Add editable() method to registry.Json.

src/registry.py

@@ -686,4 +686,18 @@ class Json(String):
     def __call__(self):
         return json.loads(super(Json, self).__call__())
 
+    class _Context:
+        def __init__(self, var):
+            self._var = var
+        def __enter__(self):
+            self._dict = self._var()
+            return self._dict
+        def __exit__(self, *args):
+            self._var.setValue(self._dict)
+
+    def editable(self):
+        """Return an editable dict usable within a 'with' statement and
+        committed to the configuration variable at the end."""
+        return self._Context(self)
+
 # vim:set shiftwidth=4 softtabstop=4 expandtab textwidth=79:

test/test_registry.py

@@ -138,6 +138,16 @@ class ValuesTestCase(SupyTestCase):
         self.assertEqual(v(), data)
         self.assertIsNot(v(), data)
 
+        with v.editable() as dict_:
+            dict_['supy'] = 'bot'
+            del dict_['qux']
+            self.assertNotIn('supy', v())
+            self.assertIn('qux', v())
+        self.assertIn('supy', v())
+        self.assertEqual(v()['supy'], 'bot')
+        self.assertIsNot(v()['supy'], 'bot')
+        self.assertNotIn('qux', v())
+
     def testNormalizedString(self):
         v = registry.NormalizedString("""foo
         bar           baz