Manage asyncio coroutines in dict - python

I want to manage some coroutines in a dict, while running a main coroutine.
Specifically I want to start endless coroutines, put the handler of them in a dict and cancel them via a dict call again. In my example I want to start 4 Coroutines and they shall cancel one by one again with the coroutine doomsday. I'm using Python 3.6.
import asyncio
import traceback
async def add_to_handler(node, func):
func_handler[node] = asyncio.ensure_future(func, loop=loop)
return
async def test_func1():
while True:
print("1 - HeNlO")
await asyncio.sleep(1)
async def test_func2():
while True:
print("2 - TestFunc2")
await asyncio.sleep(2)
async def test_func3():
while True:
print("3 - Tukan")
await asyncio.sleep(3)
async def test_func4():
while True:
print("4 - Do Coro!")
await asyncio.sleep(4)
async def doomsday():
# Cancel coroutine every 10 seconds
print("launch doomsday")
for i in range(len(func_handler)):
await asyncio.sleep(10)
print("start cancelling with {}".format(i))
func_handler[str(i + 1)].cancel()
return
async def main():
await add_to_handler("1", test_func1)
await add_to_handler("2", test_func2)
await add_to_handler("3", test_func3)
await add_to_handler("4", test_func4)
await doomsday()
while True:
print("z..z..Z..Z...Z")
print(func_handler)
await asyncio.sleep(5)
func_handler = {}
loop = asyncio.get_event_loop()
try:
loop.run_until_complete(main())
except KeyboardInterrupt:
print("stop loop")
loop.close()
I tried it with the .call_latermethod of AbstractEventLoop instead of an endless while loop, but it still doesn't want to work and it seems, that my coroutines are seen as function, but I don't know why. Where is my fault?


try to change this function:
async def add_to_handler(node, func):
func_handler[node] = asyncio.ensure_future(func(), loop=loop)
return None
pay attention on asyncio.ensure_future(func(),loop=loop)

Related

Closing a client

There is the following code:
import asyncio
import aiohttp
aut_token = ("token")
tasks = []
iter_flag = False
class WAPI:
async def receiver(WAPI_S):
async for msg in WAPI_S:
data = msg.json()
raise aiohttp.ClientError #test
async def heartbeating(WAPI_S):
while iter_flag:
await WAPI_S.send_json({
"op": 1,
"d": None
})
await asyncio.sleep(42.5)
async def event_manager():
loop = asyncio.get_running_loop()
try:
async with aiohttp.ClientSession().ws_connect("url") as WAPI_S:
task_receive = loop.create_task(WAPI.receiver(WAPI_S)); task_heartbeating = loop.create_task(WAPI.heartbeating(WAPI_S))
tasks.append(task_receive); tasks.append(task_heartbeating)
await asyncio.gather(*tasks)
except aiohttp.ClientError:
global iter_flag
iter_flag = False
await asyncio.sleep(44)
[task.cancel() for task in tasks]
try:
loop.close()
except:
loop.stop()
asyncio.run(WAPI.event_manager())
I want to correctly shutdown the client when the exception is raised. My implementation throws "RuntimeError: Event loop stopped before Future completed" exception while executing. How to do it right?

In method event_manager, the statement:
async with aiohttp.ClientSession().ws_connect("url") as WAPI_S:
needs to be replaced with:
async with aiohttp.ClientSession() as session:
async with session.ws_connect("url") as WAPI_S:
Also, it is considered anti-Pythonic to use a list comprehension for its side effects. See Is it Pythonic to use list comprehensions for just side effects? So you really should replace:
[task.cancel() for task in tasks]
with:
for task in tasks:
task.cancel()
Putting this all together:
async def event_manager():
loop = asyncio.get_running_loop()
try:
async with aiohttp.ClientSession() as session:
async with session.ws_connect("url") as WAPI_S:
task_receive = loop.create_task(WAPI.receiver(WAPI_S)); task_heartbeating = loop.create_task(WAPI.heartbeating(WAPI_S))
tasks.append(task_receive); tasks.append(task_heartbeating)
await asyncio.gather(*tasks)
except aiohttp.ClientError:
global iter_flag
iter_flag = False
await asyncio.sleep(44)
for task in tasks:
task.cancel()
try:
loop.close()
except:
loop.stop()

Python - Cancel task in asyncio?

