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
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