emit a warning when a setting has an unexpected type

rest_framework/settings.py

@@ -18,6 +18,7 @@ REST framework settings, checking for user settings first, then falling
 back to the defaults.
 """
 from __future__ import unicode_literals
+from collections import Iterable
 import warnings
 from django.conf import settings
 from django.utils import importlib, six
@@ -190,13 +191,21 @@ class APISettings(object):
             # Fall back to defaults
             val = self.defaults[attr]
         else:
-            expected_class = self.defaults[attr].__class__
+            # Verify that the user hasn't accidentally given a string instead
-            if val.__class__ != expected_class:
+            #  of an iterable, or vice-versa
+            default = self.defaults[attr]
+            if issubclass(val.__class__, Iterable) \
+                    and (not issubclass(default.__class__, Iterable)
+                         or isinstance(val, basestring)):
                 warnings.warn(
-                    "The `{val}` setting has the class {val.__class__}, "
+                    "The `{attr}` setting must be iterable".format(**locals()),
-                    "but the expected class was {expected_class}".format(
+                    RESTFrameworkSettingHasUnexpectedClassWarning,
-                        **locals()
+                    stacklevel=3
-                    ),
+                )
+            elif isinstance(default, basestring) and not \
+                    isinstance(val, basestring):
+                warnings.warn(
+                    "The `{attr}` setting must be a string".format(**locals()),
                     RESTFrameworkSettingHasUnexpectedClassWarning,
                     stacklevel=3
                 )

tests/test_settings.py

@@ -19,9 +19,10 @@ class TestSettings(TestCase):
         with self.assertRaises(ImportError):
             settings.DEFAULT_RENDERER_CLASSES
 
-    def test_bad_setting_class_raises_warning(self):
+    def test_bad_iterable_setting_class_raises_warning(self):
         """
-        Make sure warnings are emitted when settings have an unexpected class.
+        Make sure warnings are emitted when settings which should be iterable
+        are not.
         """
         settings = APISettings({
             'DEFAULT_RENDERER_CLASSES': 'rest_framework.renderers.JSONRenderer'
@@ -35,3 +36,22 @@ class TestSettings(TestCase):
             assert issubclass(
                 w[-1].category, RESTFrameworkSettingHasUnexpectedClassWarning
             )
+
+    def test_bad_string_setting_class_raises_warning(self):
+        """
+        Make sure warnings are emitted when settings which should be strings are
+        not.
+        """
+        settings = APISettings({
+            'DEFAULT_METADATA_CLASS': []
+        })
+
+        with warnings.catch_warnings(record=True) as w:
+            # Trigger a warning.
+            settings.DEFAULT_METADATA_CLASS
+
+            # Verify that a warning is thrown
+            assert len(w) == 1
+            assert issubclass(
+                w[-1].category, RESTFrameworkSettingHasUnexpectedClassWarning
+            )