xiio - really simple async runtime

xiio (Ξ-I/O) is yet another async runtime for Python, like asyncio or trio. Both of these libraries have ~10k lines of code, while this one has a few hundred lines. So I guess it is fair to say that it is really simple.

Usage

import sys
import xiio


async def greet(name):
    await xiio.sleep(len(name) / 10)
    print(f'Hello, {name}!')


async def main():
    async with xiio.timeout(10):
        name1 = (await xiio.read(sys.stdin, 32)).decode()
        name2 = (await xiio.read(sys.stdin, 32)).decode()

        await xiio.gather([
            greet(name1),
            greet(name2),
        ])


xiio.run(main())

Structured Concurrency

Similar to nurseries in trio and task groups in asyncio, xiio provides a low level primitive that controls the lifetime of subtasks. For example, gather() is just a higher level abstraction on top of that:

async def gather(coros):
    async with TaskGroup() as tg:
        tasks = [tg.add_task(coro) for coro in coros]
    return [task.result for task in tasks]

Task groups in xiio have the following properties:

• All subtasks are guaranteed to have finished when the task group exits.
• Subtasks are not started immediately. They have a chance to get started the next time the main task awaits.
• If any task in a task group raises an exception, a xiio.CancelledError is raised in all other tasks. The tasks are then responsible for cleaning up quickly. They may still await async functions if necessary.
• Any exceptions that are raised after cancellation are lost. Only the first one is raised after cleanup is done.
• Tasks are removed from the task group once they are done. If you need their results, keep the reference that is returned by TaskGroup.add_task().
• It is possible to add new tasks while the task group is already running, and even after cancellation.

Design

I spent quite some time creating meaningful commits. So if you want to understand why all the individual pieces are there and how they fit together, I encourage you to check the commit history.