python如何防止类被子类化? [英] How does python prevent a class from being subclassed?

问题描述

我在 python文档中遇到了以下内容:

I came across the following in the python docs:

bool([x])

使用标准真值测试过程将值转换为布尔值.如果x为false或省略，则返回False；否则为false.否则 返回True. bool也是一个类，它是int的子类. 班级 bool不能再进一步子类化.它的唯一实例是False和 是的.

Convert a value to a Boolean, using the standard truth testing procedure. If x is false or omitted, this returns False; otherwise it returns True. bool is also a class, which is a subclass of int. Class bool cannot be subclassed further. Its only instances are False and True.

我一生中从未想过要继承bool，但自然而然地，我立即尝试了它，而且很确定:

I've never in my life wanted to subclass bool, but naturally I immediately tried it, and sure enough:

>>> class Bool(bool):
    pass

Traceback (most recent call last):
  File "<pyshell#2>", line 1, in <module>
    class Bool(bool):
TypeError: Error when calling the metaclass bases
    type 'bool' is not an acceptable base type

所以，问题是:这是怎么做的?我是否可以使用相同的技术(或不同的技术)将自己的类标记为final，即防止它们被子类化?

So, the question: How is this done? And can I apply the same technique (or a different one) to mark my own classes as final, i.e., to keep them from being subclassed?

推荐答案

bool类型在C中定义，并且其tp_flags插槽故意不包含

The bool type is defined in C, and its tp_flags slot deliberately does not include the Py_TPFLAGS_BASETYPE flag.

C类型需要将自己明确地标记为为可归类的.

C types need to mark themselves explicitly as subclassable.

要对自定义Python类执行此操作，请使用元类:

To do this for custom Python classes, use a metaclass:

class Final(type):
    def __new__(cls, name, bases, classdict):
        for b in bases:
            if isinstance(b, Final):
                raise TypeError("type '{0}' is not an acceptable base type".format(b.__name__))
        return type.__new__(cls, name, bases, dict(classdict))

class Foo:
    __metaclass__ = Final

class Bar(Foo):
    pass

给予:

>>> class Bar(Foo):
...     pass
... 
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 5, in __new__
TypeError: type 'Foo' is not an acceptable base type

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