python面向对象中__new__和__init__区别

class Person:
    def __init__(self, name):
        print(f"2. __init__ 被调用，正在初始化: {name}")
        self.name = name

p = Person("Alice")
# 输出:
# 2. __init__ 被调用，正在初始化: Alice

class Demo:
    def __new__(cls, *args, **kwargs):
        print("1. __new__ 被调用：负责创建实例")
        # 必须调用父类的 __new__ 来真正创建对象，否则 __init__ 不会执行
        instance = super().__new__(cls)
        return instance

    def __init__(self, value):
        print("2. __init__ 被调用：负责初始化属性")
        self.value = value

obj = Demo(100)
print(f"最终对象值: {obj.value}")

class MyInt(int):
    # 想要强制让生成的整数都是偶数
    def __new__(cls, value):
        # 在创建之前就修改值
        if value % 2 != 0:
            value += 1
        return super().__new__(cls, value)
    
    # __init__ 在这里无法修改整数的值，因为 int 是不可变的

num = MyInt(5)
print(num)  # 输出 6 (自动变成了偶数)

class Singleton:
    _instance = None

    def __new__(cls, *args, **kwargs):
        if not cls._instance:
            print("创建新实例")
            cls._instance = super().__new__(cls)
        else:
            print("返回已有实例")
        return cls._instance

    def __init__(self, name):
        # 注意：即使返回旧实例，__init__ 仍会被调用，通常需要小心处理
        self.name = name

s1 = Singleton("A")
s2 = Singleton("B")

print(s1 is s2)  # True (是同一个对象)
print(s2.name)   # "B" (注意：__init__ 再次运行覆盖了名字)

class Shape:
    def __new__(cls, shape_type, *args, **kwargs):
        if shape_type == "circle":
            return super().__new__(Circle)
        elif shape_type == "square":
            return super().__new__(Square)
        return super().__new__(cls)

class Circle(Shape):
    pass

class Square(Shape):
    pass

c = Shape("circle")
print(type(c))  # <class '__main__.Circle'>