metaclass-元类

  • metaclass 就是一个用来创建其他 class 的类
  • type 类就是所有 class 默认的 metaclass
  • 可以在定义类时，声明 metaclass
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class Car(type):
    @staticmethod
    def __new__(mcs, *args):
        class_ = super().__new__(mcs, *args)
        class_.auto = True
        return class_

class Porsche(object, metaclass=Car):
    pass

print(Porsche.auto)
  • 依据传入的关键字参数，对属性进行扩展
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class Car(type):
    @staticmethod
    def __new__(mcs, *args， **kwargs):
        class_ = super().__new__(mcs, *args)
        if kwargs:
            for key, value in kwargs.items():
                setattr(class_, key, value)
        return class_

class Porsche(object, metaclass=Car, auto = True, origin = "Germany"):
    pass

print(Porsche.auto)
print(Porsche.origin)
  • 在元类中获取使用了元类的类的属性
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class PropUtil:
    def __init__(self,attr):
        self._attr = f"_{attr}"  # 避免递归调用

	def get(self,obj):
        if not hasattr(obj, self._attr):
            return None
        return getattr(obj, self._attr)

	def set(self,obj,value):
        setattr(obj,self._attr, value)

class Car(type):
    @staticmethod
    def __new__(mcs, *args， **kwargs):
        class_ = super().__new__(mcs, *args)
        for attr in class_.attrs:
            prop = PropUtil(attr)
            pObj = property(fget=prop.get, fset=prop.set)
            setattr(class_, attr, pObj)
        return class_

class Porsche(object, metaclass=Car):
    attrs = ["origin", "auto"]

panamera = Porsche()
print(panamera.auto)
panamera.auto = True
print(panamera.auto)

# 使用装饰器实现
def car(cls):
    return Car(cls.__name__, cls.__bases__, dict(cls.__dict__))

@car
class Benz:
    attrs = ["origin", "auto"]

benz = Benz()
print(benz.auto)
print(benz.origin)
benz.auto = False
print(benz.auto)

更新: 2024-01-10 00:47:48
原文: https://www.yuque.com/zacharyblock/cx2om6/ed59q8lf72v6ibfn