For the chapters in this book, you will need a running Python 3.12 environment, or for some exceptional cases in some chapters, 3.11.

In addition, install the Mypy static type checker (https://www.mypy-lang.org) by running the following:

python3.12 –m pip install -–user mypy

The examples are available in the GitHub repository here: https://github.com/PacktPublishing/Mastering-Python-Design-Patterns-Third-Edition

About the Python executable

Throughout the book, we will reference the Python executable for executing code examples as python3.12 or python. Adapt that to your specific environment, practice and/or workflow.

One of the most common challenges in software development is dealing with change. Requirements evolve, technologies advance, and user needs also change. Therefore, it is crucial to write code that can adapt without causing a ripple effect of modifications throughout your program or application. This is where the principle of Encapsulate What Varies comes into play.

What does it mean?

The idea behind this principle is straightforward: isolate the parts of your code that are most likely to change and encapsulate them. By doing so, you create a protective barrier that shields the rest of your code from these elements that are subject to change. This encapsulation allows you to make changes to one part of your system without affecting others.

Benefits

Encapsulating what varies provides several benefits, mainly the following:

• Ease of maintenance: When changes are needed, you must only modify the encapsulated parts, reducing...

In OOP, it’s tempting to create complex hierarchies of classes through inheritance. While inheritance has its merits, it can lead to tightly coupled code that is hard to maintain and extend. This is where the principle of Favor Composition Over Inheritance comes into the picture.

What does it mean?

This principle advises that you should prefer composing objects from simpler parts to inheriting functionalities from a base class. In other words, build complex objects by combining simpler ones.

Benefits

Choosing composition over inheritance offers several advantages:

• Flexibility: Composition allows you to change objects’ behavior at runtime, making your code more adaptable
• Reusability: Smaller, simpler objects can be reused across different parts of your application, promoting code reusability
• Ease of maintenance: With composition, you can easily swap out or update individual components...

In software design, it’s easy to get caught up in the specifics of how a feature is implemented. However, focusing too much on implementation details can lead to code that is tightly coupled and difficult to modify. The principle of Program to Interfaces, Not Implementations offers a solution to this problem.

What does it mean?

An interface defines a contract for classes, specifying a set of methods that must be implemented.

This principle encourages you to code against an interface rather than a concrete class. By doing so, you untie your code from the specific classes that provide the required behavior, making it easier to swap or extend implementations without affecting the rest of the system.

Benefits

Programming to interfaces offers several benefits:

• Flexibility: You can easily switch between different implementations without altering the code that uses them
• Maintainability: Losing...

As software grows in complexity, the relationships between its components can become tangled, leading to a system that is hard to understand, maintain, and extend. The principle of Loose Coupling aims to mitigate this issue.

What does it mean?

Loose coupling refers to minimizing the dependencies between different parts of a program. In a loosely coupled system, components are independent and interact through well-defined interfaces, making it easier to make changes to one part without affecting others.

Benefits

Loose coupling offers several advantages:

• Maintainability: With fewer dependencies, it’s easier to update or replace individual components
• Extensibility: A loosely coupled system can be more easily extended with new features or components
• Testability: Independent components are easier to test in isolation, improving the overall quality of your software

Techniques for loose coupling

Two primary...

We began the book with the foundational design principles that developers should follow for writing maintainable, flexible, and robust software. From encapsulating what varies to favoring composition, programming to interfaces, and aiming for loose coupling, these principles provide a strong foundation for any Python developer.

As you’ve seen, these principles are not just theoretical constructs but practical guidelines that can significantly improve the quality of your code. They set the stage for what comes next: diving deeper into more specialized sets of principles that guide object-oriented design.

In the next chapter, we will delve into the SOLID principles, a set of five design principles aimed at making software designs more understandable, flexible, and maintainable.