Type subclassing: bug or feature

[holger krekel]

To Aahz wrote:
> 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()

oops. sorry. should have checked it better (or at all<self.wink>)...

class Names(str):
    def __new__(cls, namelist):
        return str.__new__(str, ":".join(namelist))

n=Names(['tim','peter','guido'])

But hopefully my point was understandable that it is hard 
to guess a return type for 'n.capitalize()' now ...

> 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

cheers,

    holger