decorating container types (Python 2.4)

[timaranz]

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

[George Sakkis]

On Oct 11, 5:42 pm, [e-mail] 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