I have written code for async pool below. in __aexit__ i'm cancelling the _worker tasks after the tasks get finished. But when i run the code, the worker tasks are not getting cancelled and the code is running forever. This what the task looks like: <Task pending coro=<AsyncPool._worker() running at \async_pool.py:17> wait_for=<Future cancelled>>. The asyncio.wait_for is getting cancelled but not the worker tasks.
class AsyncPool:
def __init__(self,coroutine,no_of_workers,timeout):
self._loop = asyncio.get_event_loop()
self._queue = asyncio.Queue()
self._no_of_workers = no_of_workers
self._coroutine = coroutine
self._timeout = timeout
self._workers = None
async def _worker(self):
while True:
try:
ret = False
queue_item = await self._queue.get()
ret = True
result = await asyncio.wait_for(self._coroutine(queue_item), timeout = self._timeout,loop= self._loop)
except Exception as e:
print(e)
finally:
if ret:
self._queue.task_done()
async def push_to_queue(self,item):
self._queue.put_nowait(item)
async def __aenter__(self):
assert self._workers == None
self._workers = [asyncio.create_task(self._worker()) for _ in range(self._no_of_workers)]
return self
async def __aexit__(self,type,value,traceback):
await self._queue.join()
for worker in self._workers:
worker.cancel()
await asyncio.gather(*self._workers, loop=self._loop, return_exceptions =True)
To use the Asyncpool:
async def something(item):
print("got", item)
await asyncio.sleep(item)
async def main():
async with AsyncPool(something, 5, 2) as pool:
for i in range(10):
await pool.push_to_queue(i)
asyncio.run(main())
The Output in my terminal:

The problem is that your except Exception exception clause also catches cancellation, and ignores it. To add to the confusion, print(e) just prints an empty line in case of a CancelledError, which is where the empty lines in the output come from. (Changing it to print(type(e)) shows what's going on.)
To correct the issue, change except Exception to something more specific, like except asyncio.TimeoutError. This change is not needed in Python 3.8 where asyncio.CancelledError no longer derives from Exception, but from BaseException, so except Exception doesn't catch it.

When you have an asyncio task created and then cancelled, you still have the task alive that need to be "reclaimed". So you want to await worker for it. However, once you await such a cancelled task, as it will never give you back the expected return value, the asyncio.CancelledError will be raised and you need to catch it somewhere.
Because of this behavior, I don't think you should gather them but to await for each of the cancelled tasks, as they are supposed to return right away:
async def __aexit__(self,type,value,traceback):
await self._queue.join()
for worker in self._workers:
worker.cancel()
for worker in self._workers:
try:
await worker
except asyncio.CancelledError:
print("worker cancelled:", worker)

This appears to work. The event is a counting timer and when it expires it cancels the tasks.
import asyncio
from datetime import datetime as dt
from datetime import timedelta as td
import random
import time
class Program:
def __init__(self):
self.duration_in_seconds = 20
self.program_start = dt.now()
self.event_has_expired = False
self.canceled_success = False
async def on_start(self):
print("On Start Event Start! Applying Overrides!!!")
await asyncio.sleep(random.randint(3, 9))
async def on_end(self):
print("On End Releasing All Overrides!")
await asyncio.sleep(random.randint(3, 9))
async def get_sensor_readings(self):
print("getting sensor readings!!!")
await asyncio.sleep(random.randint(3, 9))
async def evauluate_data(self):
print("checking data!!!")
await asyncio.sleep(random.randint(3, 9))
async def check_time(self):
if (dt.now() - self.program_start > td(seconds = self.duration_in_seconds)):
self.event_has_expired = True
print("Event is DONE!!!")
else:
print("Event is not done! ",dt.now() - self.program_start)
async def main(self):
# script starts, do only once self.on_start()
await self.on_start()
print("On Start Done!")
while not self.canceled_success:
readings = asyncio.ensure_future(self.get_sensor_readings())
analysis = asyncio.ensure_future(self.evauluate_data())
checker = asyncio.ensure_future(self.check_time())
if not self.event_has_expired:
await readings
await analysis
await checker
else:
# close other tasks before final shutdown
readings.cancel()
analysis.cancel()
checker.cancel()
self.canceled_success = True
print("cancelled hit!")
# script ends, do only once self.on_end() when even is done
await self.on_end()
print('Done Deal!')
async def main():
program = Program()
await program.main()

Any chance to shield create_task'ed task?

