__slots_ and inheritance

Alex Martelli aleax at aleax.it
Tue Apr 15 00:05:46 CEST 2003

Graham Ashton wrote:

> 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?

Yes, I would agree they should be.  I don't know how (and indeed, if)
one can submit patches for AMK's "what's new" documents, though -- I
don't normally think of them as part of "Python's docs" (I don't even
think they are in the CVS...).


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