Mercurial > cpython-lru_cache-weakref

annotate Lib/shelve.py @ 79675:c0a2294de787 2.7

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Issue #16199: Backport 2to3 typo fix from 3.2.
author Chris Jerdonek <chris.jerdonek@gmail.com>
date Thu, 11 Oct 2012 16:15:46 -0700
parents bfd4798b1f77
children
Ignore whitespace changes - Everywhere: Within whitespace: At end of lines:
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1 """Manage shelves of pickled objects.
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2
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3 A "shelf" is a persistent, dictionary-like object. The difference
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4 with dbm databases is that the values (not the keys!) in a shelf can
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5 be essentially arbitrary Python objects -- anything that the "pickle"
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6 module can handle. This includes most class instances, recursive data
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7 types, and objects containing lots of shared sub-objects. The keys
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8 are ordinary strings.
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9
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10 To summarize the interface (key is a string, data is an arbitrary
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11 object):
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12
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13 import shelve
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14 d = shelve.open(filename) # open, with (g)dbm filename -- no suffix
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15
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16 d[key] = data # store data at key (overwrites old data if
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17 # using an existing key)
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18 data = d[key] # retrieve a COPY of the data at key (raise
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19 # KeyError if no such key) -- NOTE that this
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20 # access returns a *copy* of the entry!
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21 del d[key] # delete data stored at key (raises KeyError
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22 # if no such key)
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23 flag = d.has_key(key) # true if the key exists; same as "key in d"
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24 list = d.keys() # a list of all existing keys (slow!)
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25
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26 d.close() # close it
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27
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28 Dependent on the implementation, closing a persistent dictionary may
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29 or may not be necessary to flush changes to disk.
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30
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31 Normally, d[key] returns a COPY of the entry. This needs care when
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32 mutable entries are mutated: for example, if d[key] is a list,
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33 d[key].append(anitem)
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34 does NOT modify the entry d[key] itself, as stored in the persistent
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35 mapping -- it only modifies the copy, which is then immediately
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36 discarded, so that the append has NO effect whatsoever. To append an
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37 item to d[key] in a way that will affect the persistent mapping, use:
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38 data = d[key]
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39 data.append(anitem)
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40 d[key] = data
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41
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42 To avoid the problem with mutable entries, you may pass the keyword
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43 argument writeback=True in the call to shelve.open. When you use:
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44 d = shelve.open(filename, writeback=True)
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45 then d keeps a cache of all entries you access, and writes them all back
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46 to the persistent mapping when you call d.close(). This ensures that
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47 such usage as d[key].append(anitem) works as intended.
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48
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49 However, using keyword argument writeback=True may consume vast amount
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50 of memory for the cache, and it may make d.close() very slow, if you
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51 access many of d's entries after opening it in this way: d has no way to
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52 check which of the entries you access are mutable and/or which ones you
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53 actually mutate, so it must cache, and write back at close, all of the
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54 entries that you access. You can call d.sync() to write back all the
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55 entries in the cache, and empty the cache (d.sync() also synchronizes
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56 the persistent dictionary on disk, if feasible).
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57 """
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58
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59 # Try using cPickle and cStringIO if available.
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60
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61 try:
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62 from cPickle import Pickler, Unpickler
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63 except ImportError:
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64 from pickle import Pickler, Unpickler
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65
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66 try:
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67 from cStringIO import StringIO
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68 except ImportError:
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69 from StringIO import StringIO
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70
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71 import UserDict
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72
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73 __all__ = ["Shelf","BsdDbShelf","DbfilenameShelf","open"]
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74
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75 class _ClosedDict(UserDict.DictMixin):
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76 'Marker for a closed dict. Access attempts raise a ValueError.'
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77
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78 def closed(self, *args):
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79 raise ValueError('invalid operation on closed shelf')
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80 __getitem__ = __setitem__ = __delitem__ = keys = closed
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81
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82 def __repr__(self):
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83 return '<Closed Dictionary>'
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84
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85 class Shelf(UserDict.DictMixin):
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86 """Base class for shelf implementations.
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87
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88 This is initialized with a dictionary-like object.
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89 See the module's __doc__ string for an overview of the interface.
