Static properties

[Bengt Richter]

On Thu, 26 Aug 2004 23:01:49 +0200, aleaxit at yahoo.com (Alex Martelli) wrote:

>Per Erik Stendahl <python-spam at berrs.net> wrote:
>
>> Hello everyone,
>> 
>> Is it possible to define "static properties" in a class?
>
>Yes, but only by defining a custom metaclass to be the type of that
>class.  Properties are defined in the type, not in the instance; so, for
>a class itself to have properties, the class's type, commonly known as
>its metaclass, must be the one defining them.

Um, UIAM a property is just a peculiar descriptor, which you can define
without resorting to metaclass magic. See below ...
>
>
>>>> class metaPath(type):
>...     def curdir(cls): return os.getcwd()
>...     curdir = property(curdir)
>... 
>>>> class Path: __metaclass__ = metaPath
>... 
>>>> print Path.curdir
>/Users/alex/cb/dblite/dblite_0.5
>
>
>_However_...:
>
>>>> print Path().curdir
>Traceback (most recent call last):
>  File "<stdin>", line 1, in ?
>AttributeError: 'Path' object has no attribute 'curdir'
>>>> 
>
>if you want to be able to get at property curdir on INSTANCES of Path as
>you normally would for a staticmethod, yet with property syntax, I think
>you need to get a bit more clever than this... descriptors don't get
>looked up two metalevels up, only one...
>
But you only need one:

 >>> class Path(object):
 ...     class curdir(object):
 ...         def __get__(self, inst, cls): return os.getcwd()
 ...     curdir = curdir()
 ...
 >>> import os
 >>> Path.curdir
 'C:\\pywk\\clp'
 >>> Path().curdir
 'C:\\pywk\\clp'

I wonder if I'll beat you to this post ;-)

Regards,
Bengt Richter