Type subclassing: bug or feature

Gustavo Cordova gcordova at hebmex.com
Thu Jun 13 23:18:15 CEST 2002


> >
> > 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?
> > --
> > Aahz (aahz at pythoncraft.com)           <*>
> > http://www.pythoncraft.com/
> 
> Looks like a bug to me. How can self be something else then 
> what it is?
> Should MySocket return a socket, thus stripping off any additional
> functionality added by my derived class?
> 
> David LeBlanc
> Seattle, WA USA
> 

Hmmm...

maybe all methods of builtin types should do:

	return self.__class__(return_value)

when returning a new object (like str methods).

-gustavo





More information about the Python-list mailing list