news.py

class NewsPageForm(WagtailAdminPageForm):
    """
    This form exists to make the Summary, Header, and Body fields optional if the external_url field is populated.
    """

    def clean(self):
        cleaned_data = super().clean()

        # If there's an external_url, remove any errors that might have been added for the summary, header, and body.
        if cleaned_data.get('external_url'):
            summary_error = self._errors.pop('summary', None)
            # Since the summary field is required at the database level, we need it to have at least a placeholder.
            if summary_error:
                cleaned_data['summary'] = 'placeholder'
            header_error = self._errors.pop('header', None)
            body_error = self._errors.pop('body', None)
            # This is here to handle a weird use case. If the user already saved the page with blocks in the header
            # and body fields, but then removed them on a subsequent save, this code is required to ensure that the
            # form actually saves blank data to those fields, rather than just leaving their data intact.
            if header_error:
                cleaned_data['header'] = ''
            if body_error:
                cleaned_data['body'] = ''

        # Don't let people remove the publication_date from a live article; Unfortunately the field 'live' defaults
        # to True for a never saved article. But having a live_revision_id appears to be an accurate indicator.
        # Note: this isn't really needed since publishing will fill in an empty publication_date. But removing a
        # previous publication date rather than editing it might make the date jump in an unexpected way.
        if self.instance.live_revision_id and not cleaned_data.get('publication_date'):
            self.add_error(
                'publication_date',
                ValidationError('You may not remove the publication date from a live news article')
            )

        return cleaned_data

      
class NewsPage(Page):
    # lots of fields..
    
    
    ###################
    # EDIT FORM CONFIG
    ###################
    content_panels = [
        FieldPanel('title', classname='full title'),
        FieldPanel('external_url'),
        FieldPanel('writer'),
        FieldPanel('contact'),
        StreamFieldPanel('related_links'),
        StreamFieldPanel('header'),
        StreamFieldPanel('body')
    ]
    publishing_panels = [
        PublishingPanel(),
        FieldPanel('publication_date'),
        FieldPanel('display_locations', widget=forms.CheckboxSelectMultiple),
    ]

    edit_handler = TabbedInterface(
        [
            ObjectList(content_panels, heading='Content'),
            ObjectList(publishing_panels, heading='Publishing'),
        ],
        base_form_class=NewsPageForm
    )