settings: Fail if BACKENDS/FRONTENDS setting isn't iterable (fixes #278)

docs/changes.rst

@@ -8,6 +8,15 @@ This change log is used to track all major changes to Mopidy.
 v0.11.0 (in development)
 ========================
 
+**Settings**
+
+- The settings validator now complains if a setting which expects a tuple of
+  values (e.g. :attr:`mopidy.settings.BACKENDS`,
+  :attr:`mopidy.settings.FRONTENDS`) has a non-iterable value. This typically
+  happens because the setting value contains a single value and one has
+  forgotten to add a comma after the string, making the value a tuple. (Fixes:
+  :issue:`278`)
+
 **Spotify backend**
 
 - Add :attr:`mopidy.settings.SPOTIFY_TIMEOUT` setting which allows you to

mopidy/utils/settings.py

@@ -172,6 +172,10 @@ def validate_settings(defaults, settings):
                 'bin in OUTPUT.')
 
         elif setting in list_of_one_or_more:
+            if not hasattr(value, '__iter__'):
+                errors[setting] = (
+                    'Must be a tuple. '
+                    "Remember the comma after single values: (u'value',)")
             if not value:
                 errors[setting] = 'Must contain at least one value.'
 

tests/utils/settings_test.py

@@ -87,6 +87,14 @@ class ValidateSettingsTest(unittest.TestCase):
         self.assertEqual(
             result['BACKENDS'], 'Must contain at least one value.')
 
+    def test_noniterable_multivalue_setting_returns_error(self):
+        result = setting_utils.validate_settings(
+            self.defaults, {'FRONTENDS': ('this is not a tuple')})
+        self.assertEqual(
+            result['FRONTENDS'],
+            'Must be a tuple. '
+            "Remember the comma after single values: (u'value',)")
+
 
 class SettingsProxyTest(unittest.TestCase):
     def setUp(self):