在定义元类时，是否有任何理由选择__new__ over __init__？ [英] Is there any reason to choose __new__ over __init__ when defining a metaclass?

问题描述

我一直设置元类如下：

 类SomeMetaClass（type）：
 def __new__ （cls，name，bases，dict）：
 #do stuff here 
  

遇到一个定义如下的元类：

 类SomeMetaClass（type）：
 def __init __ ，bases，dict）：
 #do stuff here 
  

有任何理由

更新：请记住，我要询问使用 __ new __ 和 __ init __ 。我已经明白了他们在另一个类之间的区别。但在元类中，我不能使用 __ new __ 来实现缓存，因为 __ new __ 元类。

解决方案

如果要在创建类之前更改属性dict，或更改基本元组，使用 __ new __ 。到时间 __ init __ 看到参数，类对象已经存在。此外，如果要返回某个新创建的类型类以外的其他类，则必须使用 __ new __ 。

另一方面，在时间 __ init __ 运行时，类确实存在。

编辑：将改变的措辞更改为其中一个成员对象的引用。使得更清楚的是，通过对象，我的意思是类对象。

I've always set up metaclasses something like this:

class SomeMetaClass(type):
    def __new__(cls, name, bases, dict):
        #do stuff here

But I just came across a metaclass that was defined like this:

class SomeMetaClass(type):
    def __init__(self, name, bases, dict):
        #do stuff here

Is there any reason to prefer one over the other?

Update: Bear in mind that I'm asking about using __new__ and __init__ in a metaclass. I already understand the difference between them in another class. But in a metaclass, I can't use __new__ to implement caching because __new__ is only called upon class creation in a metaclass.

解决方案

If you want to alter the attributes dict before the class is created, or change the bases tuple, you have to use __new__. By the time __init__ sees the arguments, the class object already exists. Also, you have to use __new__ if you want to return something other than a newly created class of the type in question.

On the other hand, by the time __init__ runs, the class does exist. Thus, you can do things like give a reference to the just-created class to one of its member objects.

Edit: changed wording to make it more clear that by "object", I mean class-object.

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