emit a warning when a setting has an unexpected type

rest_framework/exceptions.py

@@ -145,3 +145,7 @@ class Throttled(APIException):
             self.detail += ' ' + force_text(
                 self.extra_detail % {'wait': self.wait}
             )
+
+
+class RESTFrameworkSettingHasUnexpectedClassWarning(Warning):
+    pass

rest_framework/settings.py

@@ -18,9 +18,11 @@ REST framework settings, checking for user settings first, then falling
 back to the defaults.
 """
 from __future__ import unicode_literals
+import warnings
 from django.conf import settings
 from django.utils import importlib, six
 from rest_framework import ISO_8601
+from rest_framework.exceptions import RESTFrameworkSettingHasUnexpectedClassWarning
 
 
 USER_SETTINGS = getattr(settings, 'REST_FRAMEWORK', None)
@@ -187,6 +189,17 @@ class APISettings(object):
         except KeyError:
             # Fall back to defaults
             val = self.defaults[attr]
+        else:
+            expected_class = self.defaults[attr].__class__
+            if val.__class__ != expected_class:
+                warnings.warn(
+                    "The `{val}` setting has the class {val.__class__}, "
+                    "but the expected class was {expected_class}".format(
+                        **locals()
+                    ),
+                    RESTFrameworkSettingHasUnexpectedClassWarning,
+                    stacklevel=3
+                )
 
         # Coerce import strings into classes
         if val and attr in self.import_strings:

tests/test_settings.py

@@ -1,5 +1,8 @@
 from __future__ import unicode_literals
+import warnings
 from django.test import TestCase
+from rest_framework.exceptions import \
+    RESTFrameworkSettingHasUnexpectedClassWarning
 from rest_framework.settings import APISettings
 
 
@@ -15,3 +18,20 @@ class TestSettings(TestCase):
         })
         with self.assertRaises(ImportError):
             settings.DEFAULT_RENDERER_CLASSES
+
+    def test_bad_setting_class_raises_warning(self):
+        """
+        Make sure warnings are emitted when settings have an unexpected class.
+        """
+        settings = APISettings({
+            'DEFAULT_RENDERER_CLASSES': 'rest_framework.renderers.JSONRenderer'
+        })
+
+        with warnings.catch_warnings(record=True) as w:
+            # Trigger a warning.
+            settings.DEFAULT_RENDERER_CLASSES
+            # Verify that a warning is thrown
+            assert len(w) == 1
+            assert issubclass(
+                w[-1].category, RESTFrameworkSettingHasUnexpectedClassWarning
+            )