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Level 4 / Project 06 - Backup Rotation Tool

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Estimated time: 55 minutes

Focus

  • retention windows and cleanup policies

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-4/06-backup-rotation-tool
# Create sample backup files first:
mkdir -p data/backups
for d in $(seq 1 20); do touch "data/backups/backup_2025-01-$(printf '%02d' $d).tar.gz"; done
python project.py --backup-dir data/backups --output data/rotation_report.json --daily 7 --weekly 4 --monthly 6
pytest -q

Expected terminal output

{
  "keep": [ ... ],
  "delete": [ ... ],
  "summary": { "total": 20, "keeping": 11, "deleting": 9, ... }
}
5 passed

Expected artifacts

  • data/rotation_report.json — keep/delete lists with retention reasons
  • 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 --execute flag that actually deletes the files marked for deletion (default: plan only).
  2. Add a --min-keep safety net that refuses to delete if fewer than N backups would remain.
  3. Re-run script and tests — add a test for the min-keep guard.

Break it (required) — Core

  1. Create backup files with unparseable names (no date) and confirm they land in unparseable.
  2. Set --daily 0 --weekly 0 --monthly 0 and observe whether ALL backups are scheduled for deletion.
  3. Create two backups on the same day and verify only one is counted for the daily slot.

Fix it (required) — Core

  1. Add a confirmation prompt before actual deletion (when --execute is used).
  2. Handle the edge case of monthly retention when months vary in length (28-31 days).
  3. Re-run until all tests pass.

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

Explain it (teach-back)

  1. Why does classify_backups take now as a parameter instead of calling datetime.now() internally?
  2. What is the purpose of kept_set — why not just check the keep list directly?
  3. Why does weekly retention use ISO week numbers instead of just counting 7-day intervals?
  4. How would this pattern scale to thousands of backup files?

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 Backup Rotation Tool. I got this error: [paste error]. Can you explain what this error means without giving me the fix?"
  • "I am trying to implement a retention policy that keeps daily backups for 7 days and weekly for 4 weeks. Can you explain the logic for deciding which files to keep vs delete?"
  • "Can you explain how to sort files by modification time using pathlib and os.stat?"
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