decorating container types (Python 2.4)

James Stroud jstroud at mbi.ucla.edu
Fri Oct 12 01:19:00 CEST 2007


timaranz 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
> 

The why part is that __len__ is an unbound method of the class, not an 
unbound method of the class's metaclass. Also, your code above makes 
absolutely no association between classes A and B, which is the most 
fundamental reason.

James

-- 
James Stroud
UCLA-DOE Institute for Genomics and Proteomics
Box 951570
Los Angeles, CA  90095

http://www.jamesstroud.com



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