overriding method that returns base class object

[Stuart McGraw]

"Paul Rubin" <http://phr.cx@NOSPAM.invalid> rote in message news:7xwu6mmm3v.fsf at ruckus.brouhaha.com...
> "Stuart McGraw" <smcg4191 at frii.RemoveThisToReply.com> writes:
> > Class A has a method A.a() that returns an A.  I want a 
> > identical class but with an additional property .newprop
> > and method .b()  And I want .a() to return a B, not an A.
> > 
> >   class B (A):
> >     def __init__(self, *args, **kwds):
> >       A.__init__(self, *args, **kwds)
> >       self.newprop = 99
> >     def a(self):
> >       x = A.a(self)    # x is an A
> >       x.__class__ = B
> >       return x    # I want x to be a B, i.e have b() and .newprop.
> 
> Ugh!!
> 
> > Yes, I know this is bogus.  But I am not sure what I should be doing.
> 
> I think you have to make B into a container for an A.  Something like:
> 
> class B(A):
>      def __init__(self, *args, **kwds):
>        self.newprop = 99
>        self.wrapped_A = A(*args, **kwds)
>      def a(self):
>        x = B()
>        x.wrapped_A = A.a(self.wrapped_A)
>        return     # I want x to be a B, i.e have b() and .newprop.
>      def __getattr__(self, attr):
>        # delegate all inherited operations to the wrapped A object
>        return A.__getattr__(self.wrapped_A, attr)
> 
> You might also be able to do something crazy, like change A's metaclass
> so that its __new__ operation can make a B under certain circumstances.

Ahhhh....  I did not know about delegation.  I do now, thanks.
A minor disadvantage is that, because I also delegate __setattr__
all the instance variable have to be assigned in the form
self.__dict__['var'], rather than self.var (as I discovered the hard 
way).  Ugly, but I can live with it.  Thanks again.