Lesson 19: Asynchronous Programming (async/await)

What Is Asynchronous Programming?

Asynchronous programming allows your program to perform other tasks while waiting for slow operations to finish, such as network requests, file I/O, or timers. Instead of blocking execution, Python can switch between tasks efficiently.

The asyncio Module

Python’s built‑in asyncio module provides tools for writing asynchronous code using async and await.

Defining an Async Function

import asyncio

async def greet():
    print("Hello!")

Running an Async Function

asyncio.run(greet())

Using await

await pauses the function until the awaited task completes:

async def wait_and_print():
    await asyncio.sleep(2)
    print("Done waiting!")

Combining Multiple Tasks

You can run tasks concurrently:

async def task(name, delay):
    await asyncio.sleep(delay)
    print(f"{name} finished")

async def main():
    await asyncio.gather(
        task("Task 1", 2),
        task("Task 2", 1)
    )

asyncio.run(main())

Creating Tasks Manually

async def main():
    t1 = asyncio.create_task(task("A", 2))
    t2 = asyncio.create_task(task("B", 3))

    await t1
    await t2

When to Use Async Programming

• Network requests (APIs, web scraping)
• Database queries
• File I/O
• Timers and scheduled tasks
• Any situation with waiting or latency

When NOT to Use Async

• CPU‑heavy tasks (use multiprocessing instead)
• Simple scripts that don’t involve waiting
• Codebases where async adds unnecessary complexity

Real‑World Example: Fetching Multiple URLs

import asyncio
import aiohttp

async def fetch(session, url):
    async with session.get(url) as response:
        return await response.text()

async def main():
    urls = ["https://example.com", "https://python.org"]
    async with aiohttp.ClientSession() as session:
        results = await asyncio.gather(*(fetch(session, u) for u in urls))
        print(results)

asyncio.run(main())

Why Async Matters

• Makes programs faster and more responsive.
• Handles many tasks at once without threads.
• Essential for modern web servers and network applications.

Next Steps

Now that you understand asynchronous programming, you're ready to explore how to write automated tests in Lesson 20: Introduction to Testing (unittest).

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