📐 SOLID Principles Explained Like You're 5

Five rules for maintainable code

Day 115 of 149

The LEGO Blocks Analogy

Good LEGO pieces:

Do one thing well (a wheel is just a wheel)
Can be extended without breaking
Are interchangeable (any wheel fits any axle)
Connect through standard interfaces

SOLID principles make code like well-designed LEGO - modular and maintainable.

The Five Principles

Letter	Principle	Meaning
S	Single Responsibility	One class, one job
O	Open/Closed	Open for extension, closed for modification
L	Liskov Substitution	Subtypes must be substitutable
I	Interface Segregation	Many small interfaces beat one big one
D	Dependency Inversion	Depend on abstractions, not concretions

Quick Examples

Single Responsibility

# Bad: User class does everything
class User:
    def save(self): ...
    def send_email(self): ...
    def generate_report(self): ...

# Good: Separate responsibilities
class User: ...
class UserRepository: ...
class EmailService: ...

Dependency Inversion

# Bad: Hardcoded dependency
class UserService:
    def __init__(self):
        self.db = MySQLDatabase()  # Locked in!

# Good: Inject abstraction
class UserService:
    def __init__(self, database: Database):
        self.db = database  # Flexible!