/code-quality-principles

name: code-quality-principles
description: Applies KISS, YAGNI, and SOLID principles for clean code with reduced complexity. Use when refactoring or reviewing code for over-engineering.
alwaysApply: true
category: development
tags:
- design
- principles
- clean-code
- architecture
tools: []
complexity: low
model_hint: fast
estimated_tokens: 600

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):
        se