__getattr__ feature

[Greg Ewing]

Hans Nowak wrote:
> I didn't know this was possible:
> 
> class Foo:
>     def __init__(self, obj):
>         self.obj = obj
>     def __getattr__(self, name):
>         return getattr(self.obj, name)
> 
> foo = Foo([1,2,3])
> print foo[2]    # prints 3

I think what's happening here is that invoking
the [] is causing a lookup for a __getitem__
method, and your __getattr__ is catching this
and redirecting it to self.obj, which does
have a __getitem__ method.

 > A quick test learns that this is apparently a
> feature new to 2.2;

The new feature is that built-in types now
have all the relevant special methods accessible
by __xxx__ names. Previously they only worked
on user-defined classes.

> Does anybody 
> know how/why this works and why it was added?

I don't think it was explicitly added -- it's
just a side effect of the type/class unification
stuff.

Greg Ewing, Computer Science Dept,
University of Canterbury,	
Christchurch, New Zealand
http://www.cosc.canterbury.ac.nz/~greg