wagtail/wagtail/contrib/modeladmin/forms.py

from django import forms
from django.utils.translation import gettext as _

from wagtail.models import Page


class PageChoiceField(forms.ModelChoiceField):
    def label_from_instance(self, obj):
        bits = []
        for ancestor in (
            obj.get_ancestors(inclusive=True).exclude(depth=1).specific(defer=True)
        ):
            bits.append(ancestor.get_admin_display_title())
        return " | ".join(bits)


class ParentChooserForm(forms.Form):
    parent_page = PageChoiceField(
        label=_("Parent page"),
        required=True,
        empty_label=None,
        queryset=Page.objects.none(),
        widget=forms.RadioSelect(),
    )

    def __init__(self, valid_parents_qs, *args, **kwargs):
        self.valid_parents_qs = valid_parents_qs
        super().__init__(*args, **kwargs)
        self.fields["parent_page"].queryset = self.valid_parents_qs