python 如何确定合适的元类？

Question

我可能在official doc中找到了答案，但我还是得不到。在第二种和第三种情况下，你能给我更详细或更直接的解释吗？如果你能给我一些例子，我将不胜感激！

The appropriate metaclass for a class definition is determined as follows:

if no bases and no explicit metaclass are given, then type() is used;

if an explicit metaclass is given and it is not an instance of type(), then it is used directly as the metaclass;

if an instance of type() is given as the explicit metaclass, or bases are defined, then the most derived metaclass is used.

Answer 1

假设您试图通过元类（在本例中为其中的两个）来实现单例模式。

class Singleton(type):
    _inst = {}

    def __call__(cls, *args, **kwargs):
        return cls._inst.setdefault(cls, super().__call__(*args, **kwargs))

class Singleton2(Singleton):
    def __call__(cls, *args, **kwargs):
        return super().__call__(*args, **kwargs)



class SingletonClass1(metaclass=Singleton):
    pass


class SingletonClass2(SingletonClass1, metaclass=Singleton2):
    pass


class SingletonClass3():
    pass


class SingletonClass4(SingletonClass2, metaclass=Singleton):
    pass

type(class_obj) 函数 returns class_obj 的元类。

这是您的案例：

如果没有给出基类和显式元类，则使用 type()；

type(SingletonClass3) --> <class 'type'>

~~2.如果给出了一个显式的元类，并且它不是type()的实例，那么直接作为元类使用；~~

~~`type(SingletonClass2) --> <class '__main__.Singleton2'>`~~

3. 如果 type() 的实例作为显式元类给出，或者定义了基类，则使用最派生的元类；

`type(SingletonClass4) --> <class '__main__.Singleton2'>`

如果你从SingletonClass1继承了SingletonClass2，却忘记从Singleton继承Singleton2，会导致TypeError。

The most derived metaclass is one which is a subtype of all of these candidate metaclasses. If none of the candidate metaclasses meets that criterion, then the class definition will fail with TypeError.

更新： 元类可以是函数，不是 type 的实例，因此第二种情况（如果给出了显式元类并且它不是类型 的实例）可以与这个。这是元类函数的 Singleton 实现。

def singleton_func(class_name, bases, attrs):
    _inst = {}

    def custom_new(cls):
        return _inst.setdefault(cls, object.__new__(cls))

    attrs['__new__'] = custom_new
    return type(class_name, bases, attrs)


class SingletonClass5(metaclass=singleton_func):
    pass

python 如何确定合适的元类？

How does the python determine the appropriate metaclass?

python

grammar

cpython