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

问题描述

我只是想简化我的一门课，并以与 flyweight设计模式。

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

但是，为什么总是 __ init __ 有点困惑在 __ new __ 之后调用。我没想到这一点。谁能告诉我为什么会这样，否则我如何实现此功能？ （除了将实现放到 __ new __ 中，这感觉很hacky。）

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.)

这里有个例子：

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()

输出：

NEW
INIT

EXISTS
INIT

EXISTS
INIT

为什么？

推荐答案

使用 __new __ ，当您需要控制
创建新实例时。

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

当您需要控制新的初始化时，请使用
__ init __

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

__ new __ 是实例创建的第一步。首先被称为，是
负责返回您班级的新
实例。

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

相反，
__ init __ 不返回任何内容；

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

通常，您不需要
覆盖 __ new __ ，除非您将
子类化为不可变类型，例如
str，int，unicode或tuple。

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

从2008年4月发布：何时在mail.python.org上使用 __ new __ 与 __ init __ ？。

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

您应考虑通常要使用工厂，这是最好的方法。使用 __ new __ 并不是一个好的解决方案，因此请考虑使用工厂。在这里，您有一个很好的工厂示例。

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 you have a good factory example.

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