Plugin inheritance is when you extend some existing Lino plugin by inheriting everything of it (its models, views, methods, fixtures and admin commands) and then overriding some of it.
Plugins aren't classes, they are packages, so plugin inheritance is not "real" inheritance but rather a series of guidelines and programming patterns.
Plugin inheritance is intensively used by Lino's plugin libraries.
A simple example¶
A simple example of plugin inheritance is the
lino_book.projects.min2 project: it defines a
lino_book.projects.min2.modlib.contacts plugin which inherts
lino_xl.lib.contacts by adding a series of mixins to some
of its models. Look at the code and at the resulting application!
it adds two fields
it adds another base class (the
it overrides the
save()method to add some specific behaviour
Here is the relevant application code which defines the Voga version
from lino_xl.lib.cal.models import Room from lino_xl.lib.contacts.models import ContactRelated class Room(Room, ContactRelated): tariff = dd.ForeignKey('products.Product', ...) calendar = dd.ForeignKey('cal.Calendar', ...) def save(self, *args, **kwargs): super(Room, self). save(*args, **kwargs) # add specific behaviour
For this to work, the library version of
lino_xl.lib.cal.models.Room) must have abstract=True.
But only in this special case. The general case is that when an
lino_xl.lib.cal , it gets (among others) a
wouldn't want to force every application which uses
lino_xl.lib.cal to override the Room model just to make it
There is no way in Django to make a model abstract "afterwards". When it is declared as abstact, then you must override it in order to get a concrete model. When it is not abstract, then you cannot override it by a model of same name (Django complains if you try).
In other words: The abstractness of certain models in a plugin depends on whether the plugin is going to be extended.
So how can the library version know whether the
should be abstract or not?
This is why we need a central place where models modules can ask whether it wants a given model to be abstract or not.
To solve this problem, Lino offers the
is_abstract_model method. Usage example:
class Room(dd.BabelNamed): class Meta: abstract = dd.is_abstract_model(__name__, 'Room') verbose_name = _("Room") verbose_name_plural = _("Rooms")
The trick here is that the
now contains this information in the extends_models attribute:
from lino_xl.lib.cal import Plugin class Plugin(Plugin): extends_models = ['Room']
The implementation of
is_abstract_model has evolved in time. The
first implementation used a simple set of strings in a class attribute
lino.core.site.Site. That might have been a standard
Django setting. But as things got more and more complex, it became
difficult to define this manually. And it was redundant because every
plugin does know which library models it is going to override.
But how to load that information from a plugin before actually
importing it? We then discovered that Django doesn't use the
__init__.py files of installed plugins. And of course we were
lucky to have a
lino.core.site.Site class which is being
instantiated before settings have finished to load...
Overriding other things¶
Overriding other Python objects (ChoiceList, Action, Plugin) is straightforward.
But the fixtures, config and management subdirs need special attention when doing plugin inheritance.
The config directory¶
The config subdirectories are handled automatically as expected: Lino scans first the config subdirectory of the child, then those of the parents.
Inheriting fixtures and django-admin commands¶
When you extend a plugin that has a
fixtures package, then you must
decide whether you want to inherit these fixtures.
There are good chances that you actually just want to inherit them without changing anything. In that case you must define a wrapper fixture for each fixture you want to inherit, which imports at least objects from its "parent" fixture.
For example the
fixtures package of
a suite of one-line modules, one for each fixture defined by its parent, the
lino_xl.lib.cal plugin. Each of these wrapper fixtures has just one
import statement like this:
from lino_xl.lib.cal.fixtures.demo import objects
There is currently no easier way to implement the default behaviour. Keep in mind that your fixtures may do something else, or you may decide to not inherit some fixture from your parent.
Beware the pitfall: when you create a new fixture in a plugin, then those who inherit your plugin will not automatically get notified that you added a new fixture and that they must create a wrapper if they want it.
A similar approach is necessary for django-admin commands. Django
discovers them by checking whether the plugin has a subpackage
management and then calling
os.listdir() on that module's
"commands" subdirectory. (See Django's
core/management/__init__.py file.) So when you extent a
plugin which has admin commands, you must create a pseudo command