`__init__` 能做什么而 `__new__` 不能?

问题描述

在 Python 中，__new__ 用于初始化不可变类型，__init__ 通常用于初始化可变类型.如果 __init__ 从语言中删除，什么不能再做(容易)?

In Python, __new__ is used to initialize immutable types and __init__ typically initializes mutable types. If __init__ were removed from the language, what could no longer be done (easily)?

例如，

class A:

    def __init__(self, *, x, **kwargs):
        super().__init__(**kwargs)
        self.x = x


class B(A):

    def __init__(self, y=2, **kwargs):
        super().__init__(**kwargs)
        self.y = y

可以像这样使用 __new__ 重写:

Could be rewritten using __new__ like this:

class A_N:

    def __new__(cls, *, x, **kwargs):
        obj = super().__new__(cls, **kwargs)
        obj.x = x
        return obj


class B_N(A_N):

    def __new__(cls, y=2, **kwargs):
        obj = super().__new__(cls, **kwargs)
        obj.y = y
        return obj

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澄清问题范围:这不是关于如何使用 __init__ 和 __new__ 或它们之间有什么区别的问题.这是一个关于如果从语言中删除 __init__ 会发生什么的问题.有什么会坏吗?有什么事情会变得更难或不可能吗?

Clarification for scope of question: This is not a question about how __init__ and __new__ are used or what is the difference between them. This is a question about what would happen if __init__ were removed from the language. Would anything break? Would anything become a lot harder or impossible to do?

推荐答案

你可以在 __init__ 中做的所有事情，也可以在 __new__ 中完成.

Everything you can do in __init__ can also be done in __new__.

那么，为什么要使用 __init__?
因为您不必将实例存储在变量中(示例代码中的 obj )，然后再费心返回它.你可以专注于你真正想做的事 –初始化可变对象.

Then, why use __init__?
Because you don't have to store instance in variable (obj in your example code), and later bother returning it. You can focus on what you realy want to do – initializing mutable object.

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