coding-standards

name	coding-standards
description	Use when reviewing code for quality, naming a class or function, choosing a design pattern, refactoring a code smell, or establishing coding conventions for a new project.

Coding Standards

A comprehensive reference for writing clean, consistent, maintainable code across Python, TypeScript, and Go.

When to Activate

• Reviewing a pull request for code quality
• Refactoring legacy or unclear code
• Naming a class, function, variable, or file
• Deciding which design pattern to apply
• Setting up coding standards for a new project
• Identifying code smells and their root causes

---

Naming Conventions

Construct	Python	TypeScript	Go
Variables	snake_case	camelCase	camelCase
Functions / methods	snake_case	camelCase	PascalCase (exported), camelCase (unexported)
Classes / types	PascalCase	PascalCase	PascalCase
Constants	UPPER_SNAKE_CASE	UPPER_SNAKE_CASE or camelCase	PascalCase (exported), camelCase (unexported)
Interfaces	N/A	IFoo (avoid) or just Foo (preferred)	Foo (no I prefix)
Files	snake_case.py	kebab-case.ts or camelCase.ts	snake_case.go
Test files	test_*.py	*.test.ts / *.spec.ts	*_test.go

Naming Heuristics

• Be specific: user_registration_date not date; payment_amount_cents not amount
• Avoid meaningless suffixes: UserManager, DataHelper, Utils — what does it manage/help/do?
• Boolean names: use is_, has_, can_, should_ prefix: is_active, has_permission
• Functions: verb phrases — calculate_tax(), send_email(), validate_schema()
• Avoid abbreviations except universally understood ones (id, url, http, db)
• Length proportional to scope: loop variable i is fine; module-level variable needs a full name

# BAD
def proc(d, f):
    temp = d * f
    return temp

# GOOD
def calculate_discounted_price(base_price: float, discount_factor: float) -> float:
    return base_price * discount_factor

---

SOLID Principles

S — Single Responsibility

A class should have one reason to change.

// BAD — UserService does everything
class UserService {
  createUser(data: CreateUserDto) { /* ... */ }
  sendWelcomeEmail(user: User) { /* ... */ }  // email is a separate concern
  generateReport(users: User[]) { /* ... */ } // reporting is a separate concern
}

// GOOD — separated concerns
class UserService { createUser(data: CreateUserDto) { /* ... */ } }
class EmailService { sendWelcomeEmail(user: User) { /* ... */ } }
class UserReportService { generateReport(users: User[]) { /* ... */ } }

O — Open/Closed

Open for extension, closed for modification.

// BAD — must modify to add new discount type
function calculateDiscount(type: string, price: number): number {
  if (type === 'student') return price * 0.8;
  if (type === 'senior') return price * 0.75;
  // must add else-if here for each new type
}

// GOOD — extend by adding a new class
interface DiscountStrategy { apply(price: number): number; }
class StudentDiscount implements DiscountStrategy { apply(p: number) { return p * 0.8; } }
class SeniorDiscount implements DiscountStrategy { apply(p: number) { return p * 0.75; } }

L — Liskov Substitution

Subtypes must be substitutable for their base type.

// BAD — Square overrides setWidth/setHeight inconsistently, breaking Rectangle contract
class Rectangle {
  constructor(protected width: number, protected height: number) {}
  setWidth(w: number) { this.width = w; }
  setHeight(h: number) { this.height = h; }
  area() { return this.width * this.height; }
}
class Square extends Rectangle {
  setWidth(w: number) { this.width = this.height = w; }   // violates LSP
  setHeight(h: number) { this.width = this.height = h; }  // caller expects independent dims
}

// GOOD — model separately; share via interface if needed
interface Shape { area(): number; }
class Rectangle implements Shape {
  constructor(private w: number, private h: number) {}
  area() { return this.w * this.h; }
}
class Square implements Shape {
  constructor(private side: number) {}
  area() { return this.side * this.side; }
}

I — Interface Segregation

Many specific interfaces are better than one general interface.

// BAD — not all workers can eat or sleep
interface Worker { work(): void; eat(): void; sleep(): void; }

// GOOD — split by capability
interface Workable { work(): void; }
interface Feedable { eat(): void; }
interface Restable { sleep(): void; }

class HumanWorker implements Workable, Feedable, Restable {
  work() { /* ... */ }
  eat() { /* ... */ }
  sleep() { /* ... */ }
}
class RobotWorker implements Workable {
  work() { /* ... */ }
}

D — Dependency Inversion

Depend on abstractions, not concretions.

