[Python-3000] callable()

[Greg Ewing]

Guido van Rossum wrote:

> I'd be happy to extend the convention to all such attributes --
> setting it to None to mean that the subclass doesn't want to provide
> it. That's clean, can't possibly be interpreted to mean anything else,
> and doesn't require you to actually call the attribute.

Although unless there's some special casing for it in the
interpreter, attempting to use such an attribute will
give a somewhat confusing error message -- something
like "Object of type NoneType is not callable" instead
of "Object of type <YourClass> has no __xxx__ attribute".