using an instance of Object as an empty class

[Peter Otten]

Ulrich Eckhardt wrote:

> 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?

collections.namedtuple is a convenient struct replacement -- if you don't 
mind that it is immutable.