Python中__new__()方法适应及注意事项详解

class MyClass:  
    def __new__(cls, *args, **kwargs):  
        print("Creating a new instance of MyClass")  
        instance = super(MyClass, cls).__new__(cls)  # 调用父类的 __new__ 方法来创建实例  
        return instance  
  
    def __init__(self, value):  
        print("Initializing MyClass instance")  
        self.value = value  
  
# 实例化 MyClass  
obj = MyClass(10)
输出：

Creating a new instance of MyClass  
Initializing MyClass instance

class MyClass:  
    def __new__(cls, *args, **kwargs):  
        print("Creating a new instance of MyClass")  
        # 不返回任何内容（即隐式返回 None）  
  
    def __init__(self, value):  
        print("Initializing MyClass instance")  
        self.value = value  
  
# 实例化 MyClass，注意 __init__ 不会被调用  
obj = MyClass(10)  # 只会输出 "Creating a new instance of MyClass"

class Singleton:  
    _instance = None  
  
    def __new__(cls, *args, **kwargs):  
        if not cls._instance:  
            cls._instance = super(Singleton, cls).__new__(cls)  
        return cls._instance  
  
    def __init__(self, value=None):  
        if not hasattr(self, 'initialized'):  # 确保 __init__ 只被调用一次  
            self.value = value  
            self.initialized = True  
  
# 尝试创建多个实例  
obj1 = Singleton(1)  
obj2 = Singleton(2)  
  
print(obj1 is obj2)  # 输出: True  
print(obj1.value)    # 输出: 1，因为 __init__ 只会在第一次实例化时被调用

class FactoryClass:  
    @staticmethod  
    def __new__(cls, *args, **kwargs):  
        if kwargs.get('type') == 'A':  
            return ClassA(*args, **kwargs)  
        elif kwargs.get('type') == 'B':  
            return ClassB(*args, **kwargs)  
        else:  
            raise ValueError("Unknown type")  
  
class ClassA:  
    def __init__(self, value):  
        self.value = value  
        print(f"ClassA instance created with value: {value}")  
  
class ClassB:  
    def __init__(self, value):  
        self.value = value  
        print(f"ClassB instance created with value: {value}")  
  
# 使用工厂类创建不同类型的实例  
obj_a = FactoryClass(type='A', value=10)  
obj_b = FactoryClass(type='B', value=20)