filename
)
await wget.wait()
def main(url):
loop = asyncio.get_event_loop()
future = asyncio.ensure_future(download(url, 'test.zip'), loop=loop)
print("Downloading..")
time.sleep(15)
print("Still downloading...")
loop.run_until_complete(future)
loop.close()
What I'm trying to do is witness the printing of "Downloading.." then 15 seconds later "Still downloading...", all while the download of the file has started. What I'm actually seeing is that the download of the file only starts when the code hits loop.run_until_complete(future)
My understanding is that asyncio.ensure_future should start running the code of the download coroutine, but apparently I'm missing something.
When passed a coroutine, asyncio.ensure_future converts it to a task - a special kind of future that knows how to drive the coroutine - and enqueues it in the event loop. "Enqueue" means that the code inside the coroutine will be executed by a running event loop that schedules the coroutines. If the event loop is not running, then none of the coroutines will get a chance to run either. The loop is told to run by a call to loop.run_forever() or loop.run_until_complete(some_future). In the question the event loop is only started after the call to time.sleep(), so the beginning of the download is delayed by 15 seconds.
time.sleep should never be called in a thread that runs the asyncio event loop. The correct way to sleep is with asyncio.sleep, which yields the control to the event loop while waiting. asyncio.sleep returns a future that can be submitted to the event loop or awaited from a coroutine:
# ... definition of download omitted ...
async def report():
print("Downloading..")
await asyncio.sleep(15)
print("Still downloading...")
def main(url):
loop = asyncio.get_event_loop()
dltask = loop.create_task(download(url, 'test.zip'))
loop.create_task(report())
loop.run_until_complete(dltask)
loop.close()
The above code has a different problem. When the download is shorter than 15 seconds, it results in a Task was destroyed but it is pending! warning being printed. The problem is that the report task was never canceled when the download task finished and the loop closed, it was just abandoned. This happening often indicates a bug or a misunderstanding of how asyncio works, so asyncio flags it with a warning.
The obvious way to eliminate the warning is by explicitly canceling the task of the report coroutine, but the resulting code ends up being verbose and not very elegant. An simpler and shorter fix is to change report to await the download task, specifying a timeout for displaying the "Still downloading..." message:
async def dl_and_report(dltask):
print("Downloading..")
try:
await asyncio.wait_for(asyncio.shield(dltask), 15)
except asyncio.TimeoutError:
print("Still downloading...")
# assuming we want the download to continue; otherwise
# remove the shield(), and dltask will be canceled
await dltask
def main(url):
loop = asyncio.get_event_loop()
dltask = loop.create_task(download(url, 'test.zip'))
loop.run_until_complete(dl_and_report(dltask))
loop.close()
Related
I have been trying to understand the asyncio module in order to
implement a server. I was looking at this question, which seems
to ask a similar, if not the same question, however I am still trying
to grasp the workflow happening behind the sceens.
I have the following simple program that has two coroutines, one reading from the terminal
and putting it into a Queue and one coroutine that waits for items in the queue and
simply prints them back to the screen.
import asyncio
q = asyncio.Queue()
async def put():
while True:
await q.put(input()) #Input would be normaly something like client.recv()
await asyncio.sleep(1) #This is neccessarry but I dont understand why
async def get():
while True:
print(await q.get())
def run():
loop = asyncio.get_event_loop()
task1 = loop.create_task(put())
task2 = loop.create_task(get())
loop.run_forever()
run()
This program works as expected, however when one removes the await asyncio.sleep(1)
statement from the put method, it stops working. I assume because the loop keeps eating up the thread and the message doesn't get pushed through. I don't understand why though because I would think input() would be a blocking function and the coroutine should thus suspend until a new line is available on the tty.
