Understanding SOLID Principles

SOLID is a set of five design principles that help software developers write maintainable and scalable code. Let's walk through each one.

S — Single Responsibility Principle

A class should have only one reason to change. Each class should encapsulate a single piece of functionality.

# Bad: UserService does too many things
class UserService:
    def create_user(self, data): ...
    def send_email(self, user): ...
    def generate_report(self, user): ...

# Good: Each class has one responsibility
class UserService:
    def create_user(self, data): ...

class EmailService:
    def send_email(self, user): ...

class ReportService:
    def generate_report(self, user): ...

O — Open/Closed Principle

Software entities should be open for extension but closed for modification. Use abstractions to allow new behavior without changing existing code.

L — Liskov Substitution Principle

Subtypes must be substitutable for their base types without altering the correctness of the program.

I — Interface Segregation Principle

Clients should not be forced to depend on interfaces they do not use. Prefer many small, focused interfaces over one large one.

D — Dependency Inversion Principle

High-level modules should not depend on low-level modules. Both should depend on abstractions.

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These principles form the foundation of good object-oriented design and are frequently discussed in system design interviews.