method names in __slots__ ??

[John Machin]

Rob Williscroft wrote:
> John Machin wrote in news:1167008799.074885.250770@
> 73g2000cwn.googlegroups.com in comp.lang.python:
>
> > Given a = Adder(),
> >     a.tally = 0
> > gets AttributeError: 'Adder' object attribute 'tally' is read-only
> >     a.notinslots = 1
> > gets AttributeError: 'Adder' object attribute 'notinslots' is read-only
> >
> > So is there some magic class-fu going down here, or is this just a
> > waste of memory space in the instances?
> >
>
> Haven't you, with your 2 examples above, answered your own question ?

No.

>
> Clearly from your example it doesn't make any difference if you add
> a class attribute to the slots, one way or another its as if you
> hadn't put it in there in the first place.

Clearly? Not so. It takes up memory. A list of 1 million Adder
instances takes up about 68 Mb (Python 2.5 on Windows XP). With the
method names removed from the __slots__, it takes only about 44 Mb.
[For comparison: with no __slots__ at all, it takes about 180 Mb]

>
> This will give the same error, which shows its about class attributes
> and not just methods:
>
> class Adder(object):
>
>     __slots__ = [
>           'class_name'
>         ]
>
>     class_name = 3
>
>
> a = Adder()
>
> a.class_name = 2
>
> It would seem that the interpreter removes any names it finds as class
> attribute names from the list it finds in __slots__ before it creates
> the instance.

It doesn't seem so to me. If it did that, the memory usage would not
increase.

>
> Of course if my guessing above isn't good enough, we could look at
> the documentation:
>
>   http://docs.python.org/ref/slots.html#l2h-218
>
> __slots__ are implemented at the class level by creating descriptors
> (3.4.2) for each variable name. As a result, class attributes cannot be
> used to set default values for instance variables defined by __slots__;
> otherwise, the class attribute would overwrite the descriptor assignment.

I have read that, before I posted. Asides:
(1) It would be useful if it stated the empirically determined fact
that the result is that the class attribute is thusly made read-only.
(2) The second sentence is not a model of clarity.

In any  case I can't see how the paragraph gives any support for your
next statement:

>
> So its that the __slots__ assignment makes the descriptors and then the
> subsiquent method defenitions and class attribute bindings remove them.

Errrmmm ... if the descriptors are removed, how is it that the
behaviour is read-only?

Cheers,
John