Fixed #26295 -- Allowed using i18n_patterns() in any root URLconf.

Thanks Tim for the review.

django/middleware/locale.py

@@ -5,9 +5,8 @@ from django.http import HttpResponseRedirect
 from django.urls import (
     LocaleRegexURLResolver, get_resolver, get_script_prefix, is_valid_path,
 )
-from django.utils import translation
+from django.utils import lru_cache, translation
 from django.utils.cache import patch_vary_headers
-from django.utils.functional import cached_property
 
 
 class LocaleMiddleware(object):
@@ -21,17 +20,19 @@ class LocaleMiddleware(object):
     response_redirect_class = HttpResponseRedirect
 
     def process_request(self, request):
+        urlconf = getattr(request, 'urlconf', settings.ROOT_URLCONF)
         language = translation.get_language_from_request(
-            request, check_path=self.is_language_prefix_patterns_used)
+            request, check_path=self.is_language_prefix_patterns_used(urlconf)
+        )
         translation.activate(language)
         request.LANGUAGE_CODE = translation.get_language()
 
     def process_response(self, request, response):
         language = translation.get_language()
         language_from_path = translation.get_language_from_path(request.path_info)
+        urlconf = getattr(request, 'urlconf', settings.ROOT_URLCONF)
         if (response.status_code == 404 and not language_from_path
-                and self.is_language_prefix_patterns_used):
-            urlconf = getattr(request, 'urlconf', None)
+                and self.is_language_prefix_patterns_used(urlconf)):
             language_path = '/%s%s' % (language, request.path_info)
             path_valid = is_valid_path(language_path, urlconf)
             path_needs_slash = (
@@ -52,20 +53,20 @@ class LocaleMiddleware(object):
                 )
                 return self.response_redirect_class(language_url)
 
-        if not (self.is_language_prefix_patterns_used
+        if not (self.is_language_prefix_patterns_used(urlconf)
                 and language_from_path):
             patch_vary_headers(response, ('Accept-Language',))
         if 'Content-Language' not in response:
             response['Content-Language'] = language
         return response
 
-    @cached_property
-    def is_language_prefix_patterns_used(self):
+    @lru_cache.lru_cache(maxsize=None)
+    def is_language_prefix_patterns_used(self, urlconf):
         """
         Returns `True` if the `LocaleRegexURLResolver` is used
         at root level of the urlpatterns, else it returns `False`.
         """
-        for url_pattern in get_resolver(None).url_patterns:
+        for url_pattern in get_resolver(urlconf).url_patterns:
             if isinstance(url_pattern, LocaleRegexURLResolver):
                 return True
         return False

docs/releases/1.10.txt

@@ -244,7 +244,9 @@ Generic Views
 Internationalization
 ~~~~~~~~~~~~~~~~~~~~
 
-* ...
+* The :func:`~django.conf.urls.i18n.i18n_patterns` helper function can now be
+  used in a root URLConf specified using :attr:`request.urlconf
+  <django.http.HttpRequest.urlconf>`.
 
 Management Commands
 ~~~~~~~~~~~~~~~~~~~

docs/topics/i18n/translation.txt

@@ -1328,7 +1328,7 @@ Language prefix in URL patterns
 
 .. function:: i18n_patterns(*pattern_list)
 
-This function can be used in your root URLconf and Django will automatically
+This function can be used in a root URLconf and Django will automatically
 prepend the current active language code to all url patterns defined within
 :func:`~django.conf.urls.i18n.i18n_patterns`. Example URL patterns::
 
@@ -1373,10 +1373,16 @@ function. Example::
 
 .. warning::
 
-    :func:`~django.conf.urls.i18n.i18n_patterns` is only allowed in your root
+    :func:`~django.conf.urls.i18n.i18n_patterns` is only allowed in a root
     URLconf. Using it within an included URLconf will throw an
     :exc:`~django.core.exceptions.ImproperlyConfigured` exception.
 
+.. versionchanged:: 1.10
+
+    In older version, using ``i18n_patterns`` in a root URLconf different from
+    :setting:`ROOT_URLCONF` by setting :attr:`request.urlconf
+    <django.http.HttpRequest.urlconf>` wasn't supported.
+
 .. warning::
 
     Ensure that you don't have non-prefixed URL patterns that might collide

tests/i18n/patterns/tests.py

@@ -7,6 +7,7 @@ from django.http import HttpResponsePermanentRedirect
 from django.middleware.locale import LocaleMiddleware
 from django.template import Context, Template
 from django.test import SimpleTestCase, override_settings
+from django.test.client import RequestFactory
 from django.test.utils import override_script_prefix
 from django.urls import clear_url_caches, reverse, translate_url
 from django.utils import translation
@@ -92,6 +93,18 @@ class URLDisabledTests(URLTestCaseBase):
             self.assertEqual(reverse('prefixed'), '/prefixed/')
 
 
+class RequestURLConfTests(SimpleTestCase):
+
+    @override_settings(ROOT_URLCONF='i18n.patterns.urls.path_unused')
+    def test_request_urlconf_considered(self):
+        request = RequestFactory().get('/nl/')
+        request.urlconf = 'i18n.patterns.urls.default'
+        middleware = LocaleMiddleware()
+        with translation.override('nl'):
+            middleware.process_request(request)
+        self.assertEqual(request.LANGUAGE_CODE, 'nl')
+
+
 @override_settings(ROOT_URLCONF='i18n.patterns.urls.path_unused')
 class PathUnusedTests(URLTestCaseBase):
     """