async and await

Waiting without blocking

Most of the time an HTTP client is not computing anything — it is waiting. asyncio lets one thread hold many such waits at once.

import asyncio

async def greet(name):
    await asyncio.sleep(0.1)      # a stand-in for waiting on a network
    return "Hello, " + name

asyncio.run(greet("Ada"))          # 'Hello, Ada'

Three rules:

• async def makes a coroutine function. Calling it does not run it — it returns a coroutine object.
• await runs a coroutine and waits for its result. It is only legal inside async def.
• Something has to start the whole thing. asyncio.run(main()) is that something.

The most common beginner bug is forgetting await, which leaves you holding a coroutine object instead of a value — and Python warns "coroutine was never awaited".

Your turn: write an async double(n) that waits briefly and returns n * 2, and an async main() that awaits it for 5 and returns the result. Call it with asyncio.run.