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Data Persistence</a> »</li> </ul> </div> <div class="document"> <div class="documentwrapper"> <div class="bodywrapper"> <div class="body"> <div class="section" id="module-gdbm"> <span id="gdbm-gnu-s-reinterpretation-of-dbm"></span><h1>11.9. <a class="reference internal" href="#module-gdbm" title="gdbm: GNU's reinterpretation of dbm. (Unix)"><tt class="xref py py-mod docutils literal"><span class="pre">gdbm</span></tt></a> — GNU’s reinterpretation of dbm<a class="headerlink" href="#module-gdbm" title="Permalink to this headline">¶</a></h1> <div class="admonition note"> <p class="first admonition-title">Note</p> <p class="last">The <a class="reference internal" href="#module-gdbm" title="gdbm: GNU's reinterpretation of dbm. (Unix)"><tt class="xref py py-mod docutils literal"><span class="pre">gdbm</span></tt></a> module has been renamed to <tt class="xref py py-mod docutils literal"><span class="pre">dbm.gnu</span></tt> in Python 3. The <a class="reference internal" href="../glossary.html#term-to3"><em class="xref std std-term">2to3</em></a> tool will automatically adapt imports when converting your sources to Python 3.</p> </div> <p id="index-0">This module is quite similar to the <a class="reference internal" href="dbm.html#module-dbm" title="dbm: The standard "database" interface, based on ndbm. (Unix)"><tt class="xref py py-mod docutils literal"><span class="pre">dbm</span></tt></a> module, but uses <tt class="docutils literal"><span class="pre">gdbm</span></tt> instead to provide some additional functionality. Please note that the file formats created by <tt class="docutils literal"><span class="pre">gdbm</span></tt> and <tt class="docutils literal"><span class="pre">dbm</span></tt> are incompatible.</p> <p>The <a class="reference internal" href="#module-gdbm" title="gdbm: GNU's reinterpretation of dbm. (Unix)"><tt class="xref py py-mod docutils literal"><span class="pre">gdbm</span></tt></a> module provides an interface to the GNU DBM library. <tt class="docutils literal"><span class="pre">gdbm</span></tt> objects behave like mappings (dictionaries), except that keys and values are always strings. Printing a <tt class="docutils literal"><span class="pre">gdbm</span></tt> object doesn’t print the keys and values, and the <tt class="xref py py-meth docutils literal"><span class="pre">items()</span></tt> and <tt class="xref py py-meth docutils literal"><span class="pre">values()</span></tt> methods are not supported.</p> <p>The module defines the following constant and functions:</p> <dl class="exception"> <dt id="gdbm.error"> <em class="property">exception </em><tt class="descclassname">gdbm.</tt><tt class="descname">error</tt><a class="headerlink" href="#gdbm.error" title="Permalink to this definition">¶</a></dt> <dd><p>Raised on <tt class="docutils literal"><span class="pre">gdbm</span></tt>-specific errors, such as I/O errors. <a class="reference internal" href="exceptions.html#exceptions.KeyError" title="exceptions.KeyError"><tt class="xref py py-exc docutils literal"><span class="pre">KeyError</span></tt></a> is raised for general mapping errors like specifying an incorrect key.</p> </dd></dl> <dl class="function"> <dt id="gdbm.open"> <tt class="descclassname">gdbm.</tt><tt class="descname">open</tt><big>(</big><em>filename</em><span class="optional">[</span>, <em>flag</em><span class="optional">[</span>, <em>mode</em><span class="optional">]</span><span class="optional">]</span><big>)</big><a class="headerlink" href="#gdbm.open" title="Permalink to this definition">¶</a></dt> <dd><p>Open a <tt class="docutils literal"><span class="pre">gdbm</span></tt> database and return a <tt class="docutils literal"><span class="pre">gdbm</span></tt> object. The <em>filename</em> argument is the name of the database file.</p> <p>The optional <em>flag</em> argument can be:</p> <table border="1" class="docutils"> <colgroup> <col width="17%" /> <col width="83%" /> </colgroup> <thead valign="bottom"> <tr class="row-odd"><th class="head">Value</th> <th class="head">Meaning</th> </tr> </thead> <tbody valign="top"> <tr class="row-even"><td><tt class="docutils literal"><span class="pre">'r'</span></tt></td> <td>Open existing database for reading only (default)</td> </tr> <tr class="row-odd"><td><tt class="docutils literal"><span class="pre">'w'</span></tt></td> <td>Open existing database for reading and writing</td> </tr> <tr class="row-even"><td><tt class="docutils literal"><span class="pre">'c'</span></tt></td> <td>Open database for reading and writing, creating it if it doesn’t exist</td> </tr> <tr class="row-odd"><td><tt class="docutils literal"><span class="pre">'n'</span></tt></td> <td>Always create a new, empty database, open for reading and writing</td> </tr> </tbody> </table> <p>The following additional characters may be appended to the flag to control how the database is opened:</p> <table border="1" class="docutils"> <colgroup> <col width="17%" /> <col width="83%" /> </colgroup> <thead valign="bottom"> <tr class="row-odd"><th class="head">Value</th> <th class="head">Meaning</th> </tr> </thead> <tbody valign="top"> <tr class="row-even"><td><tt class="docutils literal"><span class="pre">'f'</span></tt></td> <td>Open the database in fast mode. Writes to the database will not be synchronized.