Add derived/derived_dict_entry/derive_settings and tests.

- Enables a method of deriving Django settings from other Django
  settings after all other Django settings are stable.

openedx/core/lib/tests/test_derived.py

@@ -0,0 +1,44 @@
+"""
+Tests for derived.py
+"""
+
+import sys
+from unittest import TestCase
+from openedx.core.lib.derived import derived, derive_settings, clear_for_tests
+
+
+class TestDerivedSettings(TestCase):
+    """
+    Test settings that are derived from other settings.
+    """
+    def setUp(self):
+        super(TestDerivedSettings, self).setUp()
+        clear_for_tests()
+        self.module = sys.modules[__name__]
+        self.module.SIMPLE_VALUE = 'paneer'
+        self.module.DERIVED_VALUE = lambda settings: 'mutter ' + settings.SIMPLE_VALUE
+        self.module.ANOTHER_DERIVED_VALUE = lambda settings: settings.DERIVED_VALUE + ' with naan'
+        self.module.UNREGISTERED_DERIVED_VALUE = lambda settings: settings.SIMPLE_VALUE + ' is cheese'
+        derived('DERIVED_VALUE', 'ANOTHER_DERIVED_VALUE')
+        self.module.DICT_VALUE = {}
+        self.module.DICT_VALUE['test_key'] = lambda settings: settings.DERIVED_VALUE * 3
+        derived(('DICT_VALUE', 'test_key'))
+
+    def test_derived_settings_are_derived(self):
+        derive_settings(__name__)
+        self.assertEqual(self.module.DERIVED_VALUE, 'mutter paneer')
+        self.assertEqual(self.module.ANOTHER_DERIVED_VALUE, 'mutter paneer with naan')
+
+    def test_unregistered_derived_settings(self):
+        derive_settings(__name__)
+        self.assertTrue(callable(self.module.UNREGISTERED_DERIVED_VALUE))
+
+    def test_derived_settings_overridden(self):
+        self.module.DERIVED_VALUE = 'aloo gobi'
+        derive_settings(__name__)
+        self.assertEqual(self.module.DERIVED_VALUE, 'aloo gobi')
+        self.assertEqual(self.module.ANOTHER_DERIVED_VALUE, 'aloo gobi with naan')
+
+    def test_derived_dict_settings(self):
+        derive_settings(__name__)
+        self.assertEqual(self.module.DICT_VALUE['test_key'], 'mutter paneermutter paneermutter paneer')