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Module 05 / Project 01 — Async Basics

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— This project Walkthrough — Flashcards — —

Focus

  • async def to define coroutines
  • await to pause and resume
  • asyncio.run() to start the event loop
  • asyncio.sleep() vs time.sleep()
  • asyncio.gather() to run tasks concurrently

Why this project exists

Before you can use async libraries (aiohttp, FastAPI background tasks, etc.), you need to understand what async and await actually do. This project builds that foundation with simple, visible examples.

Run

cd projects/modules/05-async-python/01-async-basics
python project.py

Expected output

--- Sequential execution ---
Task A starting...
Task A done after 2 seconds
Task B starting...
Task B done after 1 second
Task C starting...
Task C done after 3 seconds
Sequential total: ~6 seconds

--- Concurrent execution ---
Task A starting...
Task B starting...
Task C starting...
Task B done after 1 second
Task A done after 2 seconds
Task C done after 3 seconds
Concurrent total: ~3 seconds

Alter it

  1. Add a fourth task that takes 1.5 seconds. Run it both ways and compare times.
  2. Change gather() to use asyncio.create_task() instead. What changes?
  3. Add return_exceptions=True to gather() and make one task raise an error.

Break it

  1. Call an async function without await. What happens?
  2. Use time.sleep() instead of asyncio.sleep(). Does concurrency still work? Why not?
  3. Try to call asyncio.run() from inside an already-running event loop.

Fix it

  1. Replace time.sleep() with asyncio.sleep() to restore concurrency.
  2. Add try/except around tasks to handle errors without crashing the whole group.
  3. Use asyncio.wait() with return_when=FIRST_COMPLETED instead of gather().

Explain it

  1. What is a coroutine and how is it different from a regular function?
  2. Why does time.sleep() break concurrency but asyncio.sleep() does not?
  3. What does the event loop actually do?
  4. When would you use async instead of threads?

Mastery check

You can move on when you can: - explain the difference between async def and def, - use gather() to run multiple coroutines concurrently, - predict the output order of concurrent tasks, - explain why await is required.

Next

Project 02 — Concurrent Requests