Expense report refactoring kata in Python

Practice the Open-Closed Principle by refactoring an expense report without changing its behavior. Setup is complete when all four starter tests pass.

Overview

This kata complements Clean Code: SOLID, Ep. 10 - Open-Closed Principle.

In this exercise, you'll practice refactoring code to adhere to the Open-Closed Principle (OCP). You'll be working with an expense report system that tracks different types of expenses and prints a final report.

Instructions

Exercise 1

The ExpenseReport class in the expense_report.py file generates an expense report. It iterates over a list of expenses, printing out a line for each expense and totaling up the amounts.

The current implementation, however, is complex and hard to understand, and it's not easy to modify or extend its behavior. It's your task to refactor this code to make it cleaner and more maintainable, while ensuring that it still correctly calculates and prints the report.

Make sure the program still behaves the same way after your refactoring. The unit test suite checks this at a rudimentary level by examining only the program's output.

Exercise 2

When you're done refactoring, test the quality of your refactoring by implementing two additional scenarios:

1. Extend our system to handle two more types of expenses.

   1. Transportation. Surcharge: 5% of the expense amount.
   2. Supplies. Surcharge: No surcharge.

2. Add dynamic surcharges based on the day of the week.

   • For dinners on weekends, the surcharge is 15% of the expense amount.
   • For breakfasts on weekends, the surcharge is 10% of the expense amount.
   • On weekdays, the surcharges remain the same as before (10% for dinner and 5% for breakfast).

   You may need to add a date field to the Expense class to support this requirement. The field should record when the expense occurred.

Tip

You can use the datetime module in Python to work with dates, and you can determine if a date is a weekend by using the following function:

from datetime import datetime


def is_weekend(date_str):
    date = datetime.strptime(date_str, "%Y-%m-%d")
    day_of_week = date.weekday()
    # If day of week is 5 or 6 (Saturday or Sunday), it's a weekend
    return day_of_week >= 5

date_str = "2023-06-17"  # this is a Saturday
print(is_weekend(date_str))  # this should print: True

Prerequisites

Required:

• Git
• uv

Optional:

• GNU Make, for shorter commands. Every required task also has a direct uv command.

You do not need to install Python or pytest separately. uv installs a compatible Python version and the locked project dependencies when needed.

Set up the kata

1. Clone the repository:

   git clone https://github.com/Coding-Cuddles/expense-report-refactoring-python-kata.git

2. Enter the repository directory:

   cd expense-report-refactoring-python-kata

3. Run the tests. Use Make when it is installed:

   make test

   Otherwise, run pytest through uv directly:

   uv run pytest

   The first run may install Python and the project dependencies. Setup is complete when pytest reports 4 passed.

   If the command fails with uv: command not found, install uv and repeat this step.

Work on the kata

1. Refactor ExpenseReport in expense_report.py without changing the report output.

2. Run the tests after each change. Use Make when it is installed:

   make test

   Otherwise, run pytest through uv directly:

   uv run pytest

   Continue when the test run completes without failures.

Make command reference

Make is optional. Run make or make help to list these commands in the terminal.

Command	Result
make all	Run the test suite
make help	Show the command reference
make test	Run the test suite
make format	Format tracked Python files
make format-check	Check formatting without changing files
make clean	Remove generated caches