initializing mutable class attributes

[Alex Martelli]

Bengt Richter <bokr at oz.net> wrote:
   ...
> Here's a way to auto-initialize an instance attribute to a mutable via a
class variable
> (that happens to be a descriptor ;-)
> 
>  >>> class C(object):
>  ...     class prop(object):
>  ...         def __get__(self, inst, cls=None):
>  ...             if inst: return inst.__dict__.setdefault('prop',[])
>  ...             return self
>  ...         def __set__(self, inst, value): inst.__dict__['prop'] = value
>  ...         def __delete__(self, inst): del inst.__dict__['prop']
>  ...     prop = prop()
>  ...

I think it's the most elegant approach so far, except I would further
tweak it as follows:

class auto_initialized(object):
    def __init__(self, name, callable, *args, **kwds):
        self.data = name, callable, args, kwds
    def __get__(self, ints, cls=None):
        if inst:
            name, callable, args, kwds = self.data
            self = inst.__dict__[name] = callable(*args, **kwds)
        return self

No __set__ and __del__ warranted, I believe.  Use would then be, e.g:

class C(object):
    prop = auto_initialized('prop', list)

Pity that the name must be specified, but ah well, that's descriptors'
defects -- they don't get reminded about the name upon __get__ etc.

> Any use to you? (disclaimer: not tested beyond what you see here ;-)

I could use it in the Cookbook, side by side with Dan's "overloading
__new__" idea and my custom metaclass, as various approaches to solving
a 'problem' (if that be one) of "automatically initializing mutable
instance attributes".  It would help if you and/or Dan posted these
recipes to ActiveState's online cookbook ASAP (deadline is Sat Sep 5...)
-- I'm allowed to add recipes that didn't show up on the online
cookbook, but that _is_ discouraged... (and posting the recipes myself
isn't much better, either).


Alex