Allow settings prefixed with 'CUSTOM_' (fixes #204)

docs/changes.rst

@@ -96,6 +96,9 @@ backends:
 - Make the entire code base use unicode strings by default, and only fall back
   to bytestrings where it is required. Another step closer to Python 3.
 
+- The settings validator will now allow any setting prefixed with ``CUSTOM_``
+  to exist in the settings file.
+
 **Bug fixes**
 
 - :issue:`218`: The MPD commands ``listplaylist`` and ``listplaylistinfo`` now

docs/settings.rst

@@ -200,6 +200,21 @@ can use with the ``gst-launch-0.10`` command can be plugged into
 :attr:`mopidy.settings.OUTPUT`.
 
 
+Custom settings
+===============
+
+Mopidy's settings validator will stop you from defining any settings in your
+settings file that Mopidy doesn't know about. This may sound obnoxious, but it
+helps you detect typos in your settings, and deprecated settings that should be
+removed or updated.
+
+If you're extending Mopidy in some way, and want to use Mopidy's settings
+system, you can prefix your settings with ``CUSTOM_`` to get around the
+settings validator. We recommend that you choose names like
+``CUSTOM_MYAPP_MYSETTING`` so that multiple custom extensions to Mopidy can be
+used at the same time without any danger of naming collisions.
+
+
 Available settings
 ==================
 

mopidy/utils/settings.py

@@ -121,7 +121,6 @@ def validate_settings(defaults, settings):
     errors = {}
 
     changed = {
-        'CUSTOM_OUTPUT': 'OUTPUT',
         'DUMP_LOG_FILENAME': 'DEBUG_LOG_FILENAME',
         'DUMP_LOG_FORMAT': 'DEBUG_LOG_FORMAT',
         'FRONTEND': 'FRONTENDS',
@@ -176,7 +175,7 @@ def validate_settings(defaults, settings):
             if not value:
                 errors[setting] = 'Must contain at least one value.'
 
-        elif setting not in defaults:
+        elif setting not in defaults and not setting.startswith('CUSTOM_'):
             errors[setting] = 'Unknown setting.'
             suggestion = did_you_mean(setting, defaults)
 

tests/utils/settings_test.py

@@ -29,6 +29,11 @@ class ValidateSettingsTest(unittest.TestCase):
             result['MPD_SERVER_HOSTNMAE'],
             'Unknown setting. Did you mean MPD_SERVER_HOSTNAME?')
 
+    def test_custom_settings_does_not_return_errors(self):
+        result = setting_utils.validate_settings(
+            self.defaults, {'CUSTOM_MYAPP_SETTING': 'foobar'})
+        self.assertNotIn('CUSTOM_MYAPP_SETTING', result)
+
     def test_not_renamed_setting_returns_error(self):
         result = setting_utils.validate_settings(
             self.defaults, {'SERVER_HOSTNAME': '127.0.0.1'})