On 4 January 2011 00:56, Guido van Rossum guido@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@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) _______________________________________________ Python-ideas mailing list Python-ideas@python.org http://mail.python.org/mailman/listinfo/python-ideas