python - Is it possible to run only a single step of the asyncio event loop

10
votes

I'm working on a simple graphical network application, using asyncio and tkinter. I'm running into the problem of combining the asyncio event loop with Tk's mainloop. If possible, I'd like to do it without threads, because both these libraries (but especially tkinter) aren't very thread safe. Currently, I'm using Tk.update in an asyncio coroutine, which runs only a single iteration of the tk event loop:

@asyncio.coroutine
def run_tk(tk, interval=0.1):
    try:
        while True:
            tk.update()
            yield from asyncio.sleep(interval)
    except TclError as e:
        if "application has been destroyed" not in e.args[0]:
            raise

However, in the interest of exploring all options, I was wondering if it was possible to do the reverse- if it was possible to invoke only a single iteration of an asyncio event loop inside a tk callback.

3
pythoneventstkintertkpython-asyncio
You can combine the Tk main loop with asyncio, but I don't know if that still allows you to process network traffic (i.e. how do network events flow into the Tk main loop)? - Christian Aichinger
It appears that it would work, but it cheats- It creates a TkEventLoop, which essentially runs update in a loop, but also creates a traditional event loop to do network i/o and runs it in a thread. It's the worst of both worlds; the only advantage is that direct callbacks (loop.call_later) are invoked directly in the Tk event loop. - Lucretiel
I am missing a minimal working example in the question and in the answers. - buhtz
The accepted answer shows how to run a single step of the event loop. You should be able to repeatedly call run_once in the tkinter main loop to "run" asyncio. - Lucretiel
There is loop._run_once(). However if you stick thinter to its own thread while running asyncio loop on another there is no problem with that. Most of asyncio is not thread safe either. You can schedule coroutines and callbacks from tkinter thread with loop.run_coroutine_threadsafe() and loop.call_soon_threadsafe() respectively. - Emsi

3 Answers

11
votes

The missing of public method like loop.run_once() is intentional. Not every supported event loop has a method to iterate one step. Often underlying API has methods for creating event loop and running it forever but emulating single step may be very ineffective.

If you really need it you may implement single-step iteration easy:

import asyncio


def run_once(loop):
    loop.call_soon(loop.stop)
    loop.run_forever()


loop = asyncio.get_event_loop()

for i in range(100):
    print('Iteration', i)
    run_once(loop)
1
votes

Take a look at this example.

import asyncio
from tkinter import *

class asyncTk(Tk):
    def __init__(self):
        super().__init__()
        self.running = True
        self.protocol("WM_DELETE_WINDOW", self.on_closing)

    def on_closing(self):
        self.running = False
        self.destroy()
        
    def __await__(self):
        while self.running:
            self.update()
            yield

async def asd():
    for x in range(1,10):
        await asyncio.sleep(1)
        print(x)

async def main():
    w = asyncTk()
    asyncio.create_task(asd())
    await w

asyncio.run(main())
0
votes

I using the following procedure for own run_once() and run_forever() creation.

Here's a simplified example:

import asyncio

async def worker(**kwargs):
    id = kwargs.get('id', '0.0.0.0.0.0')
    time = kwargs.get('time', 1)

    try:
        # Do stuff.
        print('start: ' + id)
    finally:
        await asyncio.sleep(time)

async def worker_forever(**kwargs):
    while True:
        await worker(**kwargs)

def init_loop(configs, forever=True):
    loop = asyncio.get_event_loop()

    if forever:
        tasks = [
            loop.create_task(worker_forever(id=conf['id'], time=conf['time'])) 
            for conf in config
        ]

    else:
        tasks = [
            asyncio.ensure_future(worker(id=conf['id'], time=conf['time'])) 
            for conf in configs
        ]

    return loop, tasks

def run_once(configs):
    print('RUN_ONCE')
    loop, futures = init_loop(configs, forever=False)
    result = loop.run_until_complete(asyncio.gather(*futures))
    print(result)

def run_forever(configs):
    print('RUN_FOREVER')
    loop, _ = init_loop(configs, forever=True)
    try:
        loop.run_forever()
    except KeyboardInterrupt:
        pass
    finally:
        print("Closing Loop")
        loop.close()

if __name__ == '__main__':
    configurations = [
        {'time': 5, 'id': '4'},
        {'time': 6, 'id': '5'},
        {'time': 1, 'id': '6'},
    ]  # TODO :: DUMMY

    run_once(configurations)
    run_forever(configurations)