====== 第十七章：设计模式 ======

===== 本章目标 =====

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

===== 创建型模式 =====

==== 单例模式 ====

<code python>
# 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 = []
</code>

==== 工厂模式 ====

<code python>
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!
</code>

===== 结构型模式 =====

==== 装饰器模式 ====

<code python>
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
</code>

===== 行为型模式 =====

==== 观察者模式 ====

<code python>
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!")
</code>

==== 策略模式 ====

<code python>
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)
</code>

===== 本章练习 =====

1. 实现建造者模式构建复杂对象
2. 实现适配器模式统一接口
3. 实现命令模式支持撤销操作
4. 实现模板方法模式

下一章：[[python_course:chapter18|第十八章：文件操作]]