20130907 (Saturday, 07 September 2013)

Should Actor.label be inherited?

Yesterday I stumbled once more over the surprising fact that lino.core.actors.Actor.label is never inherited from base classes.

This rule was necessary because of (e.g.) the following use case:

class Partner(dd.Model):
class Partners(dd.Table):
    # no explicit `label` attribute, so take model._meta.verbose_name_plural

class Person(Partner):
class Persons(Partners):
    # again no explicit `label` attribute, so take model._meta.verbose_name_plural

In such a case we want (of course) the Persons table to be labelled “Persons” (the verbose_name_plural from model Person), not to inherit the label “Partners” from Partners.

But lino_faggio.courses overrides the ActiveCourses class definition from lino.modlib.courses. In this case it is unexpected behaviour to not inherit from the parent.

The solution was to differentiate between an explicitly specified label and a label taken from the model. So I converted lino.core.actors.Actor.label into a property and new store an explicit label class attribute internally as _label. Same philosophy was required for known_values.

Note the side effect that get_actor_label now is no longer called only once at startup but each time somebody reads the label property. Which altogether seems more sane. For example, certain dynamically labelled tables in lino_welfare.modlib.cv or lino_welfare.modlib.debts no longer need to use sophisticated signal handlers, they just implement their own special get_actor_label method.

Thanks to http://stackoverflow.com/questions/128573/using-property-on-classmethods for revealing the trick to define a property on the metaclass so that it works for the class object. Remember that Actors are never instantiated.

Since all this is very subtle and was not yet completely covered by the Lino test suite, I started to write a new tested document: a tutorial about lino.core.actors.

(Notiz am Rande: in Lino Welfare sieht man von alldem nur eines: Der Menübefehl Büro ‣ Meine Notizen lautet jetzt Büro ‣ Meine Ereignisse/Notizen. Was ja konsequent ist.)

Continued on userdocs

The welfare.userdocs is getting better and better… but still not what I’d call finished.