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Level 5 / Project 14 - Change Detection Tool

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— This project — — Flashcards — Browser

Estimated time: 90 minutes

Focus

  • baseline vs current delta detection

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-5/14-change-detection-tool
python project.py --old data/old_version.txt --new data/new_version.txt --output data/change_report.json
pytest -q

Expected terminal output

Change detection: modified
7 passed

Expected artifacts

  • data/change_report.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 --summary-only flag that prints just the status and counts, not the full diff.
  2. Add percentage change calculation: what fraction of lines were added/removed.
  3. Support comparing entire directories (detect new, deleted, and modified files).
  4. Re-run script and tests.

Break it (required) — Core

  1. Compare two binary files (e.g. images) and observe what happens to line_diff.
  2. Compare a file against itself (both --old and --new pointing to the same path).
  3. Capture the first failing test or visible bad output.

Fix it (required) — Core

  1. Detect binary files (by checking for null bytes) and skip line-level diffing.
  2. Short-circuit when old and new paths are identical (status = "unchanged").
  3. Add tests for binary detection and same-path comparison.
  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. How does file_hash use SHA-256 to detect whether content changed?
  2. Why does line_diff use sets instead of comparing line-by-line in order?
  3. What information does the "modified" status include beyond just "changed"?
  4. Where do you see change detection in production (git, config drift, file integrity monitoring)?

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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