Custom 404 (#375)

apps/core/models/home.py

@@ -1,9 +1,12 @@
+from django.conf import settings
 from django.db import models
+from django.http.response import Http404
 from wagtail.admin.panels import FieldPanel
 from wagtail.fields import StreamField
 from wagtail.models import Page
 
 from ..blocks import HOME_BLOCKS
+from ..views import Custom404
 
 
 class HomePage(Page):
@@ -23,3 +26,41 @@ class HomePage(Page):
         FieldPanel("introduction"),
         FieldPanel("sections"),
     ]
+
+    def route(self, request, path_components):
+        try:
+            return super().route(request, path_components)
+        except Http404:
+            raise Custom404(
+                fallback_pages=self.get_fallback_pages(request, path_components)
+            )
+
+    def get_fallback_pages(self, request, path_components):
+        # No fallback for main locale.
+        if (language_code := self.locale.language_code) == settings.LANGUAGE_CODE:
+            return []
+
+        # Work out translations of the page requested corresponding to:
+        # 1. Same version but in english (if language requested isn't english)
+        # 2. Same language using latest version (if version requested isn't the latest)
+        # 3. Latest version in English
+        language, version = language_code.rsplit("-", maxsplit=1)
+        codes = {language, "en"}
+        versions = {version, "latest"}
+        possible_language_codes = [
+            f"{code}-{version}" for code in codes for version in versions
+        ]
+        translations = self.get_translations(inclusive=False).filter(
+            locale__language_code__in=possible_language_codes
+        )
+
+        fallback_pages = []
+        for page in translations:
+            try:
+                result = Page.route(page, request, path_components)
+            except Http404:
+                continue
+            else:
+                fallback_pages.append(result.page)
+
+        return fallback_pages

apps/core/templates/404.html

@@ -1,5 +1,5 @@
 {% extends "base.html" %}
-{% load i18n %}
+{% load i18n wagtailcore_tags %}
 
 {% block title %}{% trans 'Page not found' %}{% endblock %}
 
@@ -9,6 +9,17 @@ <h1 class="error-page__heading heading heading--display">{% trans 'Page not foun
 
         <h2 class="error-page__sub-heading">{% trans 'Sorry, this page could not be found.' %}</h2>
 
+        {% if fallback_pages %}
+            <h3 class="error-page__sub-heading">{% trans 'Try:' %}</h3>
+            <ul>
+                {% for page in fallback_pages %}
+                    <li>
+                        <h4><a href="{% pageurl page %}">{{ page }} - {{ page.locale }}</a></h4>
+                    </li>
+                {% endfor %}
+            </ul>
+        {% endif %}
+
         <svg class="error-page__icon" aria-hidden="true" viewBox="0 0 47 57" aria-hidden="true"><path d="M37.034 2.04s-4.18-1.262-5.975 0c-2.643 1.858 1.168.815-1.681 4.408-2.073 2.605-2.044 5.325-.713 8.858 10.201-.858 12.278 4.85 12.278 4.85l-1.58-6.624 2.889-4.278c-.381-4.732-4.396-7.04-5.218-7.214z" fill="#fff"/><path d="M29.958 16.705l.829-1.431c-1.502-1.167-2.538-2.969-2.538-5.09 0-3.498 2.745-6.307 6.163-6.307.88 0 1.657.159 2.382.477.363-.53.725-1.166 1.088-1.961C36.38 1.385 34.567.855 32.652.855c-5.387 0-9.789 4.453-9.789 10.019 0 5.513 4.35 10.018 9.788 10.018 1.606 0 2.33-.424 2.538-1.113l-5.23-3.074zM42.23 9.284l-2.433 3.71h6.577l-4.143-3.71zM38.296 9.602c.686 0 1.243-.57 1.243-1.273 0-.702-.557-1.272-1.243-1.272-.687 0-1.243.57-1.243 1.272 0 .703.556 1.273 1.243 1.273z"/><path d="M33.208 18.39c-1.043 1.896-3.176 5.806-3.362 6.28L15.123 38.888l15.883-2.606 6.956-13.389-4.754-4.502z" fill="#fff" stroke="#000"/><path d="M41.869 22.642c0-5.407-4.298-9.86-9.633-9.86-2.02 0-3.935.636-5.49 1.75l-2.589-8.535L4.582 40.505l5.696-1.484L.283 56.78l8.442-2.862 3.21-11.026c3.263.159 26.05.689 29.675-18.235v-.053c.052-.371.155-.742.207-1.113h-.052c.052-.318.104-.583.104-.848zM20.118 36.106l.414-.318C33.324 31.97 33.894 22.96 33.894 22.96l2.175-.742c-2.175 11.609-15.951 13.888-15.951 13.888z"/></svg>
     </div>
 {% endblock %}

apps/core/views.py

@@ -0,0 +1,16 @@
+from django.http import HttpResponseNotFound
+from django.http.response import Http404
+from django.template import loader
+
+
+class Custom404(Http404):
+    def __init__(self, *args, fallback_pages=None, **kwargs):
+        super().__init__(*args, **kwargs)
+        self.fallback_pages = fallback_pages
+
+
+def page_not_found(request, exception):
+    context = {"fallback_pages": getattr(exception, "fallback_pages", None)}
+    template = loader.get_template("404.html")
+    body = template.render(context, request)
+    return HttpResponseNotFound(body)

apps/guide/urls.py

@@ -45,3 +45,6 @@
     path("jsi18n/", JavaScriptCatalog.as_view(), name="javascript-catalog"),
     path("", include(wagtail_urls)),
 )
+
+
+handler404 = "apps.core.views.page_not_found"