--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/app/soc/logic/no_overwrite_sorted_dict.py	Sat Oct 04 04:32:28 2008 +0000
@@ -0,0 +1,72 @@
+#!/usr/bin/python2.5
+#
+# Copyright 2008 the Melange authors.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#   http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""Like Django SortedDict, but no repeated assignments to the same key.
+"""
+
+__authors__ = [
+  '"Todd Larsen" <tlarsen@google.com>',
+  ]
+
+
+from django.utils import datastructures
+
+
+class NoOverwriteSortedDict(datastructures.SortedDict):
+  """SortedDict where each key can be given a value only once.
+  
+  The purpose of this data structure is to be able to detect when
+  an attempt is made to overwrite the value of an existing key
+  in the SortedDict.  This is to catch, for example, cases such as
+  a registry where two different callers attempt to register the
+  same view, handler, etc.
+ 
+  It is still possible to pop or del a key out of the dict and then
+  add it back to the dict.
+  """
+  
+  KEY_ALREADY_PRESENT_ERROR_FMT = \
+    '%s already present, value cannot be overwritten'
+  
+  def __init__(self, data=None):
+    if data is None:
+      data = {}
+
+    # call SortedDict's parent __init__()
+    # (bypassing the __init__() of SortedDict itself, since it will not
+    # enforce our no-overwrite requirement)
+    super(datastructures.SortedDict, self).__init__(data)
+    
+    if isinstance(data, dict):
+      self.keyOrder = data.keys()
+    else:
+      self.keyOrder = []
+
+      for key, value in data:
+        if key in self.keyOrder:
+          # key has already been given a value, and that value is not
+          # permitted to be overwritten, so raise an error
+          raise KeyError(self.KEY_ALREADY_PRESENT_ERROR_FMT % key)
+
+        self.keyOrder.append(key)
+
+  def __setitem__(self, key, value):
+    if key in self.keyOrder:
+      # key has already been given a value, and that value is not permitted
+      # to be overwritten, so raise an error
+      raise KeyError(self.KEY_ALREADY_PRESENT_ERROR_FMT % key)
+
+    super(NoOverwriteSortedDict, self).__setitem__(key, value)