Inheriting from str

[ast]

Hello

class NewStr(str):
def __init__(self, s):
self.l = len(s)

Normaly str is an immutable type so it can't be modified
after creation with __new__

But the previous code is working well

obj = NewStr("qwerty")
obj.l
6

I don't understand why it's working ?


(python 3.9)

[Jon Ribbens]

The string itself is immutable. If it's a subclass then it may have
attributes that are not immutable:

>>> s = 'hello'
>>> s.jam = 3
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
AttributeError: 'str' object has no attribute 'jam'
>>> class foo(str): pass
...
>>> s = foo('hello')
>>> s.jam = 3
>>> s.jam
3

There's a lot of places in Python where you can break standard
assumptions with sufficiently evil or badly-written classes.
e.g.:

>>> class intt(int):
... def __add__(self, other):
... return int(self) + int(other) * 2
...
>>> a = intt(2)
>>> b = intt(3)
>>> a + b
8
>>> b + a
7

[Julio Di Egidio]

On Monday, 20 September 2021 at 16:27:24 UTC+2, Jon Ribbens wrote:
> On 2021-09-20, ast <ast@invalid> wrote:

<snip>

> > I don't understand why it's working ?
>
> The string itself is immutable. If it's a subclass then it may have
> attributes that are not immutable:
>
> >>> s = 'hello'
> >>> s.jam = 3
> Traceback (most recent call last):
> File "<stdin>", line 1, in <module>
> AttributeError: 'str' object has no attribute 'jam'
> >>> class foo(str): pass
> ...
> >>> s = foo('hello')
> >>> s.jam = 3
> >>> s.jam
> 3

I think you have to (re)declare the __slots__, or you inherit members but in the usual __dict__:
<< The action of a __slots__ declaration is not limited to the class where it is defined. __slots__ declared in parents are available in child classes. However, child subclasses will get a __dict__ and __weakref__ unless they also define __slots__ (which should only contain names of any additional slots). >>
<https://docs.python.org/3.9/reference/datamodel.html#notes-on-using-slots>

Julio