1 files changed, 50 insertions, 0 deletions
diff --git a/python/vyos/dicts.py b/python/vyos/dicts.py
new file mode 100644
index 000000000..79cab4a08
--- /dev/null
+++ b/python/vyos/dicts.py
@@ -0,0 +1,50 @@
+#!/usr/bin/env python3
+#
+# Copyright (C) 2019 VyOS maintainers and contributors
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License version 2 or later as
+# published by the Free Software Foundation.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+
+class FixedDict(dict):
+    """
+    FixedDict: A dictionnary not allowing new keys to be created after initialisation.
+
+    >>> f = FixedDict(**{'count':1})
+    >>> f['count'] = 2
+    >>> f['king'] = 3
+      File "...", line ..., in __setitem__
+    raise ConfigError(f'Option "{k}" has no defined default')
+    """
+
+    def __init__(self, **options):
+        self._allowed = options.keys()
+        super().__init__(**options)
+
+    def __setitem__(self, k, v):
+        """
+        __setitem__ is a builtin which is called by python when setting dict values:
+        >>> d = dict()
+        >>> d['key'] = 'value'
+        >>> d
+        {'key': 'value'}
+
+        is syntaxic sugar for
+
+        >>> d = dict()
+        >>> d.__setitem__('key','value')
+        >>> d
+        {'key': 'value'}
+        """
+        if k not in self._allowed:
+            raise ConfigError(f'Option "{k}" has no defined default')
+        super().__setitem__(k, v)