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90 """
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91
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92 def __init__(self, dict, protocol=None, writeback=False):
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93 self.dict = dict
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94 if protocol is None:
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95 protocol = 0
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96 self._protocol = protocol
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97 self.writeback = writeback
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98 self.cache = {}
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99
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100 def keys(self):
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101 return self.dict.keys()
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102
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103 def __len__(self):
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104 return len(self.dict)
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105
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106 def has_key(self, key):
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107 return key in self.dict
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108
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109 def __contains__(self, key):
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110 return key in self.dict
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111
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112 def get(self, key, default=None):
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113 if key in self.dict:
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114 return self[key]
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115 return default
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116
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117 def __getitem__(self, key):
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118 try:
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119 value = self.cache[key]
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120 except KeyError:
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121 f = StringIO(self.dict[key])
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122 value = Unpickler(f).load()
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123 if self.writeback:
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124 self.cache[key] = value
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125 return value
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126
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127 def __setitem__(self, key, value):
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128 if self.writeback:
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129 self.cache[key] = value
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130 f = StringIO()
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131 p = Pickler(f, self._protocol)
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132 p.dump(value)
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133 self.dict[key] = f.getvalue()
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134
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135 def __delitem__(self, key):
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136 del self.dict[key]
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137 try:
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138 del self.cache[key]
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139 except KeyError:
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140 pass
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141
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142 def close(self):
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143 self.sync()
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144 try:
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145 self.dict.close()
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146 except AttributeError:
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147 pass
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148 # Catch errors that may happen when close is called from __del__
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149 # because CPython is in interpreter shutdown.
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150 try:
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151 self.dict = _ClosedDict()
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152 except (NameError, TypeError):
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153 self.dict = None
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154
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155 def __del__(self):
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156 if not hasattr(self, 'writeback'):
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157 # __init__ didn't succeed, so don't bother closing
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158 return
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159 self.close()
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160
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161 def sync(self):
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162 if self.writeback and self.cache:
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163 self.writeback = False
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164 for key, entry in self.cache.iteritems():
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165 self[key] = entry
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166 self.writeback = True
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167 self.cache = {}
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168 if hasattr(self.dict, 'sync'):
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169 self.dict.sync()
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170
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171
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172 class BsdDbShelf(Shelf):
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173 """Shelf implementation using the "BSD" db interface.
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174
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175 This adds methods first(), next(), previous(), last() and
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176 set_location() that have no counterpart in [g]dbm databases.
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177
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178 The actual database must be opened using one of the "bsddb"
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179 modules "open" routines (i.e. bsddb.hashopen, bsddb.btopen or
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180 bsddb.rnopen) and passed to the constructor.
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181
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182 See the module's __doc__ string for an overview of the interface.
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183 """
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184
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185 def __init__(self, dict, protocol=None, writeback=False):
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186 Shelf.__init__(self, dict, protocol, writeback)
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187
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188 def set_location(self, key):
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189 (key, value) = self.dict.set_location(key)
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190 f = StringIO(value)
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191 return (key, Unpickler(f).load())
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192
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193 def next(self):
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194 (key, value) = self.dict.next()
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195 f = StringIO(value)
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196 return (key, Unpickler(f).load())
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197
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198 def previous(self):
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199 (key, value) = self.dict.previous()
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200 f = StringIO(value)
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201 return (key, Unpickler(f).load())
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202
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203 def first(self):
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204 (key, value) = self.dict.first()
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205 f = StringIO(value)
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206 return (key, Unpickler(f).load())
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207
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208 def last(self):
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209 (key, value) = self.dict.last()
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210 f = StringIO(value)
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211 return (key, Unpickler(f).load())
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212
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213
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214 class DbfilenameShelf(Shelf):
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215 """Shelf implementation using the "anydbm" generic dbm interface.
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216
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217 This is initialized with the filename for the dbm database.
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218 See the module's __doc__ string for an overview of the interface.
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219 """
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220
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221 def __init__(self, filename, flag='c', protocol=None, writeback=False):
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222 import anydbm
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223 Shelf.__init__(self, anydbm.open(filename, flag), protocol, writeback)
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224
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225
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226 def open(filename, flag='c', protocol=None, writeback=False):
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227 """Open a persistent dictionary for reading and writing.
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228
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229 The filename parameter is the base filename for the underlying
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230 database. As a side-effect, an extension may be added to the
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231 filename and more than one file may be created. The optional flag
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232 parameter has the same interpretation as the flag parameter of
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233 anydbm.open(). The optional protocol parameter specifies the
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234 version of the pickle protocol (0, 1, or 2).
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235
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236 See the module's __doc__ string for an overview of the interface.
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237 """
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238
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239 return DbfilenameShelf(filename, flag, protocol, writeback)