@classmethod question

[Ivan Illarionov]

On 24 апр, 07:27, Scott SA <py... at rscorp.ab.ca> wrote:
> Hi,
>
> I'm using the @classemethod decorator for some convenience methods and for some reason, either mental block or otherwise, can't seem to figure out how to elegantly detect if the call is from an instance or not.
>
> Here's the problem: Within the class definition, 'isinstance' has nothing to compare to because the class does not appear to exist.
>
> This is NOT a great example, but it outlines the the code:
>
>     class RecipieClass:
>         def __init__(self):
>             pass
>
>         @classmethod
>         def get_ingrendients(self, recipie_list=None):
>
>             if isinstnace(self,RecipieClass):
>                 return self.do_something_interesting()
>             else:
>                 return do_something_boring(recipie_list)
>
> Yes, I can test to see if the param exists, but that makes the call exclusive i.e. I can _only_ call it as an instance or with a parameter.
>
> Why am I doing this?
>
> It is a series of convenience methods, in this case I'm interacting with a database via an ORM (object-relational model). I want the ability to call a class-ojbect and get related values, or pass some criteria and get related values for them without collecting the records first as instances, then iterating them. I need to call this from several places so I want to be DRY (don't repeat yourself).
>
> The easiest way to describe this as an analogy would be like having a recipie for cookies and wanting to know all of the ingredients ahead of time. Then, at another time, wanting to know what all the ingredients would be to make cookies, cake and bread (i.e. complete shopping list).
>
>   cookie_recipie = RecipieClass.get_recipie('cookies')
>   cookie_recipie.get_ingredients()
>         2C Flour
>         0.5 C Sugar
>         ...
>
>   RecipieClass.get_ingrendients(['cookies','cake','bread'])
>         8C Flour
>         2C Sugar
>         ...
>
> Of course any suggestions on how this might be better approached would be interesting too.
>
> TIA,
>
> Scott

Hi,
It would make sense to separate instance-level and class-level
behaviour with additional 'objects' namespace. e.g.
cookie_recipie.get_ingredients() to get ingredients only for cookie
recipie and RecipieClass.objects.get_ingrendients([....]) to get all
the ingredients.

The elegant solution (AFAIK used by Django) would be to use metaclass
and the object with custom descriptor for class-object/table level
stuff.

Something like this:

class RecipieMetaclass(type):
    def __new__(cls, bases, attrs):
        new_cls = type.__new__(cls, name, bases, attrs)
        new_cls.objects = IngredientsDescriptor(IngredientsManager())
        return new_cls

class RecipieClass(object):
    __metaclass__ = RecipieMetaclass
    def get_ingredients(self, recipie_list=None):
        return self.do_something_interesting(recipie_list)

class IngredientsManager(object):
    def get_ingredients(self, recipie_list=None):
        return do_something_boring(recipie_list)

class IngredientsDescriptor(object):
    def __init__(self, ingredients_manager):
        self.ingredients_manager = ingredients_manager
    def __get__(self, instance, type=None):
        if instance is not None:
            raise AttributeError, "Access via %s instances is not
allowed" % type.__name__
        return self.ingredients_manager

Then, "at another time, wanting to know what all the ingredients would
be to make cookies, cake and bread" you would call:
RecipieClass.objects.get_ingrendients(['cookies','cake','bread'])

Both Django and Google Apps Engine API use similar concepts and you
can learn much more interesting looking in their source code.

Regards,

--
Ivan