【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：
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)
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
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
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
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
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")
七、最佳实践
- 组合优于继承：优先使用对象组合而非类继承
- 显式优于隐式：避免过度依赖魔术方法
- 保持简单：单个类职责不超过7个方法（参考SRP原则）
- 文档字符串：为每个类和方法编写docstring
- 类型注解（Python 3.5+）：
class Vector:def __init__(self, x: float, y: float):self.x = xself.y = y
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。