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Level 4 / Project 04 - Data Contract Enforcer

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Estimated time: 50 minutes

Focus

  • contract validation and drift 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-4/04-data-contract-enforcer
python project.py --contract data/contract.json --input data/sample_input.csv --output data/enforcement_report.json
pytest -q

Expected terminal output

{
  "total_rows": 8,
  "clean_rows": 3,
  "violation_count": 5,
  ...
}
6 passed

Expected artifacts

  • data/enforcement_report.json — per-row violation details
  • 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 "pattern" rule (regex) to the contract for email-like fields.
  2. Add a --strict flag that also treats extra columns as violations.
  3. Re-run script and tests — add a test for pattern enforcement.

Break it (required) — Core

  1. Remove a required column entirely from the CSV and see what missing_columns reports.
  2. Feed a value that is technically the right type but fails range AND allowed-values checks simultaneously.
  3. Create a contract with contradictory rules (e.g., min: 100, max: 50) and observe the behavior.

Fix it (required) — Core

  1. Add contract self-validation that catches contradictory rules before enforcement begins.
  2. Handle the case where a column exists in the contract but not in the CSV data headers.
  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 coerce_value try to convert strings instead of checking types directly?
  2. What is the difference between missing_columns and a required field that is empty?
  3. Why are violations collected per-row instead of per-column?
  4. How would this pattern work with a streaming data source (no full CSV in memory)?

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 Data Contract Enforcer. I got this error: [paste error]. Can you explain what this error means without giving me the fix?"
  • "I am trying to detect when a data file's structure changes unexpectedly. Can you explain what 'schema drift' means with a practical example?"
  • "Can you explain how to compare two schemas and identify the differences?"
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