Nice 404 error message

src/whitenoise/middleware.py

@@ -11,6 +11,7 @@
 from django.urls import get_script_prefix
 
 from .base import WhiteNoise
+from .responders import MissingFileError
 from .string_utils import decode_if_byte_string, ensure_leading_trailing_slash
 
 __all__ = ["WhiteNoiseMiddleware"]
@@ -68,6 +69,18 @@ def process_request(self, request):
         if static_file is not None:
             return self.serve(static_file, request)
 
+        if settings.DEBUG and request.path.startswith(settings.STATIC_URL):
+            from django.contrib.staticfiles.finders import get_finders
+            finders = get_finders()
+            app_dirs = []
+            for finder in finders:
+                for storage in finder.storages.values():
+                    app_dirs.append(storage.location)
+            app_dirs = "\n    ".join(sorted(app_dirs))
+            raise MissingFileError(f"""{request.path} not found. Searched these paths:
+
+    {app_dirs}""")
+
     @staticmethod
     def serve(static_file, request):
         response = static_file.get_response(request.method, request.META)

tests/django_urls.py

@@ -1,3 +1,12 @@
 from __future__ import annotations
 
-urlpatterns = []
+from django.urls import path
+
+
+def avoid_django_default_welcome_page():
+    pass
+
+
+urlpatterns = [
+    path('', avoid_django_default_welcome_page)
+]

tests/test_django_whitenoise.py

@@ -3,6 +3,7 @@
 import shutil
 import tempfile
 from contextlib import closing
+from pathlib import Path
 from urllib.parse import urljoin, urlparse
 
 import django
@@ -15,6 +16,7 @@
 from django.utils.functional import empty
 
 from whitenoise.middleware import WhiteNoiseFileResponse, WhiteNoiseMiddleware
+from whitenoise.responders import MissingFileError
 
 from .utils import AppServer, Files
 
@@ -210,3 +212,21 @@ def test_relative_static_url(server, static_files, _collect_static):
         url = storage.staticfiles_storage.url(static_files.js_path)
         response = server.get(url)
         assert response.content == static_files.js_content
+
+
+def test_404_in_prod(server):
+    response = server.get(settings.STATIC_URL + 'garbage')
+    assert response.status_code == 404
+
+
+@override_settings(DEBUG=True)
+def test_error_message(server):
+    response = server.get(settings.STATIC_URL + 'garbage')
+    print(response.content.decode())
+    app_dirs = Path(__file__).parent / 'test_files' / 'static'
+
+    expected = f"""{settings.STATIC_URL + 'garbage'} not found. Searched these paths:
+
+    {app_dirs}"""
+
+    assert expected in str(response.content.decode())