Gilded Rose Kata in Python

This is a refactoring exercise based on the famous GildedRose-Refactoring-Kata repository maintained by Emily Bache. This kata contains simple code in many different programming languages that need to be refactored and improve code quality.

The source program has two classes GildedRose and Item. The Item class describes the properties of an item and contains the attributes – name, sell_in, and quality. The GildedRose class has a method update_quality() that updates the sell_in date and quality of the items on a regular basis.

Requirements

The existing codebase provides support for three products, however, the addition of a new product Conjured breaks the existing business logic and hence needs remodeling. There are a few business rules and constraints that need to be taken care of while refactoring the code.

Business Rules

1. All items will have the three attributes as follows:
   1. name – Name of the item.
   2. sell_in – The number of days within which the item should be sold off.
   3. quality – The quality of the item; denoted by an integer.
2. Every day, the method update_quality() is executed, which lowers the values for sell_in and quality each.
3. Once the sell_in date has passed, the value for quality degrades twice faster.
4. There are also rules for each of the items that are currently present which are as follows.
   1. AgedBrie increases its quality with a decrease in sell_in value.
   2. BackstagePasses has the following two rules:
      1. When sell_in value is more than 10, quality increases by 1.
      2. When sell_in value is less than 10, quality increases by 2.
      3. When sell_in value is less than 5, quality increases by 3.
      4. When sell_in value is less than 0, quality is set to 0.
   3. Sulfuras is legendary, so quality is always 80 and sell_in value never decreases.
   4. A new item – Conjured is to be added with the following rule:
      1. Conjured items degrade quality twice as fast as normal items.

Constraints

There are a few constraints that we need to consider before refactoring.

1. We are not allowed to alter the Item class or the attributes of the Item class.
2. The minimum and maximum values for quality can only be 0 and 50 respectively.

For more detailed requirements, you can refer to the original text.

Reason to Refactor

If you browse through the existing code, you can see that all the business rules are implemented in a single class and inside a complex nested if-else statement. Following are a few reasons why I think the code should be refactored and developed in a modular way.

1. Readability – The current logic is very difficult to read as everything is contained in a single class with multiple nested if-else statements.
2. Duplicate logic – There are a few logics that are duplicated. This can be written in a better way by using a parent-child class relationship.
3. Implementation of the new item Conjured will need to modify the current if-else statements and that might cause harm to the existing logic.

Implementation

My approach to implement and refactor the program would be as follows.

1. Writing extensive unit test cases to preserve and validate the existing business logic.
2. For the codebases to be modular, I would break down the project structure into multiple files as follows under the app directory.
   1. gilded_rose.py – This is the refactored base file that contains the GildedRose class and the update_quality() method. I have removed the complex if-else statements from this class and implemented them separately.
   2. product_list.py – This file contains the ProductList class which is an enumerated list of all the available items. New items can be added to the list without affecting the business logic for other products.
   3. item.py – This is the definition of the class Item. It is the parent class of any item and contains methods to update the values for sell_in and quality. Any regular product which satisfies the normal business rules can be inherited from this class and if there are some specific rules, then the inherited child classes can modify the logic for the update_quality() method accordingly.
   4. products.py – This file contains a Product class that will basically route the call to the respective product classes when the instances are created. In addition to that, I have added a separate class for each of the existing items that do not follow the normal business rules. All these child classes inherit from the parent Item class.

UML Class Diagram

The UML class diagram of the refactored code is provided below.

Test Cases

In order to validate the existing logic and new business rules, several unit test cases have been written which are found under the tests directory. It contains several test files as follows.

Executing the test cases

First, you need to create a python virtual environment for the program.

python -m venv .venv
source .venv/bin/activate
pip install --upgrade pip

This will start the virtual environment and update the pip. Once the virtual environment is up and running, the test cases can be executed from the terminal as follows.

python -m unittest discover tests

If the tests are configured within the IDE (VSCode or PyCharm), then the tests can be executed from within the IDE itself.