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Level 5 / Project 04 - Config Layer Priority

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— This project — — Flashcards — Browser

Estimated time: 65 minutes

Focus

  • defaults, env, and cli precedence

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-5/04-config-layer-priority
python project.py --config data/config.json --output data/resolved_config.json
# Try with env override:
APP_LOG_LEVEL=DEBUG python project.py --config data/config.json --output data/resolved_config.json
pytest -q

Expected terminal output

Resolved config: 5 keys from 3 layers
6 passed

Expected artifacts

  • data/resolved_config.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 --set flag that allows setting individual keys from the CLI (e.g. --set log_level=TRACE).
  2. Add a --dump-sources mode that prints which layer each final value came from.
  3. Support nested config keys using dot notation (e.g. database.host).
  4. Re-run script and tests.

Break it (required) — Core

  1. Set an environment variable with a non-numeric value for a key that expects an integer (e.g. APP_MAX_CONNECTIONS=abc).
  2. Reference a config file that does not exist.
  3. Capture the first failing test or visible bad output.

Fix it (required) — Core

  1. Add type coercion with error handling that logs a warning and falls back to the default.
  2. Handle missing config file gracefully (use defaults only).
  3. Add tests for bad env var types and missing files.
  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 is env > file > defaults the standard precedence order?
  2. What happens when coerce_types encounters a value it cannot convert?
  3. How does os.environ.get with a prefix prevent collisions with other apps?
  4. Where do you see layered configuration in production systems (12-factor apps, Kubernetes)?

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.

Stuck? Ask AI

If you are stuck after trying for 20 minutes, use one of these prompts:

  • "I am working on Config Layer Priority. I got this error: [paste error]. Can you explain what this error means without giving me the fix?"
  • "I am trying to merge configuration from defaults, environment variables, and CLI arguments with proper precedence. Can you explain config layering with a simple example?"
  • "Can you explain how os.environ.get() works and when to use it vs os.environ[]?"
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