### 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](https://docs.python.org/3/library/asyncio-task.html)
  - [ ] Note what each does: `asyncio.run`, `asyncio.create_task`, `asyncio.gather`, cancellation.
- [ ] Read (practical walkthrough): [Python’s asyncio: A Hands-On Walkthrough (Real Python)](https://realpython.com/async-io-python/)
  - [ ] Write 5 bullets: when asyncio is a good fit, and when it’s not.
- [ ] Read (debugging): [Developing with asyncio — Python docs](https://docs.python.org/3/library/asyncio-dev.html#debug-mode)
  - [ ] 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`