39. Async/await with asyncio

`asyncio` provides cooperative concurrency: coroutines defined with `async def` voluntarily yield control at `await` points, letting the event loop run other coroutines while waiting on I/O.

python
import asyncio

async def fetch(name, delay):
    await asyncio.sleep(delay)
    return f"{name} done"

async def main():
    results = await asyncio.gather(
        fetch("chainA", 1),
        fetch("chainB", 0.5),
    )
    return results

asyncio.run(main())

`asyncio.gather` runs coroutines concurrently and collects results in order; unlike threads, only one coroutine actually executes Python code at any instant — concurrency comes from overlapping wait times, not parallel CPU execution.

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