trouble subclassing str

[Donn Cave]

In article <1119619726.756391.175800 at z14g2000cwz.googlegroups.com>,
 "Paul McGuire" <ptmcg at austin.rr.com> wrote:
[ ... lots of interesting discussion removed ... ]

> Most often, I see "is-a" confused with "is-implemented-using-a".  A
> developer decides that there is some benefit (reduced storage, perhaps)
> of modeling a zip code using an integer, and feels the need to define
> some class like:
> 
>     class ZipCode(int):
>         def lookupState(self):
>             ...
> 
> But zip codes *aren't* integers, they just happen to be numeric - there
> is no sense in supporting zip code arithmetic, nor in using zip codes
> as slice indices, etc.  And there are other warts, such as printing zip
> codes with leading zeroes (like they have in Maine).

I agree, but I'm not sure how easily this kind of reasoning
can be applied more generally to objects we write.  Take for
example an indexed data structure, that's generally similar
to a dictionary but may compute some values.  I think it's
common practice in Python to implement this just as I'm sure
you would propose, with composition.  But is that because it
fails your "is-a" test?  What is-a dictionary, or is-not-a
dictionary?  If you ask me, there isn't any obvious principle,
it's just a question of how we arrive at a sound implementation --
and that almost always militates against inheritance, because
of liabilities you mentioned elsewhere in your post, but in the
end it depends on the details of the implementation.

   Donn Cave, donn at u.washinton.edu