async-basics.md

Python by Example: Async Basics

Async lets you write non-blocking I/O code—useful when waiting on network or disk. Define coroutines with async def and call them with await. Use asyncio.run() to run an async function from synchronous code. Async is cooperative: a coroutine yields control when it awaits, so others can run.

What you'll learn:

• async def for coroutines
• await to call other coroutines
• asyncio.run() as the entry point

import asyncio


async def say_hello():
    print("Hello")
    await asyncio.sleep(1)
    print("World")


async def main():
    await say_hello()
    print("Done")


asyncio.run(main())

asyncio.sleep(1) doesn't block the whole program—it yields so other tasks can run. In a real app, you'd await network or file I/O instead.

To run this program:

$ python source/async-basics.py
Hello
World
Done

Tip: You can't call an async function directly—you must await it from another async function or run it with asyncio.run().

Try it: Add a second await asyncio.sleep(0.5) and see the delay.

Source: async-basics.py

Next: Async Concurrency