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Level 1 / Project 09 - JSON Settings Loader

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Concept This project — Quiz Flashcards — Browser

Estimated time: 30 minutes

Focus

  • load config and fallback behavior

Why this project exists

Load application settings from a JSON file, merge them with sensible defaults, and validate required keys. You will learn json.loads(), dictionary merging with {**defaults, **overrides}, and config validation patterns.

Run (copy/paste)

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

cd <repo-root>/projects/level-1/09-json-settings-loader
python project.py --input data/sample_input.txt
pytest -q

Expected terminal output

=== Settings Loader ===

  app_name: LearnPython (from file)
  debug:    True (from file)
  port:     8080 (default)
  host:     localhost (default)

  Loaded 4 settings (2 from file, 2 defaults)
5 passed

Expected artifacts

  • data/output.json
  • 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 whether all required keys exist without running the main logic.
  2. Add environment variable overrides (e.g. APP_PORT=9090 overrides "port": 8080 from the file).
  3. Re-run script and tests.

Break it (required) — Core

  1. Use a settings file with invalid JSON (missing a closing brace) -- does load_json() raise ValueError?
  2. Use a settings file missing a required key like "host" -- does validate_settings() catch it?
  3. Use a file that exists but is completely empty -- does the loader crash or fall back to defaults?

Fix it (required) — Core

  1. Ensure load_json() wraps json.loads() in a try/except and raises ValueError with context.
  2. Handle empty files by treating them as empty dicts (fall back to all defaults).
  3. Add a test for the empty-file case.

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

Explain it (teach-back)

  1. Why does merge_settings() create a new dict with {**defaults, **overrides} instead of modifying in place?
  2. What does json.loads() do and how does it differ from json.load() (with no 's')?
  3. Why keep a DEFAULTS dict at module level instead of hardcoding defaults inside functions?
  4. Where would settings loaders appear in real software (web servers, CLI tools, microservices)?

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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