Thanks for your *really* interesting explanations.
It seems more clear to me and I'm going to read all about
derscriptors !
Python is a facinating language ...
On 8 mar, 09:16, Masklinn <maskl...@masklinn.net> wrote:
> On 8 Mar 2010, at 09:01 , JF Simon wrote:
>
>
>
> > Hi dudes,
>
> > My object seems mysterious, as is my problem ... I'd like to
> > understand something (it's much more related to Python than Django).
>
> > Take an example :
>
> > class Image(models.Model):
> > email = models.ForeignKey('Email', verbose_name=_(u'Email'))
> > name = models.CharField(_(u'Name'), max_lenth=50,
> > help_text=_(u'Must be unique for a given email'))
> > content_id = models.CharField(_(u'Content ID'), max_length=25,
> > db_index=True, default=self.slugify_name())
> > [...]
>
> > "self" is not defined in the "default" kwarg for "content_id" field,
> > it's because I'm not in an instance method (with self as first
> > parameter). So what are those members, are they static for the class ?
> > What is the difference between them and a member instantiated in the
> > __init__ method whith self.field = [...] ?
>
> > If I replace the last field definition with :
>
> > from django.template.defaultfilters import slugify
> > [...]
> > content_id = models.CharField(_(u'Content ID'), max_length=25,
> > db_index=True, default=slugify(name))
>
> > Eclipse don't show me an error ... will it work ? Why (not) ? I'm not
> > sure to understand what I'm doing and it's weird.
>
> It will 'work', but I doubt it's going to do what you want.
>
> Here are how python class definitions work:
>
> 1. The body of the class (anything after class YourName:) is parsed and
> executed sequentially, as if it were module-level Python code (which is why
> you can have a class inside a class, for instance)
> 2. *After* the body of the class has executed, all of its content (the
> variables local to that body, which you can see by writing `print locals()`
> at the end of the class body) is collected and set as *class attributes*,
> with functions turned into methods.
>
> Because a class body is "just code", you can:
> 1. Use things defined outside the class body (models or slugify, in your
> second snippet)
> 2. *Reference things previously defined in the body* (such as `name`).
>
> Finally, you might very well be doing unnecessary work: Django already
> provides a SlugField
> (http://docs.djangoproject.com/en/dev/ref/models/fields/#slugfield) which —
> in the admin — can be prepopulated from your name using prepopulate_field
> (http://docs.djangoproject.com/en/dev/ref/contrib/admin/#django.contri...).
>
> If you want to set it in any case, you'll have to use __init__ I believe, but
> I'm sure others will be able to infirm that and show a better way.
>
> But in this case, `slugify` is going to get passed a CharField, which
> probably isn't what you want.
>
> Finally,
>
> > So what are those members, are they static for the class ?
>
> Well they are class attributes, but they're also instance attributes. They're
> one of those very-useful-but-not-necessaril-super-clear features:
> descriptors. Seehttp://users.rcn.com/python/download/Descriptor.htmfor a very
> good howto by Raymond Hettinger. Note that you could just as well go with
> "magic, gotcha" for now on that subject. They're basically class attributes
> which do special stuff when you invoke them from an instance.
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