python - “classmethod”对象在 Python 3 的元类方法中不可调用

Question

我正在尝试实现一个元类，它在创建第一个实例时初始化类变量。我想保留一个新的魔术方法 __load__，它应该作为类方法调用(如 __new__)。所以我是这样实现的:

class StaticLoad(type):
    __loaded_classes = set()

    def __call__(cls, *args, **kwargs):
        if cls not in cls.__loaded_classes:
            if hasattr(cls, '__load__'):
                cls.__load__()
            cls.__loaded_classes.add(cls)
        return super().__call__(*args, **kwargs)


class BaseClass(metaclass=StaticLoad):
    s = 0


class MyClass(BaseClass):
    @classmethod
    def __load__(cls):
        print("Loading", cls.__name__, "...")
        cls.s += 1


obj1 = MyClass()
obj2 = MyClass()
print(MyClass.s)

它工作正常并给出正确的结果:

Loading MyClass ...
1

现在我想将方法​​__load__默认实现为类方法，如__new__(无需每次都在上面键入@classmethod ).我试过这个:

class StaticLoad(type):
    __loaded_classes = set()

    def __call__(cls, *args, **kwargs):
        if cls not in cls.__loaded_classes:
            if hasattr(cls, '__load__'):
                # I try to apply classmethod routine to make
                # cls.__load__ a classmethod
                classmethod(cls.__load__)()
            cls.__loaded_classes.add(cls)
        return super().__call__(*args, **kwargs)


class BaseClass(metaclass=StaticLoad):
    s = 0


class MyClass(BaseClass):
    # @classmethod line was deleted
    def __load__(cls):
        print("Loading", cls.__name__, "...")
        cls.s += 1


obj1 = MyClass()
obj2 = MyClass()
print(MyClass.s)

我得到了错误:

Traceback (most recent call last):
  File "example.py", line 22, in <module>
    obj1 = MyClass()
  File "example.py", line 7, in __call__
    classmethod(cls.__load__)()
TypeError: 'classmethod' object is not callable

看起来 classmethod 例程仅在类定义中正确可用。

我应该如何改进我的元类以使其正常工作？我想保留我上面写的类 BaseClass 和 MyClass 的内容，将所有魔法放入 StaticLoad。

Answer 1

在@AnttiHaapala 的帮助下，解决方案很简单。而不是打电话

classmethod(cls.__load__)()

我不得不打电话

cls.__load__(cls)