SubclassingΒΆ

Subclass options may differ from superclass options. Usually they will share many options, but some may be added, and others removed. To modify the set of available options, the subclass defines its options with the add() method to the superclass options. This creates a layered effect, just like push() for an instance. The difference is, push() does not allow new options (keys) to be defined; add() does. It is also possible to assign the special null object Prohibited, which will disallow instances of the subclass from setting those values.:

options = Superclass.options.add(
    func   = None,
    prefix = Prohibited,  # was available in superclass, but not here
    suffix = Prohibited,  # ditto
)

Because some of the “additions” can be prohibitions (i.e. removing particular options from being set or used), this is “adding to” the superclass’s options in the sense of “adding a layer onto” rather than strict “adding options.”

An alternative is to copy (or restate) the superclass’s options. That suits “unlinked” cases–where the subclass is highly independent, and where changes to the superclass’s options should not effect the subclass’s options. With add(), they remain linked in the same way as instances and classes are.