design-patterns

Design Patterns (GoF Reference)

Patterns solve recurring design problems. Don't introduce a pattern to a problem that doesn't exist yet.

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Creational — Object Creation

Factory Method

Create objects without knowing the concrete class.

class NotificationFactory:
    @staticmethod
    def create(channel: str) -> Notification:
        match channel:
            case "email": return EmailNotification()
            case "sms":   return SmsNotification()
            case _: raise ValueError(f"Unknown channel: {channel}")

Use when: the exact type to create is determined at runtime, or varies by environment.

Builder

Construct complex objects step by step, separating construction from representation.

const email = new EmailBuilder()
    .to("[email protected]")
    .subject("Hello")
    .body("World")
    .build();

Use when: constructors would have 4+ parameters, especially optional ones.

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Behavioral — Object Communication

Strategy

Define a family of algorithms and make them interchangeable at runtime.

class Sorter:
    def __init__(self, strategy: SortStrategy) -> None:
        self._strategy = strategy

    def sort(self, data: list) -> list:
        return self._strategy.sort(data)

Use when: you have multiple ways to do something and want to switch them. Replace if/elif chains.

Observer (Event / Pub-Sub)

One object notifies many dependents without knowing who they are.

class EventEmitter<T> {
    private listeners = new Set<(event: T) => void>();
    on(fn: (event: T) => void) { this.listeners.add(fn); }
    emit(event: T) { this.listeners.forEach(fn => fn(event)); }
}

Use when: one thing changing should trigger reactions in multiple places, without tight coupling.

Command

Encapsulate a request as an object to support undo, logging, or queuing.

@dataclass
class CreateOrderCommand:
    user_id: str
    items: list[OrderItem]

class CommandHandler:
    def handle(self, cmd: CreateOrderCommand) -> Order: ...