Python Async and Await

What is asyncio?

asyncio is Python's standard library for writing asynchronous code. Instead of using multiple threads or processes, it runs many tasks on a single thread by pausing a task whenever it is waiting and switching to another one that is ready to run.

Coroutines with async def

You define a coroutine using async def instead of a plain def. Calling a coroutine doesn't run it immediately — it creates a coroutine object that must be awaited or scheduled to actually execute.

import asyncio

async def greet(name):
    print(f"Hello {name}")
    await asyncio.sleep(1)
    print(f"Goodbye {name}")

asyncio.run(greet("Aarav"))

The await Keyword

await pauses the current coroutine until the awaited operation finishes, without blocking the whole program — other tasks can run during that pause. You can only use await inside a function defined with async def.

import asyncio

async def fetch(name, delay):
    print(f"Start {name}")
    await asyncio.sleep(delay)
    print(f"Done {name}")

async def main():
    await asyncio.gather(
        fetch("A", 2),
        fetch("B", 1),
        fetch("C", 3),
    )

asyncio.run(main())

The Event Loop

Behind the scenes, asyncio.run() starts an event loop, which is the engine that keeps track of all pending coroutines and decides which one runs next whenever another one pauses at an await. You rarely manage the event loop directly — asyncio.run() handles it for you.

async/await vs Threads

Both approaches handle waiting-heavy tasks well, but they work very differently under the hood, and asyncio requires libraries that are specifically written to support it (like aiohttp for web requests).