overriding method that returns base class object

[Stuart McGraw]

Sorry, you are right, I wasn't clear.  I mean B inherits from
A.  Here is what I am trying to do...

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.
    def b(self):
      ...something...

Yes, I know this is bogus.  But I am not sure what 
I should be doing.  And to correct what I originally
posted, A is implented in python (but I still can't
change it for administrative reasons), but it's properties 
are declared with "__slots__ = [...]" if that makes a 
difference.  This is all in Python 2.3.3.


"Peter Hansen" <peter at engcorp.com> wrote in message news:4031481B.90B31C73 at engcorp.com...
> Stuart McGraw wrote:
> > 
> > I have a class A from a third party that I cannot change
> > and is implemented in C.  I derive my own class B from A
> > and add a couple new methods and override a method.  The
> > problem is that A has a method (call it A.f() ) that creates
> > and returns a new A object.  I need B.f() to return a B
> > object derived from A.f().  What is the best way to make
> > that happen?
> 
> If I understand this correctly, it has nothing to do with the
> fact that the parent class is implemented in C and you just
> need to know a little uncommon syntax:
> 
> class A:
>     def f(self):
>         return A()
> se
> class B(A):
>     def f(self):
>         obj = A.f(self)
>         # do whatever you want to obj here
>         return obj
> 
> The key is what you mean by "a B object derived from A.f()".  If
> by derived you mean something to do with _inheritance_, then 
> either you don't understand inheritance or you weren't clear what
> you wanted.
> 
> If you just mean you want B's f() to do something special to the 
> A object that A.f() returns, then the above code should let you
> do that properly...
> 
> -Peter