Fixed #21435 -- Improved error message when urlconf is empty.

The new error message now hints that the most likely issue
is a circular import.

Thanks to trac user elena for the report and to
bpeschier for the original patch.

django/core/urlresolvers.py

@@ -346,11 +346,17 @@ class RegexURLResolver(LocaleRegexProvider):
 
     @property
     def url_patterns(self):
+        # urlconf_module might be a valid set of patterns, so we default to it
         patterns = getattr(self.urlconf_module, "urlpatterns", self.urlconf_module)
         try:
             iter(patterns)
         except TypeError:
-            raise ImproperlyConfigured("The included urlconf %s doesn't have any patterns in it" % self.urlconf_name)
+            msg = (
+                "The included urlconf '{name}' does not appear to have any "
+                "patterns in it. If you see valid patterns in the file then "
+                "the issue is probably caused by a circular import."
+            )
+            raise ImproperlyConfigured(msg.format(name=self.urlconf_name))
         return patterns
 
     def _resolve_special(self, view_type):

tests/urlpatterns_reverse/tests.py

@@ -161,8 +161,10 @@ class NoURLPatternsTests(TestCase):
         resolver = RegexURLResolver(r'^$', self.urls)
 
         self.assertRaisesMessage(ImproperlyConfigured,
-            "The included urlconf urlpatterns_reverse.no_urls "
-            "doesn't have any patterns in it", getattr, resolver, 'url_patterns')
+            "The included urlconf 'urlpatterns_reverse.no_urls' does not "
+            "appear to have any patterns in it. If you see valid patterns in "
+            "the file then the issue is probably caused by a circular import.",
+            getattr, resolver, 'url_patterns')
 
 
 class URLPatternReverse(TestCase):