从 str 或 int 继承 [英] inheritance from str or int

问题描述

为什么我在创建从 str(或也从 int)继承的类时遇到问题

C 类(str):def __init__(self, a, b):str.__init__(self,a)自我.b = bC(a"，B")类型错误:str() 最多接受 1 个参数(给定 2 个)

如果我尝试使用 int 而不是 str，也会发生同样的情况，但它适用于自定义类.我需要使用 __new__ 而不是 __init__ 吗?为什么?

解决方案

>>>C类(str):... def __new__(cls, *args, **kw):...返回 str.__new__(cls, *args, **kw)...>>>c = C("你好世界")>>>类型(c)<类'__main__.C'>>>>c.__class__.__mro__(<class '__main__.C'>, <type 'str'>, <type 'basestring'>, <type 'object'>)

由于__init__是在对象被构造后调用的，所以修改不可变类型的值已经来不及了.注意 __new__ 是一个类方法，所以我调用了第一个参数 cls

请参阅此处了解更多信息

<预><代码>>>>C类(str):... def __new__(cls, value, meta):... obj = str.__new__(cls, value)... obj.meta = 元...返回对象...>>>c = C("你好世界", "元")>>>C'你好世界'>>>元'元'

Why I have problem creating a class inheriting from str (or also from int)

class C(str):
   def __init__(self, a, b):
     str.__init__(self,a)
     self.b = b

C("a", "B")

TypeError: str() takes at most 1 argument (2 given)

the same happens if I try to use int instead of str, but it works with custom classes. I need to use __new__ instead of __init__? why?

解决方案

>>> class C(str):
...     def __new__(cls, *args, **kw):
...         return str.__new__(cls, *args, **kw)
... 
>>> c = C("hello world")
>>> type(c)
<class '__main__.C'>

>>> c.__class__.__mro__
(<class '__main__.C'>, <type 'str'>, <type 'basestring'>, <type 'object'>)

Since __init__ is called after the object is constructed, it is too late to modify the value for immutable types. Note that __new__ is a classmethod, so I have called the first parameter cls

See here for more information

>>> class C(str):
...     def __new__(cls, value, meta):
...         obj = str.__new__(cls, value)
...         obj.meta = meta
...         return obj
... 
>>> c = C("hello world", "meta")
>>> c
'hello world'
>>> c.meta
'meta'

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