Advanced Python Patterns

Singleton, factory, observer, and other design patterns.

Overview

Implement common design patterns.

Singleton Pattern

class Singleton:
    _instance = None
    
    def __new__(cls):
        if cls._instance is None:
            cls._instance = super().__new__(cls)
        return cls._instance

a = Singleton()
b = Singleton()
print(a is b)  # True

Factory Pattern

class Animal:
    def speak(self):
        raise NotImplementedError

class Dog(Animal):
    def speak(self):
        return "Woof!"

class Cat(Animal):
    def speak(self):
        return "Meow!"

class AnimalFactory:
    @staticmethod
    def create(animal_type):
        animals = {"dog": Dog, "cat": Cat}
        return animals.get(animal_type)()

dog = AnimalFactory.create("dog")
print(dog.speak())  # Woof!

Observer Pattern

class EventSystem:
    def __init__(self):
        self.listeners = {}
    
    def subscribe(self, event, callback):
        if event not in self.listeners:
            self.listeners[event] = []
        self.listeners[event].append(callback)
    
    def emit(self, event, data):
        for callback in self.listeners.get(event, []):
            callback(data)

system = EventSystem()
system.subscribe("message", lambda msg: print(f"Received: {msg}"))
system.emit("message", "Hello!")

Practice

Implement a command pattern for a text editor.