# Working on this repo

## Tests earn their place by catching a real bug

When a bug is found, ask whether a unit test could reasonably have caught it. If
yes, write that test with the fix. If no — a rendering artefact, a browser
quirk, a data-source oddity — say so and don't add one.

The bar is "would this have failed before the fix, and would it fail again if
someone reintroduced it". Tests that restate the implementation, assert
constructor defaults, or exercise paths nothing depends on are noise; they make
the suite slow to run and expensive to change, which is how a suite stops being
trusted.

What has actually paid off here: bar aggregation and bucket boundaries, the
store's replace-vs-append rules, level and alert arithmetic, parsing real
market-data payloads (fixtures are trimmed real responses, not invented), and
the invariants that would otherwise be silent — a comment must never become a
level, a tick must never overwrite a settled bar, volume must be counted once.

Name the test after the failure, not the function: `test_a_tick_cannot_overwrite
_a_settled_bar` beats `test_put`.

## Verify UI in a real browser

Chart bugs are invisible from the outside — the API, the socket and the
frontend source can each be correct while the screen is wrong. Drive the
Playwright container against the dev stack:

```
docker exec -i chart-playwright-1 node - <<'EOF'
const { chromium } = require('/usr/lib/node_modules/playwright');
// launch with args:['--lang=en-US'] — see below
EOF
```

**Always launch Chromium with `args: ['--lang=en-US']`.** The container has no
usable locale, so Chromium reports `en-US@posix`, `Intl` throws, and the chart
renders as a blank canvas that looks exactly like a broken app.

`window.__chart` is a deliberate debug handle. Querying it separates "the data
is missing" from "the data is off-screen" — which is how a viewport bug that
three passing API checks had missed was finally found.

## Running tests

```
docker exec chart-api-1 sh -c "cd /app && python -m pytest -q"
```

pytest + pytest-asyncio, declared in `requirements-dev.txt`. Tests live in
`tests/`, import from `app.*`, and use `tmp_path` for anything that persists.
Async paths are driven with `asyncio.run(...)` directly rather than async test
markers.

## Things that will cost you an hour

- **Never write scratch `.py` files into the repo root.** It is bind-mounted, so
  `--reload` restarts the app, and startup takes ~82 seconds. Pipe throwaway
  scripts over stdin instead: `docker exec -i chart-api-1 python - <<'EOF'`.
  Screenshots into `artifacts/` are safe; only `.py` triggers the reloader.
- **Dev and production keep separate drawing stores.** Dev writes
  `data/manual_lines.json`; production has its own Coolify volume. A fix that
  "didn't land" is often the other store.
- **Rebuild the image after touching `requirements.txt`.** The bind mount makes
  source edits look live while an added dependency is simply absent.
- **A deploy resets alert cooldowns**, so production may re-alert on whatever
  price is sitting on. There is no durable state yet.
- Times are epoch seconds, UTC, everywhere. Only the display is localised —
  never shift the stored values.