Python元编程声明与初始化定制元类实例探究

class MyClass:
    x = 5
print(type(MyClass))  # 输出：<class 'type'>

class MyClass:
    def __new__(cls, *args, **kwargs):
        instance = super(MyClass, cls).__new__(cls)
        return instance
    def __init__(self, a, b):
        self.a = a
        self.b = b

class CustomMeta(type):
    def __new__(cls, name, bases, dct):
        # 自定义元类行为
        return super().__new__(cls, name, bases, dct)
class CustomClass(metaclass=CustomMeta):
    pass

class CustomMeta(type):
    def __new__(cls, name, bases, dct):
        dct['new_attribute'] = 'This is a new attribute added dynamically.'
        return super().__new__(cls, name, bases, dct)
class CustomClass(metaclass=CustomMeta):
    pass
print(CustomClass.new_attribute)  # 输出：This is a new attribute added dynamically.

class SingletonMeta(type):
    _instances = {}
    def __call__(cls, *args, **kwargs):
        if cls not in cls._instances:
            cls._instances[cls] = super().__call__(*args, **kwargs)
        return cls._instances[cls]
class SingletonClass(metaclass=SingletonMeta):
    pass
instance1 = SingletonClass()
instance2 = SingletonClass()
print(instance1 is instance2)  # 输出：True，表明两个实例是同一个对象

class MetaA(type):
    def foo(cls):
        print("Method from MetaA")

class MetaB(type):
    def bar(cls):
        print("Method from MetaB")

class MyClass(metaclass=MetaA, metaclass=MetaB):
    pass

MyClass.foo()  # 输出：Method from MetaA
MyClass.bar()  # 输出：Method from MetaB

class CustomMeta(type):
    def __new__(cls, name, bases, dct):
        # 自定义类的行为
        return super().__new__(cls, name, bases, dct)

class CustomClass(metaclass=CustomMeta):
    pass

class CustomMeta(type):
    def __new__(cls, name, bases, dct):
        return super().__new__(cls, name, bases, dct)

    def __init__(cls, name, bases, dct):
        # 初始化操作
        pass

class CustomClass(metaclass=CustomMeta):
    pass

def custom_method(self):
    return "This is a dynamically added method."

CustomClass = CustomMeta("CustomClass", (), {'custom_method': custom_method})
instance = CustomClass()
result = instance.custom_method()
print(result)  # 输出：This is a dynamically added method.

class SecureMeta(type):
    def __new__(cls, name, bases, dct):
        if 'password' in dct:
            raise ValueError("Class cannot contain 'password' attribute.")
        return super().__new__(cls, name, bases, dct)

class SecureClass(metaclass=SecureMeta):
    pass

# 会抛出异常：ValueError: Class cannot contain 'password' attribute.
class InsecureClass(SecureClass):
    password = "12345"