Python Async and Await

What Is Async Python?

Async Python helps programs handle waiting work efficiently, especially network requests, file-like I/O, timers, and many independent operations that spend time waiting.

import asyncio

async def greet():
    await asyncio.sleep(1)
    return "Hello"

asyncio.run(greet())

An async def function creates a coroutine. The coroutine runs when awaited or scheduled.

await

await pauses the current coroutine until another awaitable finishes. While it waits, the event loop can run other tasks.

async def main():
    result = await greet()
    print(result)

Use await only inside async functions.

Tasks

Tasks let multiple coroutines make progress concurrently.

async def main():
    first = asyncio.create_task(fetch("a"))
    second = asyncio.create_task(fetch("b"))

    print(await first)
    print(await second)

This is useful when both operations spend time waiting.

gather()

asyncio.gather() awaits several awaitables and returns their results.

results = await asyncio.gather(fetch("a"), fetch("b"))

The operations can run concurrently on the event loop.

Async Is Not Magic Speed

Async helps most with I/O-bound work. It does not automatically make CPU-heavy calculations faster. CPU-heavy work can block the event loop if it runs too long without yielding.

For CPU-bound work, multiprocessing or optimized algorithms may be more appropriate.

Common Mistakes

A common mistake is calling an async function without awaiting it.

greet()

This creates a coroutine object but does not run it to completion.

Another mistake is using blocking functions like time.sleep() inside async code. Use await asyncio.sleep() instead.

Quick Summary

• async def defines a coroutine function.
• await waits without blocking the event loop.
• asyncio.run() starts an async program.
• Tasks schedule coroutines concurrently.
• gather() awaits multiple awaitables.
• Async is best for I/O-bound waiting work.
• Blocking calls can freeze the event loop.

Practice Quiz

Now practice this topic with MCQs and explanations:

Start the Python Async and Await quiz