django model question ( newbie )

Question

First of all,I'm not into web programming. I bumped into django and read a bit about models. I was intrigued by the following code ( from djangoproject.com ) :

class Person(models.Model):
    first_name = models.CharField(max_length=50)
    last_name = models.CharField(max_length=50)

    def __str__(self):
        # Note use of django.utils.encoding.smart_str() here because
        # first_name and last_name will be unicode strings.
        return smart_str('%s %s' % (self.first_name, self.last_name))

By my understanding of python , first_name and last_name are class variables , right ? How is that used in code ( because I guess that setting Person.first_name or Person.last_name will affect all Person instances ) ? Why is it used that way ?

Answer 1

Yes, first_name and last_name are class variables. They define fields that will be created in a database table. There is a Person table that has first_name and last_name columns, so it makes sense for them to be at Class level at this point.

For more on models, see: http://docs.djangoproject.com/en/dev/topics/db/models/

When it comes to accessing instances of a Person in code, you are typically doing this via Django's ORM, and at this point they essentially behave as instance variables.

For more on model instances, see: http://docs.djangoproject.com/en/dev/ref/models/instances/?from=olddocs

For more on Python class variables, see: http://diveintopython.org/object_oriented_framework/class_attributes.html

Answer 2

The essence of your question is "how come these class variables (which I assign Field objects to) suddenly become instance variables (which I assign data to) in Django's ORM"? The answer to that is the magic of Python metaclasses.

A metaclass allows you to hook into and modify the process of creating a Python class (not the creation of an instance of that class, the creation of the class itself).

Django's Model object (and thus also your models, which are subclasses) has a ModelBase metaclass. It looks through all the class attributes of your model, and any that are instances of a Field subclass it moves into a fields list. That list is assigned as an attribute of the _meta object, which is a class attribute of the model. Thus you can always get to the actual Field objects via MyModel._meta.fields, or MyModel._meta.get_field('field_name').

The Model.__init__ method is then able to use the _meta.fields list to determine what instance attributes should be initialized when a model instance is created.

Don't be afraid to dive into the Django source code; it's a great source of education!