overriding method that returns base class object

[Scott David Daniels]

Stuart McGraw wrote:
> "Paul Rubin" <http://phr.cx@NOSPAM.invalid> rote in message news:7xwu6mmm3v.fsf at ruckus.brouhaha.com...
 >>...
>>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.

If you can separate the names, you can do something like:

     class B(object):
         def __init__(self, *args, **kwargs):
             self._a = A(*args, **kwargs)
             self.newprop = 99
         def __setattr__(self, name, val):
             if name.startswith('_') or name == 'newprop':
                 self.__dict__[name] = val
             else:
                 setattr(self._a, name, val)
         def getattr(self, name):
             return getattr(self._a, name)

-- 
-Scott David Daniels
Scott.Daniels at Acm.Org