【Python 面向对象】

Python 的面向对象编程（OOP）通过类（Class）和对象（Object）实现代码结构化，支持封装、继承和多态三大特性。以下是系统化指南：

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一、类与对象基础

1. 定义类

class Dog:# 类属性（所有实例共享）species = "Canis familiaris"# 构造方法（__init__是魔法方法）def __init__(self, name, age):self.name = name    # 实例属性self.age = age# 实例方法def bark(self):return f"{self.name} says: Woof!"# 创建对象
my_dog = Dog("Buddy", 3)
print(my_dog.species)  # 输出: Canis familiaris

2. 特殊方法（Magic Methods）

class Rectangle:def __init__(self, width, height):self.width = widthself.height = height# 计算面积def area(self):return self.width * self.height# 字符串表示（__str__用于用户友好显示）def __str__(self):return f"Rectangle({self.width}x{self.height})"# 运算符重载（实现加法）def __add__(self, other):return Rectangle(self.width + other.width,self.height + other.height)rect1 = Rectangle(2, 3)
rect2 = Rectangle(4, 5)
print(rect1 + rect2)  # 输出: Rectangle(6x8)

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二、继承与方法重写

1. 单继承

class Animal:def __init__(self, name):self.name = namedef speak(self):raise NotImplementedError("子类必须实现此方法")class Dog(Animal):def speak(self):return f"{self.name} says: Woof!"class Cat(Animal):def speak(self):return f"{self.name} says: Meow!"animals = [Dog("Buddy"), Cat("Whiskers")]
for animal in animals:print(animal.speak())

2. 多重继承

class Flyer:def fly(self):return "Flying!"class Swimmer:def swim(self):return "Swimming!"class Duck(Flyer, Swimmer):def __init__(self, name):self.name = nameduck = Duck("Donald")
print(duck.fly())  # 输出: Flying!
print(duck.swim()) # 输出: Swimming!

3. super() 函数

class Rectangle:def __init__(self, width, height):self.width = widthself.height = heightclass ColoredRectangle(Rectangle):def __init__(self, width, height, color):super().__init__(width, height)  # 调用父类构造方法self.color = color

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三、多态与鸭子类型

1. 运行时多态

class Shape:def area(self):passclass Circle(Shape):def __init__(self, radius):self.radius = radiusdef area(self):return 3.14 * self.radius ** 2class Square(Shape):def __init__(self, side):self.side = sidedef area(self):return self.side ** 2def print_area(shape):print(shape.area())print_area(Circle(5))  # 输出: 78.5
print_area(Square(4))  # 输出: 16

2. 鸭子类型（Duck Typing）

class Duck:def quack(self):print("Quack!")class FakeDuck:def quack(self):print("Silent quack")def make_quack(duck):duck.quack()make_quack(Duck())       # 输出: Quack!
make_quack(FakeDuck())   # 输出: Silent quack

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四、封装与访问控制

1. 属性控制

class BankAccount:def __init__(self, balance=0):self.__balance = balance  # 双下划线前缀实现名称修饰@propertydef balance(self):return self.__balance@balance.setterdef balance(self, value):if value < 0:raise ValueError("余额不能为负")self.__balance = valueaccount = BankAccount(100)
account.balance = 200    # 允许修改
# account.balance = -50  # 抛出 ValueError

2. 描述符协议

class NonNegative:def __set_name__(self, owner, name):self.name = namedef __get__(self, instance, owner):return instance.__dict__[self.name]def __set__(self, instance, value):if value < 0:raise ValueError("值不能为负")instance.__dict__[self.name] = valueclass Product:price = NonNegative()stock = NonNegative()def __init__(self, price, stock):self.price = priceself.stock = stockproduct = Product(19.99, 100)
# product.price = -5  # 抛出 ValueError

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五、高级特性

1. 元类（Metaclass）

class Singleton(type):_instances = {}def __call__(cls, *args, **kwargs):if cls not in cls._instances:cls._instances[cls] = super().__call__(*args, **kwargs)return cls._instances[cls]class Database(metaclass=Singleton):passdb1 = Database()
db2 = Database()
print(db1 is db2)  # 输出: True

2. 抽象基类（ABC）

from abc import ABC, abstractmethodclass PaymentGateway(ABC):@abstractmethoddef process_payment(self, amount):passclass PayPal(PaymentGateway):def process_payment(self, amount):print(f"PayPal 处理支付: ${amount}")# class FakeGateway(PaymentGateway): pass  # 实例化会报错

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六、设计模式应用

1. 工厂模式

class ShapeFactory:@staticmethoddef create_shape(shape_type, **kwargs):shapes = {'circle': Circle,'square': Square}return shapes[shape_type](**kwargs)circle = ShapeFactory.create_shape('circle', radius=5)

2. 单例模式

class Database:_instance = Nonedef __new__(cls, *args, **kwargs):if not cls._instance:cls._instance = super().__new__(cls)cls._instance._initialized = Falsereturn cls._instancedef __init__(self):if not self._initialized:self.connect()self._initialized = Truedef connect(self):print("建立数据库连接")

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七、最佳实践

1. 组合优于继承：优先使用对象组合而非类继承
2. 显式优于隐式：避免过度依赖魔术方法
3. 保持简单：单个类职责不超过7个方法（参考SRP原则）
4. 文档字符串：为每个类和方法编写docstring
5. 类型注解（Python 3.5+）：

   class Vector:def __init__(self, x: float, y: float):self.x = xself.y = y
   

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掌握这些面向对象技术后，可进一步探索设计模式（如MVC、观察者模式）和框架开发（如Django的类视图）。建议通过实际项目（如开发电商系统、游戏引擎）深化理解。