__slots_ and inheritance

[Graham Ashton]

On Sat, 12 Apr 2003 15:53:38 +0000, Alex Martelli wrote:

>> Alexander Schmolck <a.schmolck at gmx.net> wrote in message
>> news:<yfsistmh4ec.fsf at black132.ex.ac.uk>...
>>  
>>> __slots__ is the one addition in python2.2 that really annoys me. It
>>> seems an ill-conceived jumble of conceptually unrelated things (bug
>>> avoidance, optimization, safety, encapsulation) that screws up
>>> reflection big time for little apparent gain. As Knuth said ...
> 
> I think it's an important memory optimization, PERIOD.  The other
> conceptually unrelated things (bug avoidance -- I can't even SEE the
> others!) aren't in __slots__' own design (even though a commentator
> who's usually quite perceptive seems to have seen the "bug avoidance"
> feature that wasn't really there).

>From http://www.python.org/doc/2.2.1/whatsnew/sect-rellinks.html:

    Finally, it's possible to constrain the list of attributes that can be
    referenced on an object using the new __slots__ class attribute.
    Python objects are usually very dynamic; at any time it's possible to
    define a new attribute on an instance by just doing obj.new_attr=1.
    This is flexible and convenient, but this flexibility can also lead to
    bugs, as when you meant to write obj.template = 'a' but made a typo
    and wrote obj.templtae by accident.

    A new-style class can define a class attribute named __slots__ to
    constrain the list of legal attribute names.

This thread has pointed out rather well that __slots__ don't really behave
as suggested above in anything but the simplest of situations. It wasn't
until I read the __slots__ section in the nutshell book that I realised
that this wasn't the case. Consequently, I think the above paragraphs are
misleading. Shouldn't they be fixed?

--
Graham Ashton