[FIXED] Await call causing unexpected behavior on instance attribute

Issue

I am having some trouble to figure out why uncommenting the await asyncio.sleep(1) causes Test to be printed 10 times. It seems that the initialization of val attribute is failing when using async.

Shouldn't the initialization be respected and print only once, as it is the same instance. How can address this behavior when awaitable calls are present?

class TestAsync:

    def __init__(self):
        self.val = None

    async def some_fun(self):
        if not self.val:
            # await asyncio.sleep(1)  # Magic line
            print('Test')
            self.val = 10

async def main(loop):
    a = TestAsync()
    tasks = [a.some_fun() for _ in range(10)]
    return await asyncio.gather(*tasks)

if __name__ == '__main__':
    cur_loop = asyncio.get_event_loop()
    cur_loop.run_until_complete(main(cur_loop))

Solution

asyncio doesn't let you stop thinking about concurrency issues. You're running into pretty much exactly the same issue you would have had with threads.

Each some_fun coroutine that sees a falsy value of self.val continues into the body of the if statement. How many coroutines see such a value depends on how many coroutines reach the if test before one of them sets self.val to 10.

Without the sleep, the first coroutine sets self.val to 10 immediately, without giving any others a chance to intervene. With the sleep, each coroutine goes to sleep and lets others run, and all of them see None before any of them changes the value.

Answered By - user2357112 supports Monica