Dynamic class loading explained¶
Dynamic class loading is the foundation for making Oscar extensively customisable. It is hence worth understanding how it works, because most customisations depend on it.
from oscar.apps.shipping.repository import Repository
is replaced by:
from oscar.core.loading import get_class Repository = get_class('shipping.repository', 'Repository')
This is done for almost all classes: views, models, Application instances, etc. Every class imported by get_class can be overridden.
This structure enables a project to create a local shipping.repository module, and optionally subclass the class from oscar.app.shipping.repository. When Oscar tries to load the Repository class, it will load the one from your local project.
This way, most classes can be overridden with minimal duplication, as only the to-be-changed classes have to be altered. They can optionally inherit from Oscar’s implementation, which often amounts to little more than a few lines of custom code for changes to core behaviour.
Seen on a bigger scale, this structures enables Oscar to ship with classes with minimal assumptions about the domain, and make it easy to modify behaviour as needed.
How it works¶
The get_class function looks through your INSTALLED_APPS for a matching app and will attempt to load the custom class from the specified module. If the app isn’t overridden or the custom module doesn’t define the class, it will fall back to the default Oscar class.
For get_class to pick up the customised class, the Oscar apps need to be forked. The process is detailed and illustrated with examples in Customising Oscar. It is usually enough to call oscar_fork_app and replace the app in INSTALLED_APPS.
Using get_class in your own code¶
Generally, there is no need for get_class in your own code as the location of the module for the class is known. Some Oscar developers nonetheless use get_class when importing classes from Oscar. This means that if someday the class is overridden, it will not require code changes. Care should be taken when doing this, as this is a tricky trade-off between maintainability and added complexity. Please note that we cannot recommend ever using get_model in your own code. Especially pre-Django 1.7, model initialisation is a tricky process and it’s easy to run into circular import issues.
You can test whether your overriding worked by trying to get a class from your module:
>>> from oscar.core.loading import get_class >>> get_class('shipping.repository', 'Repository') yourproject.shipping.repository.Repository # it worked!