1 #!/usr/bin/python2.5

     2 #

     3 # Copyright 2008 the Melange authors.

     4 #

     5 # Licensed under the Apache License, Version 2.0 (the "License");

     6 # you may not use this file except in compliance with the License.

     7 # You may obtain a copy of the License at

     8 #

     9 #   http://www.apache.org/licenses/LICENSE-2.0

    10 #

    11 # Unless required by applicable law or agreed to in writing, software

    12 # distributed under the License is distributed on an "AS IS" BASIS,

    13 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.

    14 # See the License for the specific language governing permissions and

    15 # limitations under the License.

    16 

    17 """Like Django SortedDict, but no repeated assignments to the same key.

    18 """

    19 

    20 __authors__ = [

    21   '"Todd Larsen" <tlarsen@google.com>',

    22   ]

    23 

    24 

    25 from django.utils import datastructures

    26 

    27 

    28 class NoOverwriteSortedDict(datastructures.SortedDict):

    29   """SortedDict where each key can be given a value only once.

    30   

    31   The purpose of this data structure is to be able to detect when

    32   an attempt is made to overwrite the value of an existing key

    33   in the SortedDict.  This is to catch, for example, cases such as

    34   a registry where two different callers attempt to register the

    35   same view, handler, etc.

    36  

    37   It is still possible to pop or del a key out of the dict and then

    38   add it back to the dict.

    39   """

    40   

    41   KEY_ALREADY_PRESENT_ERROR_FMT = \

    42     '%s already present, value cannot be overwritten'

    43   

    44   def __init__(self, data=None):

    45     if data is None:

    46       data = {}

    47 

    48     # call SortedDict's parent __init__()

    49     # (bypassing the __init__() of SortedDict itself, since it will not

    50     # enforce our no-overwrite requirement)

    51     super(datastructures.SortedDict, self).__init__(data)

    52     

    53     if isinstance(data, dict):

    54       self.keyOrder = data.keys()

    55     else:

    56       self.keyOrder = []

    57 

    58       for key, value in data:

    59         if key in self.keyOrder:

    60           # key has already been given a value, and that value is not

    61           # permitted to be overwritten, so raise an error

    62           raise KeyError(self.KEY_ALREADY_PRESENT_ERROR_FMT % key)

    63 

    64         self.keyOrder.append(key)

    65 

    66   def __setitem__(self, key, value):

    67     if key in self.keyOrder:

    68       # key has already been given a value, and that value is not permitted

    69       # to be overwritten, so raise an error

    70       raise KeyError(self.KEY_ALREADY_PRESENT_ERROR_FMT % key)

    71 

    72     super(NoOverwriteSortedDict, self).__setitem__(key, value)