python中__new__函数的具体使用

class MyClass:
    def __new__(cls, *args, **kwargs):
        print("__new__ is called. Creating instance.")
        # 最关键的一步：调用父类的 __new__ 来创建实例
        instance = super().__new__(cls)
        # 这里可以对 instance 做一些非常早期的操作
        return instance # 必须返回实例！

    def __init__(self, value):
        print("__init__ is called. Initializing instance.")
        self.value = value

# 测试
obj = MyClass(42)
print(obj.value)

class MyClass:
    def __new__(cls, *args, **kwargs):
        print("__new__ is called. Creating instance.")

    def __init__(self, value):
        print("__init__ is called. Initializing instance.")
        self.value = value

# 测试
obj = MyClass(42)
print(obj)

class MyClass:
    def __new__(cls, *args, **kwargs):
        print("__new__ is called. Creating instance.")
        # 最关键的一步：调用父类的 __new__ 来创建实例
        super().__new__(cls)

    def __init__(self, value):
        print("__init__ is called. Initializing instance.")
        self.value = value

# 测试
obj = MyClass(42)
print(obj)

class Singleton:
    _instance = None # 类变量，用于保存唯一的实例

    def __new__(cls, *args, **kwargs):
        # 如果实例还不存在，就创建一个
        if not cls._instance:
            cls._instance = super().__new__(cls)
        # 如果已经存在，直接返回已有的实例
        # 注意：每次调用 Singleton()，__init__ 仍然会被重新执行
        return cls._instance

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

# 测试
s1 = Singleton("First")
s2 = Singleton("Second")

print(s1 is s2)  # 输出: True，它们是同一个对象
print(s1.name)    # 输出: "Second" (注意！因为 __init__ 被第二次调用覆盖了)

class PositiveInteger(int):
    def __new__(cls, value):
        # 在创建对象之前，先对值进行加工
        return super().__new__(cls, abs(value))

# 测试
num = PositiveInteger(-5)
print(num)  # 输出: 5 (是一个真正的 int 类型)
print(type(num)) # 输出: <class '__main__.PositiveInteger'>

class ClassA:
    pass

class ClassB:
    pass

class MyFactory:
    def __new__(cls, obj_type):
        if obj_type == 'A':
            return ClassA()
        elif obj_type == 'B':
            return ClassB()
        else:
            return None

# 测试
obj1 = MyFactory('A')
obj2 = MyFactory('B')

print(type(obj1)) # 输出: <class '__main__.ClassA'>
print(type(obj2)) # 输出: <class '__main__.ClassB'>