I need to shield a task been fired with create_task within an aiohttp handler process like
async def handler(request):
asyncio.create_task(long_process())
return {}, 200
So the code is
import asyncio
async def shielded(coro):
try:
await asyncio.shield(coro)
except asyncio.CancelledError:
await coro
raise
def create_task(coro) -> asyncio.Task:
task = asyncio.create_task(shielded(coro))
return task
But this test
async def test_create_task_cancel():
async def coro():
await asyncio.sleep(1)
return None
task = create_task(coro())
await asyncio.sleep(0.1)
task.cancel()
await asyncio.sleep(1)
assert task.done() and task.result() is None
with
RuntimeError: coroutine is being awaited already
aiohttp manual suggests to use aiojobs.Scheduler, but it doesn't work as expected
https://github.com/aio-libs/aiojobs/issues/148
https://github.com/aio-libs/aiojobs/issues/72

Working shield function
async def shielded(coro):
ft = asyncio.ensure_future(coro)
shielded_ft = asyncio.shield(ft)
try:
await shielded_ft
except asyncio.CancelledError:
await ft
raise

Is there a special syntax for suspending a coroutine until a condition is met?

I need to suspend a coroutine until a condition is met. Currently, I have:
class Awaiter:
def __init__(self):
self.ready = False
def __await__(self):
while not self.ready:
yield
And the caller code:
await awaiter
This works, but it requires boilerplate code. Is it necessary boilerplate or is there a special syntax to await on a predicate, such as:
await condition
which would yield until condition is false?

At the asyncio package there is a builtin Condition object that you can use.
An asyncio condition primitive can be used by a task to wait for some event to happen and then get exclusive access to a shared resource.
How to use the condition (from the same source):
cond = asyncio.Condition()
# The preferred way to use a Condition is an async with statement
async with cond:
await cond.wait()
# It can also be used as follow
await cond.acquire()
try:
await cond.wait()
finally:
cond.release()
A code example:
import asyncio
cond = asyncio.Condition()
async def func1():
async with cond:
print('It\'s look like I will need to wait')
await cond.wait()
print('Now it\'s my turn')
async def func2():
async with cond:
print('Notifying....')
cond.notify()
print('Let me finish first')
# Main function
async def main(loop):
t1 = loop.create_task(func1())
t2 = loop.create_task(func2())
await asyncio.wait([t1, t2])
if __name__ == '__main__':
l = asyncio.get_event_loop()
l.run_until_complete(main(l))
l.close()
This will results with:
It's look like I will need to wait
Notifying....
Let me finish first
Now it's my turn
An alternative way is to use the asyncio.Event.
import asyncio
event = asyncio.Event()
async def func1():
print('It\'s look like I will need to wait')
await event.wait()
print('Now it\'s my turn')
async def func2():
print('Notifying....')
event.set()
print('Let me finish first')
It will have the same results as the Condition code example.

two async tasks - one depends on another in python

I need to write a code where i need to to check in real time a status of some variable. I decited to use asyncio to create two async def functions
import asyncio
async def one():
global flag
flag = True
while flag == True:
await asyncio.sleep(0.2)
print("Doing one")
async def two():
await asyncio.sleep(2)
global flag
flag = False
async def main():
tasks = []
tasks.append(one())
tasks.append(two())
await asyncio.gather(*tasks)
loop = asyncio.get_event_loop()
try:
loop.run_until_complete(main())
finally:
loop.close()
print("Loop ended")
When loop starts, all tasks has been lauched and after 2 seconds def two() sets flag=False, which stops def one(). It's good but i want def one() to perform while loop without await asyncio.sleep(0.2) becouse i dont want to have real live update so i set await asyncio.sleep(0.0).
Is it a good practice?

Using a global variable is indeed bad practice. What you are looking for is asyncio's primitives, specifically the asyncio.Event primitive. Here is what you are doing, but with asyncio.Event:
import asyncio
async def one(event):
while event.is_set() == False:
await asyncio.sleep(0.5)
print("Hello World!")
async def two(event):
await asyncio.sleep(2)
event.set()
async def main():
event = asyncio.Event()
await asyncio.gather(*[one(event), two(event)])
asyncio.run(main())

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