Question

Archetypes API provides default_method() to populate initial values programmatically.

However, since this is a class method, it is not suitable for archetypes.schemaextender. What would be the equivalent approach with the extender?

Answer 1

If there is no field.default nor field.default_method you can use IFieldDefaultProvider adapter. See this snippet of code from Archetypes.Field.Field class, getDefault method:

if not self.default:
    default_adapter = component.queryAdapter(instance, IFieldDefaultProvider, name=self.__name__)
    if default_adapter is not None:
        return default_adapter()

and also IFieldDefaultProvider:

class IFieldDefaultProvider(Interface):
    """Register a named adapter for your content type providing
    this interface, with a name that is equal to the name of a
    field. If no default or default_method is set on that field
    explicitly, Archetypes will find and call this adapter.
    """

    def __call__():
        """Get the default value.