On Oct 12, 12:19 pm, James Stroud <[EMAIL PROTECTED]> wrote: > [EMAIL PROTECTED] 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
Just so I'm clear on this: Are you saying that the problem with my original code is that len() is equivalent to type(x).__len__(x) that looks for a class attribute rather than an instance attribute as my code requires? I don't own class A and they are generated by function calls rather than instantiated directly, that's why I'm decorating them instead of inheriting. The crux of the (academic) question is why does len() expect '__len__' to be a class attribute instead of an instance attribute, and why is this a good idea? Cheers Tim -- http://mail.python.org/mailman/listinfo/python-list