// BAD — tightly coupled to PostgresDB
class UserRepository {
  private db = new PostgresDB();  // concrete dependency
  find(id: string) { return this.db.query(/* ... */); }
}

// GOOD — depends on interface, injected
interface Database { query(sql: string, params: unknown[]): Promise<unknown[]>; }
class UserRepository {
  constructor(private db: Database) {}
  find(id: string) { return this.db.query('SELECT * FROM users WHERE id = $1', [id]); }
}

---

Design Patterns Quick Reference

Pattern	Category	One-line use case	When to avoid
Factory	Creational	Create objects without specifying concrete class	When you only have one type
Abstract Factory	Creational	Create families of related objects	Overkill for simple factories
Singleton	Creational	One shared instance (config, logger)	When it becomes global mutable state
Builder	Creational	Step-by-step construction of complex objects	Simple objects with few fields
Strategy	Behavioral	Swap algorithms at runtime	When you only have one algorithm
Observer	Behavioral	Notify dependents when state changes	When observers outlive the subject
Command	Behavioral	Encapsulate a request as an object (undo/redo)	Simple one-off calls
Template Method	Behavioral	Define skeleton, let subclasses fill steps	Deep inheritance hierarchies
Repository	Structural	Isolate data access from domain logic	When ORM already provides this
Adapter	Structural	Wrap incompatible interface	When both interfaces are yours (just fix one)
Decorator	Structural	Add behavior to objects dynamically	When subclassing is simpler
Facade	Structural	Simplified interface to complex subsystem	Over-used to hide bad design

Full Code Examples

Factory

interface Logger { log(msg: string): void; }
class ConsoleLogger implements Logger { log(msg: string) { console.log(msg); } }
class FileLogger implements Logger { log(msg: string) { fs.appendFileSync('app.log', msg); } }

function createLogger(type: 'console' | 'file'): Logger {
  if (type === 'console') return new ConsoleLogger();
  return new FileLogger();
}

// Usage
const logger = createLogger('console');
logger.log('Server started');

Strategy

interface SortStrategy { sort(data: number[]): number[]; }
class QuickSort implements SortStrategy { sort(d: number[]) { /* quicksort impl */ return d; } }
class MergeSort implements SortStrategy { sort(d: number[]) { /* mergesort impl */ return d; } }

class DataSorter {
  constructor(private strategy: SortStrategy) {}
  sort(data: number[]) { return this.strategy.sort(data); }
  setStrategy(strategy: SortStrategy) { this.strategy = strategy; }
}

// Swap strategy at runtime
const sorter = new DataSorter(new QuickSort());
sorter.setStrategy(new MergeSort());
sorter.sort([5, 3, 1, 4, 2]);

Observer

type Listener<T> = (event: T) => void;

class EventEmitter<T> {
  private listeners: Listener<T>[] = [];
  subscribe(fn: Listener<T>) { this.listeners.push(fn); }
  unsubscribe(fn: Listener<T>) { this.listeners = this.listeners.filter(l => l !== fn); }
  emit(event: T) { this.listeners.forEach(fn => fn(event)); }
}

const orderEvents = new EventEmitter<{ orderId: string; status: string }>();
orderEvents.subscribe(({ orderId }) => sendConfirmationEmail(orderId));
orderEvents.subscribe(({ orderId }) => updateInventory(orderId));
orderEvents.emit({ orderId: '123', status: 'paid' });

Repository

interface UserRepository {
  findById(id: string): Promise<User | null>;
  save(user: User): Promise<void>;
  delete(id: string): Promise<void>;
}

class PostgresUserRepository implements UserRepository {
  constructor(private db: Database) {}

  async findById(id: string): Promise<User | null> {
    const [row] = await this.db.query('SELECT * FROM users WHERE id = $1', [id]);
    return row ? User.fromRow(row) : null;
  }

  async save(user: User): Promise<void> {
    await this.db.query(
      'INSERT INTO users (id, email) VALUES ($1, $2) ON CONFLICT (id) DO UPDATE SET email = $2',
      [user.id, user.email]
    );
  }

  async delete(id: string): Promise<void> {
    await this.db.query('DELETE FROM users WHERE id = $1', [id]);
  }
}

---

Code Smells Catalog

Smell	Symptom	Refactoring
Long Method	Function > 30 lines	Extract Method
Long Class / God Class	Class > 300 lines or too many responsibilities	Extract Class
Data Clump	Same 3+ params appear together everywhere	Introduce Parameter Object
Primitive Obsession	Using strings/ints for domain concepts (money, email)	Introduce Value Object
Feature Envy	Method uses data from another class more than its own	Move Method
Shotgun Surgery	One change requires edits in many classes	Move Method/Field, consolidate
Divergent Change	Class changes for multiple unrelated reasons	Extract Class
Duplicate Code	Same logic in multiple places	Extract Method/Function
Magic Numbers	Unexplained literals in code	Replace with Named Constant
Switch/Match on Type	Repeated instanceof or type checks	Replace with Polymorphism

Smell Examples

// BAD — Primitive Obsession: money is a raw number, email is a raw string
function chargeUser(userId: string, amount: number, email: string) { /* ... */ }

// GOOD — Value Objects carry their own validation and behaviour
class Money {
  constructor(readonly cents: number, readonly currency: 'USD' | 'EUR') {
    if (cents < 0) throw new Error('Money cannot be negative');
  }
}
class Email {
  constructor(readonly value: string) {
    if (!value.includes('@')) throw new Error('Invalid email');
  }
}
function chargeUser(userId: string, amount: Money, email: Email) { /* ... */ }

// BAD — Magic Numbers
if (user.age >= 65) applyDiscount(price * 0.25);

