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Level 3 / Project 05 - Refactor Monolith Drill

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Try in Browser: Practice similar concepts online — browser exercises cover Level 2 topics

Before You Start

Recall these prerequisites before diving in: - Can you move a function from one file to another and update the import? - Can you explain what "separation of concerns" means?

Estimated time: 40 minutes

Learn Your Way

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— This project — — Flashcards — Browser

Focus

  • split large function into modules

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-3/05-refactor-monolith-drill
python project.py data/sample_input.txt
python project.py data/sample_input.txt --json
python project.py data/sample_input.txt --department Engineering
pytest -q

Expected terminal output

Company Report: 10 employees, $849,000.00 total payroll
===========================================================
Engineering (4 people)
  Salary range: $88,000 - $115,000
  ...
10 passed

Expected artifacts

  • Formatted report on stdout
  • 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 --sort-by flag (headcount, salary, tenure) to order departments.
  2. Add a median salary calculation to DepartmentStats.
  3. Add a --top-earners N flag to show the top N earners across all departments.

Break it (required) — Core

  1. Use a CSV with a missing salary column — what error appears?
  2. Pass a non-numeric value in the salary column — does it crash or skip?
  3. Filter to a department that doesn't exist — what happens?

Fix it (required) — Core

  1. Add validation that all required CSV columns exist before processing.
  2. Handle non-numeric salary/years values gracefully (skip with warning).
  3. Show a clear message when --department matches nothing.

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

Explain it (teach-back)

  1. What is the "monolith" anti-pattern and why is it hard to test?
  2. How does decomposing into parse -> group -> compute -> format help?
  3. Why is parse_csv separate from load_employees?
  4. What is the Single Responsibility Principle?

Mastery check

You can move on when you can: - decompose a large function into small, testable units, - explain the Single Responsibility Principle, - test each function independently, - build a data pipeline with clear step separation.

Stuck? Ask AI

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

  • "I am working on Refactor Monolith Drill. I got this error: [paste error]. Can you explain what this error means without giving me the fix?"
  • "I have a large function (100+ lines) and need to split it into smaller functions. Can you give me a strategy for deciding where to split?"
  • "Can you explain relative imports in Python packages with a simple two-file example?"
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