__init__ 与 __new__

__new__负责创建实例，__init__负责初始化实例：

class Person:
    # 创建实例时调用
    def __new__(cls, name):
        print(f"Creating new {cls.__name__} instance")
        return super().__new__(cls)
    
    # 初始化实例时调用
    def __init__(self, name):
        print(f"Initializing person: {name}")
        self.name = name

p = Person("Alice")
# 输出:
# Creating new Person instance
# Initializing person: Alice

__str__ 与 __repr__

__str__提供用户友好字符串表示，__repr__提供开发者友好字符串表示：

class Point:
    def __init__(self, x, y):
        self.x = x
        self.y = y
        
    # 用户友好表示
    def __str__(self):
        return f"Point({self.x}, {self.y})"
    
    # 开发者友好表示（通常可用来重新创建对象）
    def __repr__(self):
        return f"Point(x={self.x}, y={self.y})"

p = Point(3, 4)
print(str(p))   # 输出: Point(3, 4)
print(repr(p))  # 输出: Point(x=3, y=4)

算术运算方法

实现自定义对象的算术运算：

class Vector:
    def __init__(self, x, y):
        self.x = x
        self.y = y
        
    # 实现向量加法
    def __add__(self, other):
        return Vector(self.x + other.x, self.y + other.y)
    
    # 实现向量乘法
    def __mul__(self, scalar):
        return Vector(self.x * scalar, self.y * scalar)
    
    def __str__(self):
        return f"Vector({self.x}, {self.y})"

v1 = Vector(2, 3)
v2 = Vector(4, 5)
print(v1 + v2)  # 输出: Vector(6, 8)
print(v1 * 3)   # 输出: Vector(6, 9)

容器方法示例

class ShoppingCart:
    def __init__(self):
        self.items = []
    
    def add_item(self, item):
        self.items.append(item)
    
    # 实现len(cart)
    def __len__(self):
        return len(self.items)
    
    # 实现cart[0]
    def __getitem__(self, index):
        return self.items[index]
    
    # 实现cart[0] = 'new item'
    def __setitem__(self, index, value):
        self.items[index] = value

cart = ShoppingCart()
cart.add_item("Apple")
cart.add_item("Banana")
print(len(cart))  # 输出: 2
print(cart[1])    # 输出: Banana
cart[1] = "Orange"
print(cart[1])    # 输出: Orange