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trouble subclassing str

 
 
Donn Cave
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      06-24-2005
In article <(E-Mail Removed) .com>,
"Paul McGuire" <(E-Mail Removed)> wrote:
....
> This reminds me of some maddening O-O discussions I used to
> have at a former place of employment, in which one developer cited
> similar behavior for not having Square inherit from Rectangle - calling
> Square.setWidth() would have to implicitly call setHeight() and vice
> versa, in order to maintain its squarishness, and thereby broke Liskov.
> I withdrew from the debate, citing lack of context that would have
> helped resolve how things should go. At best, you can *probably* say
> that both inherit from Shape, and can be drawn, have an area, a
> bounding rectangle, etc., but not either inherits from the other.


This Squares and Rectangles issue sounds debatable in a
language like C++ or Java, where it's important because
of subtype polymorphism. In Python, does it matter?
As a user of Square, I'm not supposed to ask about its
parentage, I just try to be clear what's expected of it.
There's no static typing to notice whether Square is a
subclass of Rectangle, and if it gets out that I tried
to discover this issubclass() relationship, I'll get a
lecture from folks on comp.lang.python who suspect I'm
confused about polymorphism in Python.

This is a good thing, because as you can see it relieves
us of the need to debate abstract principles out of context.
It doesn't change the real issues - Square is still a lot
like Rectangle, it still has a couple of differences, and
the difference could be a problem in some contexts designed
for Rectangle - but no one can fix that. If you need Square,
you'll implement it, and whether you choose to inherit from
Rectangle is left as a matter of implementation convenience.

Donn Cave, http://www.velocityreviews.com/forums/(E-Mail Removed)
 
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Bengt Richter
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      06-26-2005
On 23 Jun 2005 21:27:20 -0700, "Paul McGuire" <(E-Mail Removed)> wrote:

>Dang, that class should be:
>
>class PaddedStr(str):
> def __new__(cls,s,l,padc=' '):
> if l > len(s):
> s2 = "%s%s" % (s,padc*(l-len(s)))
> return str.__new__(cls,s2)
> else:
> return str.__new__(cls,s)
>

Or you could write

>>> class PaddedStr2(str):

... def __new__(cls,s,l,padc=' '):
... return str.__new__(cls, s+padc*(l-len(s)))
...

Which gives

>>> print '>%s<' % PaddedStr2('xxx',5,'.')

>xxx..<
>>> print '>%s<' % PaddedStr2('xxx',3,'.')

>xxx<
>>> print '>%s<' % PaddedStr2('xxx',2,'.')

>xxx<


(Taking advantage of multipliers <=0 working like 0 for strings):

>>> for i in xrange(-3,4): print '%2s: >%s<'% (i, 'xxx'+'.'*i)

...
-3: >xxx<
-2: >xxx<
-1: >xxx<
0: >xxx<
1: >xxx.<
2: >xxx..<
3: >xxx...<

Regards,
Bengt Richter
 
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