Add custom attributes to a Wagtail page - but only when creating.

models.py

class CourseIndexPage(Page):
    # ..... fields ......
    base_form_class=CourseIndexPageForm
    
    
class CourseIndexPageForm(WagtailAdminPageForm):

    def __init__(self, *args, **kwargs):
        """
        Sets up the Course selector to treat selecting null for the Edition as setting it to "current".
        Also makes show_in_menus default to True.
        """
        instance = kwargs.get('instance')
        if instance is None or instance.pk is None:
            kwargs.update({'initial': {'show_in_menus': True}})
        super().__init__(*args, **kwargs)

        # Changes the "-------" string that Django uses by default for the NULL ForeignKey choice to "current".
        self.fields['default_catalog_edition'].empty_label = 'current'