The event loop and TaskGroup

Coroutines, the loop, and TaskGroup

async def defines a coroutine function; calling it creates a coroutine object that does nothing until awaited or scheduled. The event loop runs scheduled coroutines to completion, switching between them at every await (suspension point).

Cooperative multitasking means one rule: never block the loop. A bare time.sleep(1) inside a coroutine freezes every other task; await asyncio.sleep(1) yields properly.

import asyncio

async def fetch(name, delay):
    await asyncio.sleep(delay)          # stand-in for I/O
    return f"{name}-done"

async def main():
    results = []
    async with asyncio.TaskGroup() as tg:          # 3.11+
        t1 = tg.create_task(fetch("a", 0.1))
        t2 = tg.create_task(fetch("b", 0.2))
    return t1.result(), t2.result()    # both done when the block exits

TaskGroup (3.11+) is the structured-concurrency primitive: tasks created in the group are guaranteed finished (or the group raised) when the async with exits. If any task raises, the group cancels the others and raises ExceptionGroup (unwrap single errors with except* or group.exceptions).

Legacy asyncio.gather(*aws) still exists and returns results in order; it does NOT cancel siblings on failure by default. New code prefers TaskGroup.