Asyncio -- delayed calculation

[Steve D'Aprano]

I'm a complete and utter newbie when it comes to asynchronous programming,
so I may have the entire concept backwards here. But treat this as a
learning exercise rather than something I'd really do.

Suppose I have a bunch of calculations to do: count down from 10. So I have
a bunch of objects:

class Counter:
    def __init__(self):
        self.count = 10
    def count_down(self):
        print(self, "starting")
        while self.count > 0:
            # simulate a computation
            time.sleep(0.5)
            self.count -= 1
        print(self, "completed")

pool = [Counter() for i in range(5)]

for obj in pool:
    obj.count_down()



Since that's all blocking, I wait for the first Counter to count down to
zero before I move on to the second.

Let's pretend that the computation can be performed asynchronously, so that
I can have all five Counter objects counting down in parallel. I have this:


import asyncio

class Counter:
    def __init__(self):
        self.count = 10
    async def count_down(self):
        print(self, "starting")
        while self.count > 0:
            # simulate a computation
            await asyncio.sleep(0.5)
            self.count -= 1
        print(self, "completed")

async def main():
    pool = [Counter() for i in range(5)]
    for obj in pool:
        obj.count_down()

loop = asyncio.get_event_loop()
loop.run_until_complete(main())




When I try running that, I get no output. No error, no exception, the
run_until_complete simply returns instantly.

What am I doing wrong?

What I expected is that the pool of Counter objects would be created, each
one would have their count_down() method called without blocking, so I'd
have something like:

# IDs are simulated for ease of comprehension
<__main__.Counter object at 0x0123> starting
<__main__.Counter object at 0x0246> starting
<__main__.Counter object at 0x048c> starting
<__main__.Counter object at 0x0918> starting
<__main__.Counter object at 0x1230> starting
<__main__.Counter object at 0x0123> completed
<__main__.Counter object at 0x0246> completed
<__main__.Counter object at 0x048c> completed
<__main__.Counter object at 0x0918> completed
<__main__.Counter object at 0x1230> completed



-- 
Steve
“Cheer up,” they said, “things could be worse.” So I cheered up, and sure
enough, things got worse.