Is there a way to 'mask out' inherited methods?

[Alex Martelli]

Ralf Juengling wrote:
        ...
> I'm fiddling around with some new 2.2.-features. I want to
> inherit from a builtin-type but want to use only a subset
> of its functionality:
> 
> class Vector(list):
        ...
> I.e. Vector should keep or inherit methods meaningful to vectors
> (e.g. '__str__', '__getitem__', '__cmp__' and so on), and should
> somehow undefine or mask out any methods not meaningful to it
> ('append', 'reverse', ..).
> 
> Any idea how to achieve that easily?

"Easily" within the constraints you stated, only one way: add a
__getattribute__ method that selects only the attributes (note that
methods are also attributes) you want to expose, e.g.:

class Vector(list):
    mymethods = '__str__', '__getitem__', '__cmp__'
    def __getattribute__(self, name):
        if name in self.mymethods:
            return object.__getattribute__(self, name)
        else: raise AttributeError, name

Forget *SPEED*, of course -- when you define __getattribute__
Python calls it for EVERY attribute request, so your code will just
crawl.  For speed, your choice is: either drop the constraint that
Vector must subclass list, and delegate instead; or, if the constraint
MUST be kept, override all methods you want to 'mask out' and
have each of them raise AttributeError (not quite the same thing,
of course -- getattr would still 'see' them, for example).  A custom
metaclass might help (not trivial).

My advice: forget subclassing when you don't want to expose all
of the parent-class interface -- use delegation instead.


Alex