5đź‘Ť
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
24đź‘Ť
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!
- How to remove the language identifier from django-cms 2.4 URLs?
- Django admin, section without "model"?
1đź‘Ť
Not a real answer, but for enrichment:
Person.first_name
won’t work
p = Person.objects.get(pk=x)
p.first_name
will work. so an object instance of person has a first and last name, but static context Person does not.
Also note: Django has Model Managers which are allow “Person” to do static queryset operations. (https://docs.djangoproject.com/en/dev/topics/db/managers/#managers).
so for example
peoples = Person.objects.all()