Table of Contents

• KISS (Keep It Simple, Stupid)
• YAGNI (You Aren't Gonna Need It)
• SOLID Principles
• Quick Reference
• When Principles Conflict
• Integration with Code Review

Code Quality Principles

Guidance on KISS, YAGNI, and SOLID principles with language-specific examples.

When To Use

• Improving code readability and maintainability
• Applying SOLID, KISS, YAGNI principles during refactoring

When NOT To Use

• Throwaway scripts or one-time data migrations
• Performance-critical code where readability trades are justified

KISS (Keep It Simple, Stupid)

Principle: Avoid unnecessary complexity. Prefer obvious solutions over clever ones.

Guidelines

Prefer	Avoid
Simple conditionals	Complex regex for simple checks
Explicit code	Magic numbers/strings
Standard patterns	Clever shortcuts
Direct solutions	Over-abstracted layers

Python Example

# Bad: Overly clever one-liner
users = [u for u in (db.get(id) for id in ids) if u and u.active and not u.banned]

# Good: Clear and readable
users = []
for user_id in ids:
    user = db.get(user_id)
    if user and user.active and not user.banned:
        users.append(user)

Rust Example

// Bad: Unnecessary complexity
fn process(data: &[u8]) -> Result<Vec<u8>, Box<dyn std::error::Error>> {
    data.iter()
        .map(|&b| b.checked_add(1).ok_or("overflow"))
        .collect::<Result<Vec<_>, _>>()
        .map_err(|e| e.into())
}

// Good: Simple and clear
fn process(data: &[u8]) -> Result<Vec<u8>, &'static str> {
    let mut result = Vec::with_capacity(data.len());
    for &byte in data {
        result.push(byte.checked_add(1).ok_or("overflow")?);
    }
    Ok(result)
}

YAGNI (You Aren't Gonna Need It)

Principle: Don't implement features until they are actually needed.

Guidelines

Do	Don't
Solve current problem	Build for hypothetical futures
Add when 3rd use case appears	Create abstractions for 1 use case
Delete dead code	Keep "just in case" code
Minimal viable solution	Premature optimization

Python Example

# Bad: Premature abstraction for one use case
class AbstractDataProcessor:
    def process(self, data): ...
    def validate(self, data): ...
    def transform(self, data): ...

class CSVProcessor(AbstractDataProcessor):
    def process(self, data):
        return self.transform(self.validate(data))

# Good: Simple function until more cases appear
def process_csv(data: list[str]) -> list[dict]:
    return [parse_row(row) for row in data if row.strip()]

TypeScript Example

// Bad: Over-engineered config system
interface ConfigProvider<T> {
  get<K extends keyof T>(key: K): T[K];
  set<K extends keyof T>(key: K, value: T[K]): void;
  watch<K extends keyof T>(key: K, callback: (v: T[K]) => void): void;
}

// Good: Simple config for current needs
const config = {
  apiUrl: process.env.API_URL || 'http://localhost:3000',
  timeout: 5000,
};

SOLID Principles

Single Responsibility Principle

Each module/class should have one reason to change.

# Bad: Multiple responsibilities
class UserManager:
    def create_user(self, data): ...
    def send_welcome_email(self, user): ...  # Email responsibility
    def generate_report(self, users): ...     # Reporting responsibility

# Good: Separated responsibilities
class UserRepository:
    def create(self, data): ...

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

class UserReportGenerator:
    def generate(self, users): ...

Open/Closed Principle

Open for extension, closed for modification.

# Bad: Requires modification for new types
def calculate_area(shape):
    if shape.type == "circle":
        return 3.14 * shape.radius ** 2
    elif shape.type == "rectangle":
        return shape.width * shape.height
    # Must modify to add new shapes

# Good: Extensible without modification
from abc import ABC, abstractmethod

class Shape(ABC):
    @abstractmethod
    def area(self) -> float: ...

class Circle(Shape):
    def __init__(self, radius: float):
        self.radius = radius
    def area(self) -> float:
        return 3.14 * self.radius ** 2

Liskov Substitution Principle

Subtypes must be substitutable for their base types.

# Bad: Violates LSP - Square changes Rectangle behavior
class Rectangle:
    def set_width(self, w): self.width = w
    def set_height(self, h): self.height = h

class Square(Rectangle):  # Breaks when used as Rectangle
    def set_width(self, w):
        self.width = self.height = w  # Unexpected side effect

# Good: Separate types with common interface
class Shape(ABC):
    @abstractmethod
    def area(self) -> float: ...

class Rectangle(Shape):
    def __init__(self, width: float, height: float): ...

class Square(Shape):
    def __init__(self, side: float): ...

Interface Segregation Principle

Clients shouldn't depend on interfaces they don't use.

// Bad: Fat interface
interface Worker {
  work(): void;
  eat(): void;
  sleep(): void;
}

// Good: Segregated interfaces
interface Workable {
  work(): void;
}

interface Feedable {
  eat(): void;
}

// Clients only implement what they need
class Robot implements Workable {
  work(): void { /* ... */ }
}

Dependency Inversion Principle

Depend on abstractions, not concretions.

# Bad: Direct dependency on concrete class
class OrderService:
    def __init__(self):
        self.db = PostgresDatabase()  # Tight coupling

# Good: Depend on abstraction
from abc import ABC, abstractmethod

class Database(ABC):
    @abstractmethod
    def save(self, data): ...

class OrderService:
    def __init__(self, db: Database):
        self.db = db  # Injected abstraction

Quick Reference

Principle	Question to Ask	Red Flag
KISS	"Is there a simpler way?"	Complex solution for simple problem
YAGNI	"Do I need this right now?"	Building for hypothetical use cases
SRP	"What's the one reason to change?"	Class doing multiple jobs
OCP	"Can I extend without modifying?"	Switch statements for types
LSP	"Can subtypes replace base types?"	Overridden methods with side effects
ISP	"Does client need all methods?"	Empty method implementations
DIP	"Am I depending on abstractions?"	new keyword in business logic

When Principles Conflict

1. KISS vs SOLID: For small projects, KISS wins. Add SOLID patterns as complexity grows.
2. YAGNI vs DIP: Don't add abstractions until you have 2+ implementations.
3. Readability vs DRY: Prefer slight duplication over wrong abstraction.

Integration with Code Review

When reviewing code, check:

• No unnecessary complexity (KISS)
• No speculative features (YAGNI)
• Each class has single responsibility (SRP)
• No god classes (> 500 lines)
• Dependencies are injected, not created (DIP)

Verification: Run wc -l <file> to check line counts and grep -c "class " <file> to count classes per file.