Manual Setup: Tutorial 11 — Safe Refactoring with Mikado

1. Create the project skeleton

mkdir -p expense-tracker/src expense-tracker/tests
cd expense-tracker

2. Create src/expenses.py

The expense tracker is intentionally messy — you'll analyze the code smells in Step 2 of the tutorial and refactor them later. Don't clean it up by hand.

# src/expenses.py
import json
from datetime import datetime

class ExpenseManager:

    def __init__(self):
        self.expenses = []
        self.tax_rate = 0.21  # hardcoded tax rate
        self.currency = "USD"  # hardcoded currency

    def add_expense(self, amount, category, description=""):
        if amount <= 0:
            raise ValueError("Amount must be positive")
        if category not in ["food", "transport", "office", "other"]:
            raise ValueError("Invalid category")
        expense = {
            "amount": amount,
            "category": category,
            "description": description,
            "date": datetime.now().isoformat(),
            "amount_with_tax": round(amount * (1 + 0.21), 2),  # duplicated tax rate
        }
        self.expenses.append(expense)
        return expense

    def get_total(self):
        total = 0
        for e in self.expenses:
            total += e["amount"]
        return total

    def get_total_with_tax(self):
        total = 0
        for e in self.expenses:
            total += e["amount"] * (1 + 0.21)  # duplicated tax rate again
        return round(total, 2)

    def get_report(self):
        lines = []
        lines.append("=== Expense Report ===")
        lines.append(f"Currency: USD")  # hardcoded currency again
        for e in self.expenses:
            lines.append(f"  {e['category']:12s} ${e['amount']:.2f}  {e['description']}")
        lines.append(f"  {'SUBTOTAL':12s} ${self.get_total():.2f}")
        lines.append(f"  {'TAX (21%)':12s} ${self.get_total() * 0.21:.2f}")  # duplicated
        lines.append(f"  {'TOTAL':12s} ${self.get_total_with_tax():.2f}")
        lines.append("=" * 22)
        return "\n".join(lines)

    def save_to_file(self, path):
        with open(path, "w") as f:
            json.dump(self.expenses, f, indent=2)

    def load_from_file(self, path):
        with open(path) as f:
            self.expenses = json.load(f)

    def get_by_category(self, category):
        result = []
        for e in self.expenses:
            if e["category"] == category:
                result.append(e)
        return result

    def get_category_total(self, category):
        total = 0
        for e in self.expenses:
            if e["category"] == category:
                total += e["amount"]
        return total

    def get_category_total_with_tax(self, category):
        total = 0
        for e in self.expenses:
            if e["category"] == category:
                total += e["amount"] * (1 + 0.21)  # duplicated tax rate yet again
        return round(total, 2)

3. Create tests/test_expenses.py

# tests/test_expenses.py
import pytest
from src.expenses import ExpenseManager

@pytest.fixture
def manager():
    em = ExpenseManager()
    em.add_expense(10.00, "food", "Lunch")
    em.add_expense(25.50, "transport", "Taxi")
    em.add_expense(5.00, "food", "Coffee")
    return em

def test_add_expense(manager):
    assert len(manager.expenses) == 3

def test_add_expense_negative_raises():
    em = ExpenseManager()
    with pytest.raises(ValueError, match="positive"):
        em.add_expense(-5, "food")

def test_add_expense_invalid_category():
    em = ExpenseManager()
    with pytest.raises(ValueError, match="Invalid category"):
        em.add_expense(10, "vacation")

def test_get_total(manager):
    assert manager.get_total() == 40.50

def test_get_total_with_tax(manager):
    # The mathematical answer is 49.005; IEEE 754 rounding can land on
    # either 49.00 or 49.01 depending on the platform — use approx to be robust.
    assert manager.get_total_with_tax() == pytest.approx(49.005, abs=0.01)

def test_get_by_category(manager):
    food = manager.get_by_category("food")
    assert len(food) == 2

def test_get_category_total(manager):
    assert manager.get_category_total("food") == 15.00

def test_get_category_total_with_tax(manager):
    assert manager.get_category_total_with_tax("food") == 18.15

def test_get_report(manager):
    report = manager.get_report()
    assert "Expense Report" in report
    assert "TOTAL" in report
    # Compute the expected string dynamically — the report formats whatever
    # get_total_with_tax() returns (49.00 or 49.01 depending on float rounding).
    assert f"${manager.get_total_with_tax():.2f}" in report

4. Create conftest.py

So pytest can find the src module:

echo 'import sys; sys.path.insert(0, ".")' > conftest.py

5. Create the virtualenv and install pytest

python3 -m venv .venv
source .venv/bin/activate
pip install pytest

Windows users: Replace source .venv/bin/activate with .venv\Scripts\activate.

6. Initialize git with the messy starting state

echo ".venv/" > .gitignore
echo "__pycache__/" >> .gitignore
git init
git add -A
git commit -m "feat: messy expense tracker (starting point for refactoring)"

Verify

Run the tests — all 9 should pass:

pytest tests/ -v --no-header

You should now have:

• An expense-tracker/ directory with src/expenses.py, tests/test_expenses.py, conftest.py, pyproject-free Python layout
• A .venv/ virtual environment with pytest installed
• A clean git repository with one initial commit
• 9 passing tests

You're ready to start the refactoring tutorial.