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55 | <H1><A NAME="SECTION0051700000000000000000"> | |
56 | 3.17 <tt class="module">shelve</tt> -- | |
57 | Python object persistence</A> | |
58 | </H1> | |
59 | ||
60 | <P> | |
61 | <A NAME="module-shelve"></A> | |
62 | ||
63 | <P> | |
64 | A ``shelf'' is a persistent, dictionary-like object. The difference | |
65 | with ``dbm'' databases is that the values (not the keys!) in a shelf | |
66 | can be essentially arbitrary Python objects -- anything that the | |
67 | <tt class="module"><a href="module-pickle.html">pickle</a></tt> module can handle. This includes most class | |
68 | instances, recursive data types, and objects containing lots of shared | |
69 | sub-objects. The keys are ordinary strings. | |
70 | <a id='l2h-668' xml:id='l2h-668'></a> | |
71 | ||
72 | <P> | |
73 | <dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"> | |
74 | <td><nobr><b><tt id='l2h-666' xml:id='l2h-666' class="function">open</tt></b>(</nobr></td> | |
75 | <td><var>filename</var><big>[</big><var>,flag='c'</var><big>[</big><var>,protocol=<code>None</code></var><big>[</big><var>,writeback=<code>False</code></var><big>[</big><var>,binary=<code>None</code></var><big>]</big><var></var><big>]</big><var></var><big>]</big><var></var><big>]</big><var></var>)</td></tr></table></dt> | |
76 | <dd> | |
77 | Open a persistent dictionary. The filename specified is the base filename | |
78 | for the underlying database. As a side-effect, an extension may be added to | |
79 | the filename and more than one file may be created. By default, the | |
80 | underlying database file is opened for reading and writing. The optional | |
81 | <var>flag</var> parameter has the same interpretation as the <var>flag</var> | |
82 | parameter of <tt class="function">anydbm.open</tt>. | |
83 | ||
84 | <P> | |
85 | By default, version 0 pickles are used to serialize values. | |
86 | The version of the pickle protocol can be specified with the | |
87 | <var>protocol</var> parameter. | |
88 | <span class="versionnote">Changed in version 2.3: | |
89 | The <var>protocol</var> | |
90 | parameter was added. The <var>binary</var> parameter is deprecated | |
91 | and provided for backwards compatibility only.</span> | |
92 | ||
93 | <P> | |
94 | By default, mutations to persistent-dictionary mutable entries are not | |
95 | automatically written back. If the optional <var>writeback</var> parameter | |
96 | is set to <var>True</var>, all entries accessed are cached in memory, and | |
97 | written back at close time; this can make it handier to mutate mutable | |
98 | entries in the persistent dictionary, but, if many entries are | |
99 | accessed, it can consume vast amounts of memory for the cache, and it | |
100 | can make the close operation very slow since all accessed entries are | |
101 | written back (there is no way to determine which accessed entries are | |
102 | mutable, nor which ones were actually mutated). | |
103 | ||
104 | <P> | |
105 | </dl> | |
106 | ||
107 | <P> | |
108 | Shelve objects support all methods supported by dictionaries. This eases | |
109 | the transition from dictionary based scripts to those requiring persistent | |
110 | storage. | |
111 | ||
112 | <P> | |
113 | One additional method is supported: | |
114 | <dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"> | |
115 | <td><nobr><b><tt id='l2h-667' xml:id='l2h-667' class="method">sync</tt></b>(</nobr></td> | |
116 | <td><var></var>)</td></tr></table></dt> | |
117 | <dd> | |
118 | Write back all entries in the cache if the shelf was opened with | |
119 | <var>writeback</var> set to <var>True</var>. Also empty the cache and synchronize | |
120 | the persistent dictionary on disk, if feasible. This is called automatically | |
121 | when the shelf is closed with <tt class="method">close()</tt>. | |
122 | </dl> | |
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172 | <span class="release-info">Release 2.4.2, documentation updated on 28 September 2005.</span> | |
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