Fix override_settings compat (#5668)

* Add test checking override_settings compat

* Refresh APISettings, rather than replace

Fix suggested by @daggaz https://github.com/encode/django-rest-framework/issues/2466#issuecomment-344297213

rest_framework/settings.py

@@ -200,6 +200,7 @@ class APISettings(object):
             self._user_settings = self.__check_user_settings(user_settings)
         self.defaults = defaults or DEFAULTS
         self.import_strings = import_strings or IMPORT_STRINGS
+        self._cached_attrs = set()
 
     @property
     def user_settings(self):
@@ -223,6 +224,7 @@ class APISettings(object):
             val = perform_import(val, attr)
 
         # Cache the result
+        self._cached_attrs.add(attr)
         setattr(self, attr, val)
         return val
 
@@ -233,15 +235,21 @@ class APISettings(object):
                 raise RuntimeError("The '%s' setting has been removed. Please refer to '%s' for available settings." % (setting, SETTINGS_DOC))
         return user_settings
 
+    def reload(self):
+        for attr in self._cached_attrs:
+            delattr(self, attr)
+        self._cached_attrs.clear()
+        if hasattr(self, '_user_settings'):
+            delattr(self, '_user_settings')
+
 
 api_settings = APISettings(None, DEFAULTS, IMPORT_STRINGS)
 
 
 def reload_api_settings(*args, **kwargs):
-    global api_settings
-    setting, value = kwargs['setting'], kwargs['value']
+    setting = kwargs['setting']
     if setting == 'REST_FRAMEWORK':
-        api_settings = APISettings(value, DEFAULTS, IMPORT_STRINGS)
+        api_settings.reload()
 
 
 setting_changed.connect(reload_api_settings)

tests/test_settings.py

@@ -1,8 +1,8 @@
 from __future__ import unicode_literals
 
-from django.test import TestCase
+from django.test import TestCase, override_settings
 
-from rest_framework.settings import APISettings
+from rest_framework.settings import APISettings, api_settings
 
 
 class TestSettings(TestCase):
@@ -28,6 +28,23 @@ class TestSettings(TestCase):
                 'MAX_PAGINATE_BY': 100
             })
 
+    def test_compatibility_with_override_settings(self):
+        """
+        Ref #5658 & #2466: Documented usage of api_settings
+        is bound at import time:
+
+            from rest_framework.settings import api_settings
+
+        setting_changed signal hook must ensure bound instance
+        is refreshed.
+        """
+        assert api_settings.PAGE_SIZE is None, "Checking a known default should be None"
+
+        with override_settings(REST_FRAMEWORK={'PAGE_SIZE': 10}):
+            assert api_settings.PAGE_SIZE == 10, "Setting should have been updated"
+
+        assert api_settings.PAGE_SIZE is None, "Setting should have been restored"
+
 
 class TestSettingTypes(TestCase):
     def test_settings_consistently_coerced_to_list(self):