Async/Await in Practice

Asyncio Core

Async/Await Crash Course

Coroutines let you write sequential-looking code that cooperatively multitasks.

Structure async codeGather tasks

Structure

import asyncio

async def fetch_order(order_id):
    await asyncio.sleep(0.1)
    return {'id': order_id}

async def main():
    order = await fetch_order('123')
    print(order)

asyncio.run(main())

• async declares coroutines; await pauses until awaited tasks complete.
• Use asyncio.run to bootstrap top-level coroutines.

Gathering tasks

async def main():
    orders = await asyncio.gather(
        fetch_order('123'),
        fetch_order('456'),
        fetch_order('789'),
        return_exceptions=True,
    )
    print(orders)

asyncio.run(main())

• asyncio.gather waits for all tasks; pass return_exceptions=True to collect exceptions as results instead of raising them immediately.
• asyncio.create_task schedules background work; keep references so you can await or cancel later.

Task groups (Python 3.11+)

async def main():
    async with asyncio.TaskGroup() as tg:
        t1 = tg.create_task(fetch_order('123'))
        t2 = tg.create_task(fetch_order('456'))
    print(t1.result(), t2.result())

asyncio.run(main())

• asyncio.TaskGroup is the recommended structured-concurrency approach for new code: the async with block waits for all tasks, and if one fails the siblings are cancelled and the failures are raised as an ExceptionGroup.
• Prefer it over bare gather when tasks should succeed or fail together.

Error handling

• Wrap awaits in try/except just like synchronous code.
• Use asyncio.wait_for to enforce deadlines; on Python 3.11+ prefer the async with asyncio.timeout(10): context manager. Since 3.11, asyncio.TimeoutError is an alias of the builtin TimeoutError.
• Cancel tasks with task.cancel() to clean up when shutting down.

Next up in your learning path

aiohttp

Apply async to HTTP clients/servers.

Practical

threading vs async

Revisit the comparison when building real apps.

Decision

Event loop internals

Deepen your understanding of scheduling.

Concept