The second problem is, if I use asyncio.get_event_loop() in the run() call, the interpreter warns me that there is no active loop, however, as stated in the documentation this call is deprecated and thus I tried replacing it with asyncio.new_event_loop(). The programm still works the same, however I get a traceback upon KeyboardInterrupt (which does not happen when calling asyncio.get_event_loop())
Task was destroyed but it is pending!
task: <Task pending name='Task-1' coro=<put() running at test.py:10> wait_for=<Future pending cb=[Task.task_wakeup()]>>
Task was destroyed but it is pending!
task: <Task pending name='Task-2' coro=<get() running at test.py:15> wait_for=<Future pending cb=[Task.task_wakeup()]>>
Exception ignored in: <coroutine object get at 0x7f32f51369d0>
Traceback (most recent call last):
File "test.py", line 15, in get
File "/usr/lib64/python3.10/asyncio/queues.py", line 161, in get
File "/usr/lib64/python3.10/asyncio/base_events.py", line 745, in call_soon
File "/usr/lib64/python3.10/asyncio/base_events.py", line 510, in _check_closed
RuntimeError: Event loop is closed
A third variant I tried was to make the run method itself async and call it via the asyncio.run(run()) call.
import asyncio
q = asyncio.Queue()
async def put():
while True:
await q.put(input())
await asyncio.sleep(1)
async def get():
while True:
print(await q.get())
async def run():
loop = asyncio.get_event_loop()
task1 = loop.create_task(put())
task2 = loop.create_task(get())
await task1
asyncio.run(run())
This works just fine as well, however if I replace await task1 with await task2,
again I get errors when I interrupt the program.
Why is the execution different between these and how is it supposed to be done in the end?
As stated in the comments, with StreamReader (your original code) problem #1 will work flawlessly and cause no issue. input() does not give a chance for aio to switch coroutines, and you can try and limit the queue to a certain length if StreamReader constantly has data.
For problem #2, during cleanup, Python uses the assigned loop for the current thread. Under the hood, asyncio.run() and asyncio.get_event_loop() assigns the loop to the main thread. When it doesn't find a loop, all hell breaks loose.
If you wish to assign it yourself, you can do so like that:
def run():
loop = asyncio.new_event_loop()
asyncio.set_event_loop(loop)
task1 = loop.create_task(put())
task2 = loop.create_task(get())
loop.run_forever()
Keep in mind you're still missing some manual cleanup (i.e. shutdown_asyncgens and shutdown_executor) but that's for a different topic. Overall, using asyncio.run() is usually the correct choice.
I'm unable to reproduce problem #3, both await task1 and await task2 work flawlessly.
I am trying to do something similar like C# ManualResetEvent but in Python.
I have attempted to do it in python but doesn't seem to work.
import asyncio
cond = asyncio.Condition()
async def main():
some_method()
cond.notify()
async def some_method():
print("Starting...")
await cond.acquire()
await cond.wait()
cond.release()
print("Finshed...")
main()
I want the some_method to start then wait until signaled to start again.
This code is not complete, first of all you need to use asyncio.run() to bootstrap the event loop - this is why your code is not running at all.
Secondly, some_method() never actually starts. You need to asynchronously start some_method() using asyncio.create_task(). When you call an "async def function" (the more correct term is coroutinefunction) it returns a coroutine object, this object needs to be driven by the event loop either by you awaiting it or using the before-mentioned function.
Your code should look more like this:
import asyncio
async def main():
cond = asyncio.Condition()
t = asyncio.create_task(some_method(cond))
# The event loop hasn't had any time to start the task
# until you await again. Sleeping for 0 seconds will let
# the event loop start the task before continuing.
await asyncio.sleep(0)
cond.notify()
# You should never really "fire and forget" tasks,
# the same way you never do with threading. Wait for
# it to complete before returning:
await t
async def some_method(cond):
print("Starting...")
await cond.acquire()
await cond.wait()
cond.release()
print("Finshed...")
asyncio.run(main())
My Source Code:
import asyncio
async def mycoro(number):
print(f'Starting {number}')
await asyncio.sleep(1)
print(f'Finishing {number}')
return str(number)
c = mycoro(3)
task = asyncio.create_task(c)
loop = asyncio.get_event_loop()
loop.run_until_complete(task)
loop.close()
Error:
RuntimeError: no running event loop
sys:1: RuntimeWarning: coroutine 'mycoro' was never awaited
I was watching a tutorial and according to my code it was never awaited when I did and it clearly does in the video I was watching.
