Re: Constants in model, that point to certain objects in database

On 24 jan, 21:20, Filip Gruszczyński <grusz...@gmail.com> wrote:
>
> > You could write a custom descriptor then. But the whole idea of a
> > class-level pseudo-constant pointing to a model instance still looks
> > rather backward to me.
>
> What is a custom descriptor? I have never used it. Is it something
> like a property?

The descriptor protocol is the mechanism that allows both property and
method:
http://docs.python.org/reference/datamodel.html#implementing-descriptors
http://users.rcn.com/python/download/Descriptor.htm

In your case, something like the following could do:

class ModelInstanceConst(object):
def __init__(self, **lookup):
self.lookup = lookup
def __get__(self, instance, cls=None):
if instance is None and cls is None:
# shouldn't happen but well...
return self
if cls is None:
cls = type(instance)
return cls.objects.get(**self.lookup)
def __set__(self, instance, value):
raise AttributeError("attribute is read-only")
def __delete__(self, instance):
raise AttributeError("attribute is read-only")

class MyModel(models.Model):
CONST = ModelInstanceConst(id=1)

NB : not tested.

> > Oh, and while were at it: the Python standard for the current class
> > param name in a classmethod is 'cls', not 'Class' ;)
>
> I have seen both cls and klass in django code. I prefer mine, because
> it clearly indicates, that the objects is not an instance, but a class

It's still an instance (of it's metaclass). If it's a newstyle class,
at least ;)

> (and therefore type instance, right?).

Python 2.6.2 (release26-maint, Apr 19 2009, 01:56:41)
[GCC 4.3.3] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> class Foo: pass
...
>>> isinstance(Foo, type)
False
>>>

But anyway...

> > Yes they do - as long as their metaclass derives from the appropriate
> > metaclass (ModelBase IIRC but better to check it out).
>
> I'll definitely try that.

I'm not sure it's necessary here - the custom descriptor solution
might be enough.

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