Introduction
Software design is a crucial discipline in the development of robust applications and systems. There are various principles that guide how to structure the code effectively to ensure quality, maintainability, and scalability. These principles not only ensure the software functions correctly but also facilitate its evolution over time.

In this article, we will explore some of the most well-known and widely applied software design principles, with a real-world example in Python.

Common Software Design Principles

  • Single Responsibility Principle (SRP):
    This principle states that a class or module should have only one reason to change. In other words, each component should be responsible for only one functionality.

  • Open/Closed Principle (OCP):
    A software entity (class, module, function) should be open for extension but closed for modification. This allows you to add new features without altering existing code.

  • Liskov Substitution Principle (LSP):
    Derived classes should be able to substitute their base classes without affecting the correctness of the program.

  • Interface Segregation Principle (ISP):
    Clients should not be forced to depend on interfaces they do not use. This encourages the creation of smaller, more focused interfaces.

  • Dependency Inversion Principle (DIP):
    High-level modules should not depend on low-level modules. Both should depend on abstractions. Abstractions should not depend on details. Details should depend on abstractions.

Real-World Analogy
Imagine a car manufacturing company that designs different car models. Each model has specific components, such as engines, wheels, and transmissions. The company applies design principles to ensure that these components can be easily modified or replaced without disrupting the entire manufacturing process. For example, if a new type of engine is introduced, the car models should be able to adapt without significant changes to the overall structure.

Similarly, in software design, applying these principles helps ensure that the system remains flexible and maintainable, even as requirements change or new features are added.

Real-World Example in Code
Let’s look at a simple example in Python where we apply some of these principles. Imagine we’re developing a simple system for processing bank transactions. Initially, we might start with a simple, monolithic function. As we grow the system, we apply design principles to improve its structure.

account.py

class Account:
    def __init__(self, owner, balance):
        self.owner = owner
        self.balance = balance

    def deposit(self, amount):
        if amount <= 0:
            raise ValueError("Deposit amount must be positive.")
        self.balance += amount

    def withdraw(self, amount):
        if amount <= 0:
            raise ValueError("Withdrawal amount must be positive.")
        if self.balance < amount:
            raise ValueError("Insufficient funds.")
        self.balance -= amount

transaction_processor.py

from account import Account  
class TransactionProcessor:
    def __init__(self, account):
        self.account = account

    def process_transaction(self, transaction_type, amount):
        if transaction_type == "deposit":
            self.account.deposit(amount)
        elif transaction_type == "withdraw":
            self.account.withdraw(amount)
        else:
            raise ValueError("Invalid transaction type.")
        print(f"Transaction successful! New balance: {self.account.balance}")

# Example of usage
account = Account("Alice", 1000)
transaction_processor = TransactionProcessor(account)

transaction_processor.process_transaction("deposit", 200)
transaction_processor.process_transaction("withdraw", 100)

Explanation

We have a Account class that handles account-related operations like deposit and withdraw.

We separate the responsibility of processing transactions into a TransactionProcessor class. This helps adhere to the Single Responsibility Principle (SRP) because each class has one distinct responsibility.

The Open/Closed Principle (OCP) is adhered to, as we can easily add new transaction types without modifying the existing classes.

The Liskov Substitution Principle (LSP) is also respected, as we could replace or extend the Account class with subclasses (e.g., SavingsAccount) without breaking the functionality.

Conclusion
Software design principles help ensure that code is not only functional but also maintainable and scalable. Applying principles like SRP, OCP, LSP, ISP, and DIP enables developers to write code that can easily adapt to new requirements and changes. As systems grow more complex, these principles become essential to managing the complexity and ensuring that the software remains flexible and easy to maintain.

You can find the full code for this article in my GitHub repository: https://github.com/fabipm/Research01_SI889_U1_Poma