[Python-ideas] @classproperty, @abc.abstractclasspropery, etc.

[Michael Foord]

On 4 January 2011 00:56, Guido van Rossum <guido at python.org> wrote:

> > On 20110103 13:22, Dirkjan Ochtman wrote:
> >> Do you have actual use cases for these?
>
> On Mon, Jan 3, 2011 at 2:06 PM, K. Richard Pixley <rich at noir.com> wrote:
> > Yes.  Here's a toy example for abstractclassproperty:
>
> Um, a toy example is pretty much the opposite of a use case. :-(
>
> That said, I am sure there are use cases for static property and class
> property -- I've run into them myself.
>
> An example use case for class property: in App Engine, we have a Model
> class (it's similar to Django's Model class). A model has a "kind"
> which is a string (it's the equivalent of an SQL table name). The kind
> is usually the class name but sometimes there's a need to override it.
> However once the class is defined it should be considered read-only.
> Currently our choices are to make this an instance property (but there
> are some situations where we don't have an instance, e.g. when
> creating a new instance using a class method); or to make it a class
> attribute (but this isn't read-only); or to make it a class method
> (which requires the user to write M.kind() instead of M.kind). If I
> had class properties I'd use one here.
>


A class property that can be fetched is very easy to implement. Because of
asymmetry in the descriptor protocol I don't think you can create a class
property with behaviour on set though (unless you use a metaclass I guess).


class classproperty(object):
   def __init__(self, function):
       self._function = function
   def __get__(self, instance, owner):
       return self._function(owner)

All the best,

Michael


> --
> --Guido van Rossum (python.org/~guido <http://python.org/%7Eguido>)
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