第 6 章 面向对象编程（OOP）

class Dog:
    """一只狗（这是 docstring）"""

    species = "Canis lupus"        # 类属性：所有实例共享

    def __init__(self, name, age):
        """构造方法：创建实例时自动调用"""
        self.name = name            # 实例属性：每个实例独有
        self.age = age

    def bark(self):                 # 实例方法：第一个参数永远是 self
        return f"{self.name} 汪汪!"

dog = Dog("旺财", 3)     # 创建实例（不需要 new）
dog.bark()               # '旺财 汪汪!'
dog.name                 # '旺财'
Dog.species              # 'Canis lupus'

class Counter:
    count = 0                    # 类属性

    def __init__(self):
        Counter.count += 1       # 修改类属性要显式用类名

Counter(); Counter()
Counter.count     # 2

class Account:
    def __init__(self, balance):
        self._balance = balance      # 单下划线：内部使用（约定，仍可访问）
        self.__pin = "1234"          # 双下划线：触发名称改写（name mangling）

acct = Account(100)
acct._balance        # 能访问，但这是"别碰"的信号
acct.__pin           # AttributeError！实际被改名为 _Account__pin

class Account:
    def __init__(self, balance):
        self._balance = balance

    @property
    def balance(self):               # 读：acct.balance
        return self._balance

    @balance.setter
    def balance(self, value):        # 写：acct.balance = 200
        if value < 0:
            raise ValueError("余额不能为负")
        self._balance = value

class Animal:
    def __init__(self, name):
        self.name = name

    def speak(self):
        return "..."

class Cat(Animal):                   # Cat 继承 Animal
    def speak(self):                 # 重写（override）
        return "喵"

class Kitten(Cat):
    def __init__(self, name):
        super().__init__(name)       # 调用父类方法（推荐写法）
        self.age = 0

animals = [Animal("A"), Cat("B"), Kitten("C")]
for a in animals:
    print(a.speak())    # ... / 喵 / 喵 —— 多态：同一接口不同行为

class FlyingFish(Swimmer, Flyer):    # 支持多继承
    ...

FlyingFish.__mro__    # 方法解析顺序：C3 线性化，从左到右查找

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

    def __repr__(self):              # repr() 和调试显示
        return f"Vector({self.x}, {self.y})"

    def __str__(self):               # str() 和 print()
        return f"({self.x}, {self.y})"

    def __eq__(self, other):         # == 
        return self.x == other.x and self.y == other.y

    def __add__(self, other):        # +
        return Vector(self.x + other.x, self.y + other.y)

    def __len__(self):               # len()
        return 2

    def __getitem__(self, i):        # v[0]  —— 支持 for 遍历也靠它
        return (self.x, self.y)[i]

v1, v2 = Vector(1, 2), Vector(3, 4)
print(v1 + v2)      # (4, 6)

class Pizza:
    def __init__(self, ingredients):
        self.ingredients = ingredients

    @classmethod
    def margherita(cls):             # 第一个参数是类本身
        return cls(["番茄", "芝士"])  # 常用作替代构造函数

    @staticmethod
    def is_vegetarian(ingredients):  # 不接收 self/cls
        return "肉" not in ingredients

p = Pizza.margherita()

from dataclasses import dataclass, field

@dataclass
class Point:
    x: float
    y: float
    tags: list[str] = field(default_factory=list)   # 可变默认值的正确写法

p = Point(1.0, 2.0)
print(p)          # Point(x=1.0, y=2.0, tags=[])
p == Point(1.0, 2.0)   # True