Type subclassing: bug or feature

[Anders J. Munch]

"Aahz" <aahz at pythoncraft.com> wrote:
> Consider the following code:
> 
> class MyStr(str):
>     def contains(self, value):
>         return self.find(value) >= 0
> 
> s = MyStr("hello, world!")
> s = s.capitalize()
> if s.contains('Hello'):
>     print "Found it!"
> 
> It fails with an AttributeError when it calls s.contains(), because
> s.capitalize() returned a str instead of a MyStr.  Anyone want to take a
> whack at defending this as the correct behavior?

Inheriting from a class/type with value semantics is usually bad
design.  Use containment or helper functions instead.

it-so-happens-that-Python-has-support-for-creating-thusly-flawed-designs-
but-that-doesn't-make-it-a-good-idea-ly y'rs, Anders