了解__init_subclass__ [英] Understanding __init_subclass__

问题描述

我终于升级了我的python版本，发现了新功能.除其他事项外，我为新的 __init_subclass__ 方法.从文档中:

I finally upgraded my python version and I was discovering the new features added. Among other things, I was scratching my head around the new __init_subclass__ method. From the docs:

只要包含类被子类化，就会调用此方法. cls 然后是新的子类.如果定义为普通实例方法，则此 方法隐式转换为类方法.

This method is called whenever the containing class is subclassed. cls is then the new subclass. If defined as a normal instance method, this method is implicitly converted to a class method.

因此，按照文档中的示例，我开始进行一些操作:

So I started to playing around with it a little bit, following the example in the docs:

class Philosopher:
    def __init_subclass__(cls, default_name, **kwargs):
        super().__init_subclass__(**kwargs)
        print(f"Called __init_subclass({cls}, {default_name})")
        cls.default_name = default_name

class AustralianPhilosopher(Philosopher, default_name="Bruce"):
    pass

class GermanPhilosopher(Philosopher, default_name="Nietzsche"):
    default_name = "Hegel"
    print("Set name to Hegel")

Bruce = AustralianPhilosopher()
Mistery = GermanPhilosopher()
print(Bruce.default_name)
print(Mistery.default_name)

产生此输出:

Called __init_subclass(<class '__main__.AustralianPhilosopher'>, 'Bruce')
'Set name to Hegel'
Called __init_subclass(<class '__main__.GermanPhilosopher'>, 'Nietzsche')
'Bruce'
'Nietzsche'

我知道该方法在子类定义之后称为 ，但是我的问题特别是关于此功能的用法.我阅读了 PEP 487 文章也是，但是对我没有多大帮助.这种方法在哪里有帮助?它用于:

I understand that this method is called after the subclass definition, but my questions are particularly about the usage of this feature. I read the PEP 487 article as well, but didn't help me much. Where would this method be helpful? Is it for:

• 在创建时注册子类的超类?
• 强制子类在定义时设置字段?

此外，我是否需要了解 __set_name__ 完全了解其用法?

Also, do I need to understand the __set_name__ to fully comprehend its usage?

推荐答案

__init_subclass__和__set_name__是正交机制-它们并不相互绑定，仅在同一PEP中进行了描述.两者都是以前需要全功能元类的功能. PEP 487解决了元类最常见的用法 2 :

__init_subclass__ and __set_name__ are orthogonal mechanisms - they're not tied to each other, just described in the same PEP. Both are features that needed a full-featured metaclass before. The PEP 487 addresses 2 of the most common uses of metaclasses:

• 如何让父母知道何时将其子类化(__init_subclass__)
• 如何让描述符类知道其用于(__set_name__)的属性的名称

• how to let the parent know when it is being subclassed (__init_subclass__)
• how to let a descriptor class know the name of the property it is used for (__set_name__)

正如PEP所说:

尽管有许多种使用元类的方法，但大多数用例仅分为三类:一些初始化代码在类创建后运行，描述符的初始化以及保持类属性的顺序定义.

可以通过对类的创建进行简单的钩子轻松地实现前两个类别:

The first two categories can easily be achieved by having simple hooks into the class creation:

• 一个__init_subclass__钩子，用于初始化给定类的所有子类.
• 在创建类时，在类中定义的所有属性(描述符)上调用一个__set_name__钩子，并且

• An __init_subclass__ hook that initializes all subclasses of a given class.
• upon class creation, a __set_name__ hook is called on all the attribute (descriptors) defined in the class, and

第三类是另一个PEP的主题， PEP 520 .

The third category is the topic of another PEP, PEP 520.

还请注意，虽然__init_subclass__替代了在此类的继承树中使用元类，但__set_name__在 descriptor类中却替代了使用具有 descriptor实例作为属性的类的元类.

Notice also, that while __init_subclass__ is a replacement for using a metaclass in this class's inheritance tree, __set_name__ in a descriptor class is a replacement for using a metaclass for the class that has an instance of the descriptor as an attribute.

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