Type subclassing: bug or feature

[holger krekel]

Aahz 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?

class Names(str):
    def __init__(self, namelist):
        str.__init__(self, ":".join(namelist))

what would you expect 

    Names([1,2,3]).capitalize()

to return? An exception? I think it's dangerous to assume anything
about the subtyped classes' constructor. You *do* need a constructor
because strings are immutable so 'str.capitalize' can't modify
anything in place.

if-you-subtype-cake-you-gotta-eat-it-all-ly y'rs, holger