1 #!/usr/bin/python2.5 |
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2 # |
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3 # Copyright 2008 the Melange authors. |
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4 # |
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5 # Licensed under the Apache License, Version 2.0 (the "License"); |
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6 # you may not use this file except in compliance with the License. |
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7 # You may obtain a copy of the License at |
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8 # |
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9 # http://www.apache.org/licenses/LICENSE-2.0 |
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10 # |
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11 # Unless required by applicable law or agreed to in writing, software |
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12 # distributed under the License is distributed on an "AS IS" BASIS, |
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13 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. |
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14 # See the License for the specific language governing permissions and |
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15 # limitations under the License. |
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16 |
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17 """Like Django SortedDict, but no repeated assignments to the same key. |
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18 """ |
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19 |
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20 __authors__ = [ |
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21 '"Todd Larsen" <tlarsen@google.com>', |
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22 ] |
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23 |
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24 |
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25 from django.utils import datastructures |
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26 |
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27 |
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28 class NoOverwriteSortedDict(datastructures.SortedDict): |
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29 """SortedDict where each key can be given a value only once. |
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30 |
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31 The purpose of this data structure is to be able to detect when |
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32 an attempt is made to overwrite the value of an existing key |
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33 in the SortedDict. This is to catch, for example, cases such as |
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34 a registry where two different callers attempt to register the |
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35 same view, handler, etc. |
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36 |
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37 It is still possible to pop or del a key out of the dict and then |
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38 add it back to the dict. |
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39 """ |
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40 |
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41 KEY_ALREADY_PRESENT_ERROR_FMT = \ |
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42 '%s already present, value cannot be overwritten' |
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43 |
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44 def __init__(self, data=None): |
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45 if data is None: |
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46 data = {} |
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47 |
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48 # call SortedDict's parent __init__() |
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49 # (bypassing the __init__() of SortedDict itself, since it will not |
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50 # enforce our no-overwrite requirement) |
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51 super(datastructures.SortedDict, self).__init__(data) |
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52 |
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53 if isinstance(data, dict): |
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54 self.keyOrder = data.keys() |
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55 else: |
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56 self.keyOrder = [] |
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57 |
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58 for key, value in data: |
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59 if key in self.keyOrder: |
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60 # key has already been given a value, and that value is not |
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61 # permitted to be overwritten, so raise an error |
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62 raise KeyError(self.KEY_ALREADY_PRESENT_ERROR_FMT % key) |
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63 |
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64 self.keyOrder.append(key) |
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65 |
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66 def __setitem__(self, key, value): |
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67 if key in self.keyOrder: |
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68 # key has already been given a value, and that value is not permitted |
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69 # to be overwritten, so raise an error |
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70 raise KeyError(self.KEY_ALREADY_PRESENT_ERROR_FMT % key) |
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71 |
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72 super(NoOverwriteSortedDict, self).__setitem__(key, value) |
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