Fixed #19031 -- Added a warning when using override_settings with 'DATABASES'

django/test/signals.py

@@ -1,5 +1,6 @@
 import os
 import time
+import warnings
 
 from django.conf import settings
 from django.db import connections
@@ -9,11 +10,14 @@ from django.utils.functional import empty
 
 template_rendered = Signal(providing_args=["template", "context"])
 
-setting_changed = Signal(providing_args=["setting", "value"])
+setting_changed = Signal(providing_args=["setting", "value", "enter"])
 
 # Most setting_changed receivers are supposed to be added below,
 # except for cases where the receiver is related to a contrib app.
 
+# Settings that may not work well when using 'override_settings' (#19031)
+COMPLEX_OVERRIDE_SETTINGS = set(['DATABASES'])
+
 
 @receiver(setting_changed)
 def update_connections_time_zone(**kwargs):
@@ -74,8 +78,15 @@ def language_changed(**kwargs):
         if kwargs['setting'] == 'LOCALE_PATHS':
             trans_real._translations = {}
 
+
 @receiver(setting_changed)
 def file_storage_changed(**kwargs):
     if kwargs['setting'] in ('MEDIA_ROOT', 'DEFAULT_FILE_STORAGE'):
         from django.core.files.storage import default_storage
         default_storage._wrapped = empty
+
+
+@receiver(setting_changed)
+def complex_setting_changed(**kwargs):
+    if kwargs['enter'] and kwargs['setting'] in COMPLEX_OVERRIDE_SETTINGS:
+        warnings.warn("Overriding setting %s can lead to unexpected behaviour." % kwargs['setting'])

django/test/utils.py

@@ -228,7 +228,7 @@ class override_settings(object):
         settings._wrapped = override
         for key, new_value in self.options.items():
             setting_changed.send(sender=settings._wrapped.__class__,
-                                 setting=key, value=new_value)
+                                 setting=key, value=new_value, enter=True)
 
     def disable(self):
         settings._wrapped = self.wrapped
@@ -236,7 +236,7 @@ class override_settings(object):
         for key in self.options:
             new_value = getattr(settings, key, None)
             setting_changed.send(sender=settings._wrapped.__class__,
-                                 setting=key, value=new_value)
+                                 setting=key, value=new_value, enter=False)
 
 
 def compare_xml(want, got):