method names in __slots__ ??

[Rob Williscroft]

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 ?

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.

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.

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. 

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

Rob.
-- 
http://www.victim-prime.dsl.pipex.com/