Fixed #22057 -- Ensured reverse_lazy can be used in settings

And without causing a circular import. Thanks Akis Kesoglou for
the report.

django/db/models/sql/aggregates.py

@@ -5,16 +5,12 @@ import copy
 
 from django.db.models.fields import IntegerField, FloatField
 from django.db.models.lookups import RegisterLookupMixin
+from django.utils.functional import cached_property
 
 
 __all__ = ['Aggregate', 'Avg', 'Count', 'Max', 'Min', 'StdDev', 'Sum', 'Variance']
 
 
-# Fake fields used to identify aggregate types in data-conversion operations.
-ordinal_aggregate_field = IntegerField()
-computed_aggregate_field = FloatField()
-
-
 class Aggregate(RegisterLookupMixin):
     """
     Default SQL Aggregate.
@@ -61,14 +57,23 @@ class Aggregate(RegisterLookupMixin):
 
         while tmp and isinstance(tmp, Aggregate):
             if getattr(tmp, 'is_ordinal', False):
-                tmp = ordinal_aggregate_field
+                tmp = self._ordinal_aggregate_field
             elif getattr(tmp, 'is_computed', False):
-                tmp = computed_aggregate_field
+                tmp = self._computed_aggregate_field
             else:
                 tmp = tmp.source
 
         self.field = tmp
 
+    # Two fake fields used to identify aggregate types in data-conversion operations.
+    @cached_property
+    def _ordinal_aggregate_field(self):
+        return IntegerField()
+
+    @cached_property
+    def _computed_aggregate_field(self):
+        return FloatField()
+
     def relabeled_clone(self, change_map):
         clone = copy.copy(self)
         if isinstance(self.col, (list, tuple)):

tests/admin_scripts/tests.py

@@ -40,7 +40,7 @@ custom_templates_dir = os.path.join(os.path.dirname(__file__), 'custom_templates
 
 
 class AdminScriptTestCase(unittest.TestCase):
-    def write_settings(self, filename, apps=None, is_dir=False, sdict=None):
+    def write_settings(self, filename, apps=None, is_dir=False, sdict=None, extra=None):
         if is_dir:
             settings_dir = os.path.join(test_dir, filename)
             os.mkdir(settings_dir)
@@ -51,6 +51,8 @@ class AdminScriptTestCase(unittest.TestCase):
         with open(settings_file_path, 'w') as settings_file:
             settings_file.write('# -*- coding: utf-8 -*\n')
             settings_file.write('# Settings file automatically generated by admin_scripts test case\n')
+            if extra:
+                settings_file.write("%s\n" % extra)
             exports = [
                 'DATABASES',
                 'ROOT_URLCONF',

tests/urlpatterns_reverse/tests.py

@@ -15,6 +15,8 @@ from django.shortcuts import redirect
 from django.test import TestCase, override_settings
 from django.utils import six
 
+from admin_scripts.tests import AdminScriptTestCase
+
 from . import urlconf_outer, middleware, views
 from .views import empty_view
 
@@ -301,6 +303,24 @@ class ReverseLazyTest(TestCase):
         self.assertEqual(response.status_code, 200)
 
 
+class ReverseLazySettingsTest(AdminScriptTestCase):
+    """
+    Test that reverse_lazy can be used in settings without causing a circular
+    import error.
+    """
+    def setUp(self):
+        self.write_settings('settings.py', extra="""
+from django.core.urlresolvers import reverse_lazy
+LOGIN_URL = reverse_lazy('login')""")
+
+    def tearDown(self):
+        self.remove_settings('settings.py')
+
+    def test_lazy_in_settings(self):
+        out, err = self.run_manage(['test'])
+        self.assertOutput(err, "Ran 0 tests")
+
+
 class ReverseShortcutTests(TestCase):
     urls = 'urlpatterns_reverse.urls'