// GOOD — Named Constants
const SENIOR_AGE_THRESHOLD = 65;
const SENIOR_DISCOUNT_RATE = 0.25;
if (user.age >= SENIOR_AGE_THRESHOLD) applyDiscount(price * SENIOR_DISCOUNT_RATE);

---

Language-Specific Idioms

Python

# Dataclass instead of verbose __init__
from dataclasses import dataclass, field

@dataclass
class Order:
    id: str
    items: list[str] = field(default_factory=list)
    total: float = 0.0

# Context manager for resource management
with open('file.txt') as f:
    data = f.read()  # file auto-closed, even on exception

# Generator for memory-efficient processing
def process_large_file(path: str):
    with open(path) as f:
        for line in f:           # reads line-by-line, not all into memory
            yield parse(line)

# Walrus operator for readability
if (match := pattern.search(text)) is not None:
    print(match.group(0))

# BAD — unpacking ignored with throwaway names
result = get_user_and_role()
user = result[0]
role = result[1]

# GOOD — structured unpacking
user, role = get_user_and_role()

TypeScript

// Discriminated union — exhaustive type narrowing
type Result<T> =
  | { success: true; data: T }
  | { success: false; error: string };

function handleResult<T>(result: Result<T>) {
  if (result.success) {
    console.log(result.data);    // TypeScript knows this is T
  } else {
    console.error(result.error); // TypeScript knows this is string
  }
}

// satisfies — validate type without widening
const config = {
  host: 'localhost',
  port: 5432,
} satisfies Record<string, string | number>;
// config.host is still `string` (not widened to `string | number`)

// Utility types
type PartialUser         = Partial<User>;                  // all fields optional
type ReadonlyUser        = Readonly<User>;                  // all fields readonly
type UserPreview         = Pick<User, 'id' | 'name'>;       // subset of fields
type UserWithoutPassword = Omit<User, 'password'>;          // exclude field

// BAD — casting away the type
const user = response.data as any;

// GOOD — parse and validate at the boundary
import { z } from 'zod';
const UserSchema = z.object({ id: z.string(), name: z.string() });
const user = UserSchema.parse(response.data);

Go

// Functional options pattern — flexible, forward-compatible constructors
type Server struct {
    host    string
    port    int
    timeout time.Duration
}

type Option func(*Server)

func WithTimeout(d time.Duration) Option {
    return func(s *Server) { s.timeout = d }
}

func NewServer(host string, port int, opts ...Option) *Server {
    s := &Server{host: host, port: port, timeout: 30 * time.Second}
    for _, opt := range opts {
        opt(s)
    }
    return s
}

// Usage
srv := NewServer("localhost", 8080, WithTimeout(5*time.Second))

// Error wrapping — preserve context, enable errors.Is / errors.As
if err := db.Query(); err != nil {
    return fmt.Errorf("fetching user %s: %w", userID, err)
}

// Table-driven tests — idiomatic Go
func TestAdd(t *testing.T) {
    cases := []struct{ a, b, want int }{
        {1, 2, 3},
        {0, 0, 0},
        {-1, 1, 0},
    }
    for _, tc := range cases {
        t.Run(fmt.Sprintf("%d+%d", tc.a, tc.b), func(t *testing.T) {
            got := Add(tc.a, tc.b)
            if got != tc.want {
                t.Errorf("got %d, want %d", got, tc.want)
            }
        })
    }
}

---

Red Flags

• God class with Manager, Helper, or Utils suffix — a class named UserManager with 15 methods is a single-responsibility violation waiting to be split; name by what it does, not that it manages things
• Boolean parameter flags controlling fundamentally different code paths — send_email(user, urgent=True) means two functions in disguise; split into send_urgent_email and send_email with distinct call sites
• Deep inheritance hierarchies (more than 2 levels) — subclasses become entangled with grandparent internals; prefer composition and interfaces, using inheritance only for true IS-A relationships
• Returning None on error instead of raising an exception or returning a typed Result — callers silently ignore None and propagate nulls deep into the call stack before crashing; be explicit about failure
• Catching a broad exception to log-and-swallow — except Exception: logger.error(...) hides failures silently; catch the specific exception type and let unexpected ones propagate
• any type in TypeScript at a module boundary — any disables type checking for every caller downstream; use unknown and narrow explicitly, or define a proper interface
• Mutable default arguments in Python — def process(items=[]) shares the same list across all calls; use def process(items=None) and initialize inside the function body
• Long positional argument lists (more than 3 parameters) — hard to read at call sites and easy to transpose; introduce a parameter object or dataclass

Checklist

• Naming follows language convention (snake_case / camelCase / PascalCase per language)
• Functions have verb-phrase names; booleans use is_ / has_ / can_ prefix
• Each function has a single, clear responsibility (< 30 lines as a guide)
• No magic numbers — named constants used for all literal values
• Dependencies injected (not instantiated inside the class)
• Design pattern applied only where it genuinely simplifies the code
• Code smells reviewed: no God Classes, Primitive Obsession, or Data Clumps
• Language-specific idioms used (dataclasses, discriminated unions, functional options)
• No duplicate logic — extracted into shared function or module
• PR reviewer can understand code intent without asking the author