decorating container types (Python 2.4)

[George Sakkis]

On Oct 11, 5:42 pm, timar... at gmail.com wrote:
> Hi,
>
> I have a container class A and I want to add functionality to it by
> using a decorator class B, as follows:
>
> class A(object):
>     def __len__(self):
>         return 5
>
> class B(object):
>     def __init__(self, a):
>         self._a = a
>
>     def __getattr__(self, attr):
>         return getattr(self._a, attr)
>
>     def other_methods(self):
>         blah blah blah
>
> I was expecting len(B) to return 5 but I get
> AttributeError: type object 'B' has no attribute '__len__'
> instead.
> I was expecting len() to call B.__len__() which would invoke
> B.__getattr__ to call A.__len__ but __getattr__ is not being called.
> I can work around this, but I am curious if anyone knows _why_
> __getattr__ is not being called in this situation.
>
> Thanks
> Tim

Unfortunately __getattr__ is not called for special attributes; I'm
not sure if this is by design or a technical limitation. You have to
manually delegate all special methods (or perhaps write a metaclass
that does this for you).

George