Zaur Nasibov Zaur Nasibov - 8 days ago 5
Python Question

Python asyncio: reader callback and coroutine communication

I am trying to implement a simple idea of passing a data from stdin to a coroutine:

import asyncio
import sys

event = asyncio.Event()

def handle_stdin():
data = sys.stdin.readline()
event.data = data # NOTE: data assigned to the event object
event.set()

@asyncio.coroutine
def tick():
while 1:
print('Tick')
yield from asyncio.sleep(1)

if event.is_set():
data = event.data # NOTE: data read from the event object
print('Data received: {}'.format(data))
event.clear()

def main():
loop = asyncio.get_event_loop()
loop.add_reader(sys.stdin, handle_stdin)
loop.run_until_complete(tick())

if __name__ == '__main__':
main()


This code works fine, however a simplified version of it with a variable instead of an
Event
object works too:

data = None

def handle_stdin():
global data
data = sys.stdin.readline()

@asyncio.coroutine
def tick():
while 1:
print('Tick')
yield from asyncio.sleep(1)

global data
if data is not None:
print('Data received: {}'.format(data))
data = None


My questions are: is the approach with
Event
correct? Or is there a better way with another asyncio objects to handle this kind of problem?
Then, if the approach with
Event
is fine, is using a variable is fine as well?

Thank you.

Answer

I think asyncio.Queue is much better suited for this kind of producer/consumer relationship:

import asyncio
import sys

queue = asyncio.Queue()

def handle_stdin():
    data = sys.stdin.readline()
    asyncio.async(queue.put(data)) # Queue.put is a coroutine, so you can't call it directly.

@asyncio.coroutine
def tick():
    while 1:
        data = yield from queue.get()
        print('Data received: {}'.format(data))

def main(): 
    loop = asyncio.get_event_loop()
    loop.add_reader(sys.stdin, handle_stdin)
    loop.run_until_complete(tick())    

if __name__ == '__main__':
    main()

There's less logic involved than with an Event, which you need to make sure you set/unset properly, and there's no need for a sleep, wakeup, check, go back to sleep, loop, like with the global variable. So the the Queue approach is simpler, smaller, and blocks the event loop less than your other possible solutions. The other solutions are technically correct, in that they will function properly (as long as you don't introduce any yield from calls inside if if event.is_set() and if data is not None: blocks). They're just a bit clunky.