Software Design Principles: Practical Implementation with User Roles

I. Introduction Software design is a fundamental pillar for developing scalable, maintainable, and flexible applications. To achieve this, we use the SOLID principles, a set of best practices in object-oriented programming. In this document, we focus on two specific SOLID principles: Liskov Substitution Principle (LSP) Interface Segregation Principle (ISP) Through a practical implementation based on user roles, we will see how these principles improve the structure and maintainability of the code. II. SOLID Principles The SOLID acronym represents five key principles in object-oriented programming: S - Single Responsibility Principle (SRP): A class should have a single responsibility. O - Open/Closed Principle (OCP): Code should be open for extension but closed for modification. L - Liskov Substitution Principle (LSP): Subclasses should be able to replace their base class without altering functionality. I - Interface Segregation Principle (ISP): Interfaces should be specific rather than general. D - Dependency Inversion Principle (DIP): High-level modules should not depend on low-level modules but on abstractions. In this project, we apply LSP and ISP to design a well-structured user role system. III. Implementing LSP and ISP in a User Role System In a system where users have different roles (Reader, Editor, Administrator), applying LSP and ISP is key to ensuring flexibility and maintainability. A. Applying Liskov Substitution Principle (LSP) The Liskov Substitution Principle states that a subclass should be able to replace its base class without affecting expected behavior. How we apply it in our implementation: We create a base class User with common attributes (name and email). The Reader, Editor, and Admin classes inherit from User, making them interchangeable in the code. Each subclass implements only the necessary functionalities without modifying base behavior. This ensures that we can substitute a User with any subclass (Reader, Editor, Admin) without affecting the overall functionality of the system. B. Applying Interface Segregation Principle (ISP) The Interface Segregation Principle states that it's preferable to have several specific interfaces rather than a single general one with many responsibilities. How we apply it in our implementation: Instead of a single general interface for all users, we create separate interfaces: IReader: For users with reading permissions. IEditor: For users with editing permissions. IAdmin: For users with administrative permissions. Each class implements only the interfaces it needs, avoiding unnecessary methods. This reduces unnecessary dependencies and improves system scalability. IV. Implementation Code from abc import ABC, abstractmethod # Specific Interfaces (ISP) class IUser(ABC): @abstractmethod def get_name(self) -> str: pass class IReader(ABC): @abstractmethod def read_content(self) -> str: pass class IEditor(ABC): @abstractmethod def edit_content(self) -> str: pass class IAdmin(ABC): @abstractmethod def delete_content(self) -> str: pass # Base Class (LSP) class User(IUser): def __init__(self, name): self.name = name def get_name(self) -> str: return self.name # Subclasses applying LSP and ISP class Reader(User, IReader): def read_content(self) -> str: return f"{self.name} is reading." class Editor(Reader, IEditor): def edit_content(self) -> str: return f"{self.name} has edited content." class Admin(Editor, IAdmin): def delete_content(self) -> str: return f"{self.name} has deleted content." V. Conclusion By applying Liskov Substitution Principle (LSP) and Interface Segregation Principle (ISP), we have built a more flexible, scalable, and maintainable user role system. LSP ensures that subclasses can replace their base class without altering the system’s expected behavior. ISP ensures that classes implement only the necessary interfaces, avoiding bloated and unnecessary dependencies. These principles contribute to clean code, facilitating future modifications and reducing complexity in large-scale applications. For more details and source code, visit the GitHub repository: SOLID User Roles.

from abc import ABC, abstractmethod

# Specific Interfaces (ISP)
class IUser(ABC):
    @abstractmethod
    def get_name(self) -> str: pass

class IReader(ABC):
    @abstractmethod
    def read_content(self) -> str: pass

class IEditor(ABC):
    @abstractmethod
    def edit_content(self) -> str: pass

class IAdmin(ABC):
    @abstractmethod
    def delete_content(self) -> str: pass

# Base Class (LSP)
class User(IUser):
    def __init__(self, name): self.name = name
    def get_name(self) -> str: return self.name

# Subclasses applying LSP and ISP
class Reader(User, IReader):
    def read_content(self) -> str: return f"{self.name} is reading."

class Editor(Reader, IEditor):
    def edit_content(self) -> str: return f"{self.name} has edited content."

class Admin(Editor, IAdmin):
    def delete_content(self) -> str: return f"{self.name} has deleted content."

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