第十七章：设计模式

完成本章学习后，你将能够：

理解常用设计模式
在Python中实现经典设计模式
选择合适的设计模式解决问题

# 方式1：使用__new__
class Singleton:
    _instance = None
 
    def __new__(cls, *args, **kwargs):
        if cls._instance is None:
            cls._instance = super().__new__(cls)
        return cls._instance
 
# 方式2：使用装饰器
def singleton(cls):
    instances = {}
    def wrapper(*args, **kwargs):
        if cls not in instances:
            instances[cls] = cls(*args, **kwargs)
        return instances[cls]
    return wrapper
 
@singleton
class Logger:
    def __init__(self):
        self.logs = []

class Animal:
    def speak(self):
        pass
 
class Dog(Animal):
    def speak(self):
        return "Woof!"
 
class Cat(Animal):
    def speak(self):
        return "Meow!"
 
class AnimalFactory:
    @staticmethod
    def create(animal_type):
        if animal_type == "dog":
            return Dog()
        elif animal_type == "cat":
            return Cat()
        raise ValueError(f"Unknown animal type: {animal_type}")
 
# 使用
animal = AnimalFactory.create("dog")
print(animal.speak())  # Woof!

class Coffee:
    def cost(self):
        return 5
    def description(self):
        return "Coffee"
 
class Decorator(Coffee):
    def __init__(self, coffee):
        self._coffee = coffee
 
    def cost(self):
        return self._coffee.cost()
 
    def description(self):
        return self._coffee.description()
 
class Milk(Decorator):
    def cost(self):
        return self._coffee.cost() + 1
    def description(self):
        return self._coffee.description() + ", Milk"
 
class Sugar(Decorator):
    def cost(self):
        return self._coffee.cost() + 0.5
    def description(self):
        return self._coffee.description() + ", Sugar"
 
# 使用
coffee = Coffee()
coffee = Milk(coffee)
coffee = Sugar(coffee)
print(coffee.description())  # Coffee, Milk, Sugar
print(coffee.cost())         # 6.5

class Subject:
    def __init__(self):
        self._observers = []
 
    def attach(self, observer):
        self._observers.append(observer)
 
    def detach(self, observer):
        self._observers.remove(observer)
 
    def notify(self, message):
        for observer in self._observers:
            observer.update(message)
 
class Observer:
    def update(self, message):
        pass
 
class EmailObserver(Observer):
    def update(self, message):
        print(f"Email notification: {message}")
 
class SMSObserver(Observer):
    def update(self, message):
        print(f"SMS notification: {message}")
 
# 使用
subject = Subject()
subject.attach(EmailObserver())
subject.attach(SMSObserver())
subject.notify("New order received!")

from abc import ABC, abstractmethod
 
class PaymentStrategy(ABC):
    @abstractmethod
    def pay(self, amount):
        pass
 
class CreditCardPayment(PaymentStrategy):
    def pay(self, amount):
        print(f"Paying ${amount} using Credit Card")
 
class PayPalPayment(PaymentStrategy):
    def pay(self, amount):
        print(f"Paying ${amount} using PayPal")
 
class ShoppingCart:
    def __init__(self):
        self.items = []
        self._strategy = None
 
    def set_payment_strategy(self, strategy):
        self._strategy = strategy
 
    def checkout(self, amount):
        if self._strategy:
            self._strategy.pay(amount)
        else:
            raise ValueError("Payment strategy not set")
 
# 使用
cart = ShoppingCart()
cart.set_payment_strategy(CreditCardPayment())
cart.checkout(100)
cart.set_payment_strategy(PayPalPayment())
cart.checkout(50)