class attribute

[Todd Dembrey]

On 28/01/16 13:15, ast wrote:
> hello
>
> Here is a class from django framework
>
>
> from django.db import models
>
> class Article(models.Model):
>
>    titre = models.CharField(max_length=100)
>    auteur = models.CharField(max_length=42)
>    contenu = models.TextField(null=True)
>    date = models.DateTimeField(auto_now_add=True, auto_now=False,
>                                verbose_name="Date de parution")
>
>    def __str__(self):
>        return self.titre
>
> From a Python point of view, what are titre, auteur, contenu and date ?
> Are they class attributes, so common to all instance of Article ?
> It seems so to me.
>
> But if i do in a django shell (run with py manage.py shell)
>
>>>> Article.titre
>
> it doesnt work,
> AttributeError: type object 'Article' has no attribute 'titre'
> why ?
>
> if I test on a small class
>
>>>> class MyClass:
>>>>    i=0
>>>>
>>>> MyClass.i
>>>> 0
>
> works
>
>
>
> When we create an object of class Article
>
> article = Article(titre="Bonjour", auteur="Maxime")
> article.contenu = "Les crêpes bretonnes sont trop bonnes !"
>
> we use the same names titre, auteur, contenu, which should be instance
> attribute this time. This is confusing to me
>
> thx

Django Model classes are very different from a basic Python classes. It 
is worth having a look inside django/db/models/base.py for more 
information. But in summary, see below.

The Model class has a metaclass of ModelBase which on __new__() returns 
a class constructed from the attributes which you have defined in your 
Model class (Article in this example). All the information is retained, 
it is just filed away neatly for other purposes. Have a look in the 
Article._meta attribute. You can find the fields you provided through 
Article._meta.fields.

When you create Article (as above) you run the __init__() which reaches 
into the ._meta attribute of the class in order to create an object, 
which is then stored within the 'database'.

This is done because you don't want to access the attributes on the 
Model, so they are removed when the Model is created. What you actually 
want to do is access them on the objects within the Model. Writing 
Article.objects.first().titre  in Django Models is mostly equivalent to 
Article.titre from Python classes.


Todd