using an instance of Object as an empty class

[Ulrich Eckhardt]

Peter Otten wrote:
> Examples for classes that don't accept attributes are builtins
> like int, tuple, and -- obviously -- dict. You can make your own
> using the __slot__ mechanism:
> 
>>>> class A(object):
> ...     __slots__ = ["x", "y"]
> ...
>>>> a = A()
>>>> a.x = 42
>>>> a.y = "yadda"
>>>> a.z = 123
> Traceback (most recent call last):
>   File "<stdin>", line 1, in <module>
> AttributeError: 'A' object has no attribute 'z'

Wow. This is something I've been missing dearly in my toolbox until now! 
Typically, when in C++ I would have used a struct, I always created a class 
that set the according attributes in the init function instead, which is 
quite a bit more cumbersome. Or I used a tuple and hoped to get the position 
of the elements correct.

Now, follow-up question:
1. The slots are initialized to None, right? Or are they just reserved? IOW, 
would "print a.x" right after creation of the object print anything or raise 
an AttributeError?

2. Is there a convenient syntax to init an instance of such a class? Can I 
convert it from e.g. a dict or do I still have to write a constructor or 
manually fill in the slots?


Thank you!


Uli

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