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Level 5 / Project 12 - Fail Safe Exporter

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— This project — — Flashcards — Browser

Estimated time: 85 minutes

Focus

  • atomic writes and export integrity

Why this project exists

This project gives you level-appropriate practice in a realistic operations context. Goal: run the baseline, alter behavior, break one assumption, recover safely, and explain the fix.

Run (copy/paste)

Use <repo-root> as the folder containing this repository's README.md.

cd <repo-root>/projects/level-5/12-fail-safe-exporter
python project.py --input data/sample_input.json --output data/exported.json --format json
python project.py --input data/sample_input.json --output data/exported.csv --format csv
pytest -q

Expected terminal output

Exported 4 records to data/exported.json (atomic write)
Exported 4 records to data/exported.csv (atomic write)
6 passed

Expected artifacts

  • data/exported.json or data/exported.csv
  • Passing tests
  • Updated notes.md

Checkpoint: Baseline code runs and all tests pass. Commit your work before continuing.

Alter it (required) — Extension

  1. Add a --validate flag that checks each record has required fields before exporting.
  2. Add a backup: before overwriting, copy the existing file to .bak.
  3. Add a --dry-run flag that validates and reports but does not write.
  4. Re-run script and tests.

Break it (required) — Core

  1. Pass an output path to a read-only directory (or a path with invalid characters).
  2. Pass input data where records have inconsistent keys (some missing columns).
  3. Capture the first failing test or visible bad output.

Fix it (required) — Core

  1. Catch OSError / PermissionError during atomic write and report clearly.
  2. Pad missing keys with empty strings during CSV export.
  3. Add tests for write failures and inconsistent records.
  4. Re-run until output and tests are deterministic.

Checkpoint: All modifications done, tests still pass. Good time to review your changes.

Explain it (teach-back)

  1. Why does atomic_write_json write to a .tmp file and then rename?
  2. What happens if the process crashes between writing .tmp and renaming?
  3. How does atomic_write_csv determine CSV headers from the records?
  4. Where do you see atomic writes in production (databases, log rotation, config updates)?

Mastery check

You can move on when you can: - run baseline without docs, - explain one core function line-by-line, - break and recover in one session, - keep tests passing after your change.

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