Async iterators zip in python

Question

I have two iterators:

async def iter1():
   for i in range(5):
       await asyncio.sleep(0.1)
       yield 1

async def iter2():
       for i in range(10):
       await asyncio.sleep(1)
       yield 2

I want to create a function async_zip_stream(*args, default_value=0).

It should iterate over all the passed iterators simultaneously without synchronization, yielding tuples of values. If any iterator doesn't generate a value at some point, it should return the default_value.

So in this case, the output would be:

1, 0
1, 0
1, 0 
1, 0
1, 0 
0, 2
....

I don't quite understand how this can be done without synchronization between the iterators.

Answer 1

async def async_zip_stream(*iterators, default_value=0):
    tasks = [asyncio.create_task(iterator.__anext__()) for iterator in iterators]
    done = [False] * len(iterators)
    
    while not all(done):
        results = []
        
        for i, task in enumerate(tasks):
            if done[i]:
                results.append(default_value)
            elif task.done():
                try:
                    results.append(task.result())
                    tasks[i] = asyncio.create_task(iterators[i].__anext__())
                except StopAsyncIteration:
                    done[i] = True
                    results.append(default_value)
            else:
                results.append(default_value)
        
        yield tuple(results)
        await asyncio.sleep(0.01)  # Slight delay to allow other tasks to progress