为什么不自动调用超类 __init__ 方法? [英] Why aren't superclass __init__ methods automatically invoked?

问题描述

为什么 Python 设计者决定子类的 __init__() 方法不会像在其他一些语言中那样自动调用其超类的 __init__() 方法?Pythonic 和推荐的成语真的像下面这样吗?

Why did the Python designers decide that subclasses' __init__() methods don't automatically call the __init__() methods of their superclasses, as in some other languages? Is the Pythonic and recommended idiom really like the following?

class Superclass(object):
    def __init__(self):
        print 'Do something'

class Subclass(Superclass):
    def __init__(self):
        super(Subclass, self).__init__()
        print 'Do something else'

推荐答案

Python 的 __init__ 与其他语言 构造函数 之间的关键区别在于 __init__ 是不是一个构造函数:它是一个初始化器(实际的构造函数(如果有的话，但是，见后面；-)是__new__ 并再次以完全不同的方式工作).虽然构造所有超类(毫无疑问，在继续向下构造之前这样做")显然是在说您正在构造一个子类的实例，这显然是不是初始化的情况，因为有很多用例需要跳过、更改、控制超类的初始化——如果有的话，发生在子类初始化的中间"，等等.

The crucial distinction between Python's __init__ and those other languages constructors is that __init__ is not a constructor: it's an initializer (the actual constructor (if any, but, see later;-) is __new__ and works completely differently again). While constructing all superclasses (and, no doubt, doing so "before" you continue constructing downwards) is obviously part of saying you're constructing a subclass's instance, that is clearly not the case for initializing, since there are many use cases in which superclasses' initialization needs to be skipped, altered, controlled -- happening, if at all, "in the middle" of the subclass initialization, and so forth.

基本上，初始化器的超类委托在 Python 中不是自动的，原因完全相同，这种委托对于任何其他方法也不是自动的——并注意那些其他语言"不也不要为任何其他方法执行自动超类委托...只是为构造函数(如果适用，析构函数)，正如我所提到的，不是Python 的 __init__ 是什么.(__new__ 的行为也很奇特，虽然实际上与您的问题没有直接关系，因为 __new__ 是一个如此奇特的构造函数，它实际上并不一定需要构造任何东西-- 可以完美地返回现有实例，甚至是非实例......很明显，Python 为您提供了比您心目中的其他语言"更多很多 的机制控制，这 很多em>还包括在 __new__ 本身中没有自动委派！-).

Basically, super-class delegation of the initializer is not automatic in Python for exactly the same reasons such delegation is also not automatic for any other methods -- and note that those "other languages" don't do automatic super-class delegation for any other method either... just for the constructor (and if applicable, destructor), which, as I mentioned, is not what Python's __init__ is. (Behavior of __new__ is also quite peculiar, though really not directly related to your question, since __new__ is such a peculiar constructor that it doesn't actually necessarily need to construct anything -- could perfectly well return an existing instance, or even a non-instance... clearly Python offers you a lot more control of the mechanics than the "other languages" you have in mind, which also includes having no automatic delegation in __new__ itself!-).

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