[Python-Dev] __getattr__

[Guido van Rossum]

(Changing subject)

> > > _fields = {'encode':0,'decode':1,'reader':2,'writer':3}
> > > class CodecInfo(tuple):
> > >     __dynamic__ = 0
> > >     def __getattr__(self, name):
> > >         try:
> > >             return self[_fields[name]]
> > >         except KeyError:
> > >             raise AttributeError, name
> > 
> > You want to change that raise statement into
> > 
> >               return tuple.__getattr__(self, name)
> > 
> > Remember, new-style __getattr__ *replaces* the built-in getattr
> > operation
> 
> Originally, I had _fields as a class attribute, and was using
> self._fields inside getattr, which caused a StackOverflow. I could not
> figure out why it wouldn't just find _fields in the class before
> invoking __getattr__...
> 
> > (I'm still considering whether this is too backwards incompatible; we
> > could have two getattr hooks, one old-style and one new-style.)
> 
> So far, it is not really incompatible, since it only applies to
> new-style classes. It is confusing to long-time users, so it deserves
> documentation.

It seems to be the number one point of confusion.  It's also one of
the few places where you have to change your code when porting a class
to the new style -- which is otherwise very simple, either place a
``__metaclass__ = type'' line in your module or make all your base
classes derive from object.

--Guido van Rossum (home page: http://www.python.org/~guido/)