Is the async / await lesson free?

Yes. The first 5 lessons on PyRun are free forever — no signup, no card. async / await runs real Python in your browser via Pyodide.

How long does the async / await lesson take?

About 9 minutes end-to-end. It's a hands-on lesson — you read a paragraph, edit the code, and run it in the same tab.

Do I need to install Python to try the async / await lesson?

No install required. PyRun runs real CPython in your browser via Pyodide (Python compiled to WebAssembly). Works on Chrome, Firefox, Safari, and Edge — desktop or mobile.

What level is the async / await lesson?

Advanced level. Assumes comfort with classes, functions, and standard library. Not for first-time Python learners.

← All lessons

Advanced·9 min·advanced · concurrency

async / await

You’ll be able to

Write coroutines with async def and pause them with await
Run async functions with asyncio.run
Fire off many coroutines concurrently with asyncio.gather
Recognize when async helps (I/O-bound) vs when it doesn't (CPU-bound)

Why this matters

FastAPI, httpx, asyncpg, and Playwright are async-first — a single blocking requests.get in an async endpoint tanks throughput for the whole event loop. Senior engineers know when to reach for asyncio.gather, asyncio.TaskGroup (3.11+), and run_in_executor for CPU-bound or legacy sync calls.

Common pitfalls

Calling a blocking library inside an async handler — freezes the loop; wrap in asyncio.to_thread(...).
Forgetting to await a coroutine — returns a coroutine object and emits RuntimeWarning: never awaited.
Sharing one httpx.AsyncClient across tasks without an async with — connection pool leaks under load.

A model for code that waits: network I/O, timers, queues. While one coroutine is waiting, the event loop runs another.

Three keywords

async def — defines a coroutine (a function that returns a coroutine object)
await x — pause this coroutine until x completes
asyncio.gather(*coros) — run many coroutines concurrently and wait for all

In Pyodide

The browser's event loop is asyncio's event loop. await at the top level Just Works.

Common pitfall

Calling async_fn() without await returns a coroutine object — it does not run. Always await or pass to gather.

Try it

Convert the three fetch_thing calls to run sequentially with await — observe the time difference.
Use asyncio.create_task and check the difference vs gather.

Practice

3 graded exercises

Write the code, click Check. We’ll tell you exactly what to fix.

Progress0 / 3

Exercise 1
Define an async def hello(name) that returns the string "Hello, <name>". Then run it with asyncio.run(hello("Ada")) and print the result. Expected: Hello, Ada.
Exercise 2
Write an async def task(n) that awaits asyncio.sleep(0) and returns n * n. In a main() coroutine, use asyncio.gather(task(2), task(3), task(4)) and print the result list. Expected: [4, 9, 16].
Exercise 3
In an async main coroutine, launch two coroutines that each await asyncio.sleep(0.05) and return their arg. Use asyncio.gather and print the tuple/list of results. Expected includes both "a" and "b".

← PreviousIterators Next →NumPy: arrays at speed

async / await

You’ll be able to

Write coroutines with async def and pause them with await
Run async functions with asyncio.run
Fire off many coroutines concurrently with asyncio.gather
Recognize when async helps (I/O-bound) vs when it doesn't (CPU-bound)

Why this matters

Common pitfalls

Calling a blocking library inside an async handler — freezes the loop; wrap in asyncio.to_thread(...).
Forgetting to await a coroutine — returns a coroutine object and emits RuntimeWarning: never awaited.
Sharing one httpx.AsyncClient across tasks without an async with — connection pool leaks under load.

A model for code that waits: network I/O, timers, queues. While one coroutine is waiting, the event loop runs another.

Three keywords

async def — defines a coroutine (a function that returns a coroutine object)
await x — pause this coroutine until x completes
asyncio.gather(*coros) — run many coroutines concurrently and wait for all

In Pyodide

The browser's event loop is asyncio's event loop. await at the top level Just Works.

Common pitfall

Calling async_fn() without await returns a coroutine object — it does not run. Always await or pass to gather.

Try it

Convert the three fetch_thing calls to run sequentially with await — observe the time difference.
Use asyncio.create_task and check the difference vs gather.

Practice

3 graded exercises

Write the code, click Check. We’ll tell you exactly what to fix.

Progress0 / 3

Exercise 1
Define an async def hello(name) that returns the string "Hello, <name>". Then run it with asyncio.run(hello("Ada")) and print the result. Expected: Hello, Ada.
Exercise 2
Write an async def task(n) that awaits asyncio.sleep(0) and returns n * n. In a main() coroutine, use asyncio.gather(task(2), task(3), task(4)) and print the result list. Expected: [4, 9, 16].
Exercise 3
In an async main coroutine, launch two coroutines that each await asyncio.sleep(0.05) and return their arg. Use asyncio.gather and print the tuple/list of results. Expected includes both "a" and "b".