career-progression-2026/week-01-python-internals/day-002-asyncio/week-01-python-internals_day-02-asyncio_notes.md

Day 2 — asyncio fundamentals (event loop, tasks, gather)

Goals (what "done" means)

• I can explain the event loop + coroutines + tasks in plain language.
• I can write a small asyncio program using create_task + gather.
• I can add timeouts, cancellation handling, and concurrency limits.

Reading (official first)

• Read (core usage): Coroutines and Tasks — Python docs
  • Note what each does: asyncio.run, asyncio.create_task, asyncio.gather, cancellation.
• Read (practical walkthrough): Python’s asyncio: A Hands-On Walkthrough (Real Python)
  • Write 5 bullets: when asyncio is a good fit, and when it’s not.
• Read (debugging): Developing with asyncio — Python docs
  • Note 2 debugging tips you can reuse later.

Hands-on (build incrementally)

1. Minimal concurrency demo (no external libs)

• Write async def worker(i): that does await asyncio.sleep(...) and returns a value
• Run N workers concurrently with asyncio.gather
• Print start/end timestamps to show concurrency

2. Add concurrency limit

• Add asyncio.Semaphore(k) and wrap the worker body so only k run at once
• Demonstrate by running N=50 with k=5 and showing batching behavior

3. Add timeouts + error isolation

• Wrap each worker with asyncio.wait_for(..., timeout=...)
• Make some workers intentionally exceed timeout
• Use gather(..., return_exceptions=True) and summarize:
  • successes
  • timeouts
  • other exceptions

4. Cancellation behavior

• Trigger cancellation of a running task and confirm you handle asyncio.CancelledError cleanly
• Write 3 bullets: what cancellation means and how to design for it

Write-up (in notes)

• "Mental model" section (8–10 bullets):
  • coroutine vs task
  • scheduling
  • awaiting vs creating tasks
  • why blocking calls break concurrency
• "Pitfalls" section (at least 5 bullets):
  • blocking I/O
  • CPU-bound work in asyncio
  • forgetting to await
  • unhandled exceptions in tasks
  • cancellation edge cases

Deliverables (to commit)

• notes.md updated with:
  • mental model bullets
  • code snippets or links to your local scripts
  • brief results/observations
• asyncio_demo.py (or similar) added with instructions to run

Suggested commits

• day02: add asyncio notes + minimal gather demo
• day02: add semaphore, timeout, and cancellation examples