为什么 __init__() 总是在 __new__() 之后调用? [英] Why is __init__() always called after __new__()?

问题描述

我只是想简化我的一个类，并以与 相同的风格引入了一些功能享元设计模式.

然而，我有点困惑为什么 __init__ 总是在 __new__ 之后被调用.我没想到这一点.谁能告诉我为什么会发生这种情况以及我如何才能实现此功能?(除了将实现放入 __new__ 中，这感觉很hacky.)

这是一个例子:

A 类(对象):_dict = dict()def __new__(cls):如果 A._dict 中的键":打印存在"返回 A._dict['key']别的:打印新"返回 super(A, cls).__new__(cls)def __init__(self):打印初始化"A._dict['key'] = self打印 ""a1 = A()a2 = A()a3 = A()

输出:

NEW在里面存在在里面存在在里面

为什么?

解决方案

在需要控制时使用__new__创建新实例.

<块引用>

使用__init__ 当您需要控制新实例的初始化时.

__new__ 是实例创建的第一步.它首先被调用，并且是负责退回一个新的类的实例.

<块引用>

相比之下，__init__ 不返回任何内容；它只负责初始化实例创建后.

一般来说，您不需要覆盖 __new__ 除非你子类化一个不可变类型，如str、int、unicode 或元组.

来自 2008 年 4 月的帖子:何时使用 __new__ vs. __init__? 在 mail.python.org 上.

您应该考虑到您尝试执行的操作通常是通过 Factory 完成的，那就是最好的方法来做到这一点.使用 __new__ 不是一个好的干净的解决方案，所以请考虑使用工厂.这是一个很好的例子:ActiveState Fᴀᴄᴛᴏʀʏ ᴘᴀᴛᴛᴇʀɴ食谱.

I'm just trying to streamline one of my classes and have introduced some functionality in the same style as the flyweight design pattern.

However, I'm a bit confused as to why __init__ is always called after __new__. I wasn't expecting this. Can anyone tell me why this is happening and how I can implement this functionality otherwise? (Apart from putting the implementation into the __new__ which feels quite hacky.)

Here's an example:

class A(object):
    _dict = dict()

    def __new__(cls):
        if 'key' in A._dict:
            print "EXISTS"
            return A._dict['key']
        else:
            print "NEW"
            return super(A, cls).__new__(cls)

    def __init__(self):
        print "INIT"
        A._dict['key'] = self
        print ""

a1 = A()
a2 = A()
a3 = A()

Outputs:

NEW
INIT

EXISTS
INIT

EXISTS
INIT

Why?

解决方案

Use __new__ when you need to control the creation of a new instance.

Use __init__ when you need to control initialization of a new instance.

__new__ is the first step of instance creation. It's called first, and is responsible for returning a new instance of your class.

In contrast, __init__ doesn't return anything; it's only responsible for initializing the instance after it's been created.

In general, you shouldn't need to override __new__ unless you're subclassing an immutable type like str, int, unicode or tuple.

From April 2008 post: When to use __new__ vs. __init__? on mail.python.org.

You should consider that what you are trying to do is usually done with a Factory and that's the best way to do it. Using __new__ is not a good clean solution so please consider the usage of a factory. Here's a good example: ActiveState Fᴀᴄᴛᴏʀʏ ᴘᴀᴛᴛᴇʀɴ Recipe.

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