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Level 3 / Project 10 - Dependency Boundary Lab

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— This project — — Flashcards — Browser

Estimated time: 50 minutes

Focus

  • separate io from business logic

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-3/10-dependency-boundary-lab
python project.py records.json output.json --config config.json
pytest -q

Expected terminal output

Processed: 4 in, 3 out, 1 filtered
10 passed

Expected artifacts

  • Processed JSON output file
  • 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 sort_by config option to sort output records by a field.
  2. Add a CsvFileReader that reads CSV instead of JSON.
  3. Add a --dry-run flag that shows what would be processed without writing.

Break it (required) — Core

  1. Pass a config with a rename map that creates duplicate keys — what happens?
  2. Pass a JSON file that contains a string instead of an array — what error appears?
  3. Use InMemoryReader with no records — does the pipeline handle empty input?

Fix it (required) — Core

  1. Validate config keys before running the pipeline.
  2. Handle non-list JSON input by wrapping in a list.
  3. Add clear error messages when reader/writer operations fail.

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

Explain it (teach-back)

  1. What is "dependency inversion" and why does it make code testable?
  2. How do InMemoryReader/InMemoryWriter replace real files in tests?
  3. Why is the run() orchestrator the ONLY place that touches I/O?
  4. What is a Python Protocol and how does it define an interface?

Mastery check

You can move on when you can: - separate I/O from business logic, - test business logic without touching the filesystem, - use the Protocol pattern for dependency boundaries, - build a configurable data processing pipeline.

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