[FIXED] Don't Run Async Function if Lock is Locked - Python

Issue

Suppose that the function takes_time is called every 0.1 seconds on the arguments 0, 1, 2, 3, ...

async def takes_time(i):
    time.sleep(1)

After the first second, takes_time(0) will have been run once and takes_time(1), takes_time(2), ..., takes_time(9) are waiting to occur. I want to change this behavior so that those 9 calls are ignored, and takes_time only runs on the arguments 0, 10, 20, 30, etc. It does not matter to me whether they are off by 1 or 2 - I don't care about exactly which numbers are chosen - the main thing is that I want calls to takes_time to be ignored when another call is still running.

Here is my solution:

lock = asyncio.Lock()
async def takes_time(i):
    if lock.locked():
        return
    async with lock:
        time.sleep(1)

This solution works partially. It ensures that only one call to takes_time is running at a time, but it does not skip arguments 1 through 9 - even though the lock is locked when they are called. Why does this happen, and how can I fix it?

Solution

As mentioned in the comments, you should never use blocking calls like time.sleep() in asyncio code. (The same applies for blocking network IO like requests - there are asyncio libraries for that.) Since you don't need to wait for the lock, you probably don't need a full-blown lock in the first place, an ordinary boolean variable would suffice:

locked = False

async def takes_time(i):
    global locked
    if locked:
        return
    locked = True
    try:
        await asyncio.sleep(1)
    finally:
        locked = False

In a multi-threaded program, this kind of approach would be subject to a race condition because multiple threads could check locked and find it false, both proceeding to set locked to true and defeating exclusivity. In asyncio a task switch can occur only on await (and additionally async for and async with). As the code between if locked and locked = True doesn't await anything, it is executed atomically.

Answered By - user4815162342