Simply run the coroutine directly without creating a task for it:
import asyncio
async def mycoro(number):
print(f'Starting {number}')
await asyncio.sleep(1)
print(f'Finishing {number}')
return str(number)
c = mycoro(3)
loop = asyncio.get_event_loop()
loop.run_until_complete(c)
loop.close()
The purpose of asyncio.create_task is to create an additional task from inside a running task. Since it directly starts the new task, it must be used inside a running event loop – hence the error when using it outside.
Use loop.create_task(c) if a task must be created from outside a task.
In more recent version of asyncio, use asyncio.run to avoid having to handle the event loop explicitly:
c = mycoro(3)
asyncio.run(c)
In general, use asyncio.create_task only to increase concurrency. Avoid using it when another task would block immediately.
# bad task usage: concurrency stays the same due to blocking
async def bad_task():
task = asyncio.create_task(mycoro(0))
await task
# no task usage: concurrency stays the same due to stacking
async def no_task():
await mycoro(0)
# good task usage: concurrency is increased via multiple tasks
async def good_task():
tasks = [asyncio.create_task(mycoro(i)) for i in range(3)]
print('Starting done, sleeping now...')
await asyncio.sleep(1.5)
await asyncio.gather(*tasks) # ensure subtasks finished
Change the line
task = asyncio.Task(c)
I try to cancel process when timeout but asyncio.wait_for not working. How do i cancel process when reached time out. My code below:
import asyncio
async def process():
# do something take a long time like this
for i in range(0,10000000000,1):
for j in range(0,10000000000,1):
continue
print('done!')
async def main():
# I want to cancel process when reached timeout
try:
await asyncio.wait_for(process(), timeout=1.0)
except asyncio.TimeoutError:
print('timeout!')
loop = asyncio.get_event_loop()
loop.run_until_complete(main())
This doesn't work because your process function is async in name only - it doesn't await anything. That means that it finishes in its entirety without giving the event loop a chance to interrupt it. Since asyncio is cooperative (as are other async/await based systems), such a function is not a correctly written async function and cannot be interrupted.
If you add an await asyncio.sleep(0.001) into the inner loop (or anything else that awaits something that actually suspends), your code will work fine.
I was wondering how concurrency works in python 3.6 with asyncio. My understanding is that when the interpreter executing await statement, it will leave it there until the awaiting process is complete and then move on to execute the other coroutine task. But what I see here in the code below is not like that. The program runs synchronously, executing task one by one.
What is wrong with my understanding and my impletementation code?
import asyncio
import time
async def myWorker(lock, i):
print("Attempting to attain lock {}".format(i))
# acquire lock
with await lock:
# run critical section of code
print("Currently Locked")
time.sleep(10)
# our worker releases lock at this point
print("Unlocked Critical Section")
async def main():
# instantiate our lock
lock = asyncio.Lock()
# await the execution of 2 myWorker coroutines
# each with our same lock instance passed in
# await asyncio.wait([myWorker(lock), myWorker(lock)])
tasks = []
for i in range(0, 100):
tasks.append(asyncio.ensure_future(myWorker(lock, i)))
await asyncio.wait(tasks)
# Start up a simple loop and run our main function
# until it is complete
loop = asyncio.get_event_loop()
loop.run_until_complete(main())
print("All Tasks Completed")
loop.close()
Invoking a blocking call such as time.sleep in an asyncio coroutine blocks the whole event loop, defeating the purpose of using asyncio.
Change time.sleep(10) to await asyncio.sleep(10), and the code will behave like you expect.
asyncio use a loop to run everything, await would yield back the control to the loop so it can arrange the next coroutine to run.