On Thu, Feb 18, 2021 at 1:00 PM Christopher Barker <python...@gmail.com> wrote:
>
> > On Wed, Feb 17, 2021 at 9:11 PM Sven R. Kunze <srku...@mail.de> wrote:
>>
>> >> Still think that "object()" should be writable since this seems like an
>> >> arbitrary restriction
>
> ...
>>
>> But I guess there's been discussion around this already.
>
>
> > ... but changing object would be problematic.
>
> Well, yes, due to backward compatibility -- though how much code is counting
> on not being able to add attributes to an instance of object?
>
> I think someone on this thread (sorry can't find it now) said something like:
>
> if you could add attributes to object(), then you'd be able to add attributes
> to subclasses of object. -- but you can already:
>
> Isn't every class a subclass of object?
>
> class C:
> pass
You can add attributes to THIS subclass of object. But you can't add
attributes to EVERY subclass of object. For instance:
x = 1
x.something = 123
If object() could have attributes added, then 1 would have to be able
to have attributes added too. And that would cause all manner of
problems.
> It seems the difference is that both a new class and class instances get a
> __dict__. But given that all classes derive from object, and object is of
> type type, and classes are of type type -- I still have no idea why we can't
> add things to an instance of object.
>
When you create a class as you did above, it gets a __dict__. But if
you set __slots__, the only valid attributes are those listed in the
slots. These are inherited:
>>> class X: __slots__ = 'a', 'b'
...
>>> class Y(X): __slots__ = 'q', 'w'
...
>>> Y.__slots__
('q', 'w')
>>> Y().a = 1
>>> Y().q = 1
>>> Y().z = 1
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
AttributeError: 'Y' object has no attribute 'z'
But if object() could get arbitrary attributes, then __slots__ wouldn't work.
ChrisA
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