</td> </tr> <tr class="row-odd"><td><tt class="docutils literal"><span class="pre">'s'</span></tt></td> <td>Synchronized mode. This will cause changes to the database to be immediately written to the file.</td> </tr> <tr class="row-even"><td><tt class="docutils literal"><span class="pre">'u'</span></tt></td> <td>Do not lock database.</td> </tr> </tbody> </table> <p>Not all flags are valid for all versions of <tt class="docutils literal"><span class="pre">gdbm</span></tt>. The module constant <tt class="xref py py-const docutils literal"><span class="pre">open_flags</span></tt> is a string of supported flag characters. The exception <a class="reference internal" href="#gdbm.error" title="gdbm.error"><tt class="xref py py-exc docutils literal"><span class="pre">error</span></tt></a> is raised if an invalid flag is specified.</p> <p>The optional <em>mode</em> argument is the Unix mode of the file, used only when the database has to be created. It defaults to octal <tt class="docutils literal"><span class="pre">0666</span></tt>.</p> </dd></dl> <p>In addition to the dictionary-like methods, <tt class="docutils literal"><span class="pre">gdbm</span></tt> objects have the following methods:</p> <dl class="function"> <dt id="gdbm.firstkey"> <tt class="descclassname">gdbm.</tt><tt class="descname">firstkey</tt><big>(</big><big>)</big><a class="headerlink" href="#gdbm.firstkey" title="Permalink to this definition">¶</a></dt> <dd><p>It’s possible to loop over every key in the database using this method and the <a class="reference internal" href="#gdbm.nextkey" title="gdbm.nextkey"><tt class="xref py py-meth docutils literal"><span class="pre">nextkey()</span></tt></a> method. The traversal is ordered by <tt class="docutils literal"><span class="pre">gdbm</span></tt>‘s internal hash values, and won’t be sorted by the key values. This method returns the starting key.</p> </dd></dl> <dl class="function"> <dt id="gdbm.nextkey"> <tt class="descclassname">gdbm.</tt><tt class="descname">nextkey</tt><big>(</big><em>key</em><big>)</big><a class="headerlink" href="#gdbm.nextkey" title="Permalink to this definition">¶</a></dt> <dd><p>Returns the key that follows <em>key</em> in the traversal. The following code prints every key in the database <tt class="docutils literal"><span class="pre">db</span></tt>, without having to create a list in memory that contains them all:</p> <div class="highlight-python"><div class="highlight"><pre><span class="n">k</span> <span class="o">=</span> <span class="n">db</span><span class="o">.</span><span class="n">firstkey</span><span class="p">()</span> <span class="k">while</span> <span class="n">k</span> <span class="o">!=</span> <span class="bp">None</span><span class="p">:</span> <span class="k">print</span> <span class="n">k</span> <span class="n">k</span> <span class="o">=</span> <span class="n">db</span><span class="o">.</span><span class="n">nextkey</span><span class="p">(</span><span class="n">k</span><span class="p">)</span> </pre></div> </div> </dd></dl> <dl class="function"> <dt id="gdbm.reorganize"> <tt class="descclassname">gdbm.</tt><tt class="descname">reorganize</tt><big>(</big><big>)</big><a class="headerlink" href="#gdbm.reorganize" title="Permalink to this definition">¶</a></dt> <dd><p>If you have carried out a lot of deletions and would like to shrink the space used by the <tt class="docutils literal"><span class="pre">gdbm</span></tt> file, this routine will reorganize the database. <tt class="docutils literal"><span class="pre">gdbm</span></tt> will not shorten the length of a database file except by using this reorganization; otherwise, deleted file space will be kept and reused as new (key, value) pairs are added.</p> </dd></dl> <dl class="function"> <dt id="gdbm.sync"> <tt class="descclassname">gdbm.</tt><tt class="descname">sync</tt><big>(</big><big>)</big><a class="headerlink" href="#gdbm.sync" title="Permalink to this definition">¶</a></dt> <dd><p>When the database has been opened in fast mode, this method forces any unwritten data to be written to the disk.</p> </dd></dl> <div class="admonition-see-also admonition seealso"> <p class="first admonition-title">See also</p> <dl class="last docutils"> <dt>Module <a class="reference internal" href="anydbm.html#module-anydbm" title="anydbm: Generic interface to DBM-style database modules."><tt class="xref py py-mod docutils literal"><span class="pre">anydbm</span></tt></a></dt> <dd>Generic interface to <tt class="docutils literal"><span class="pre">dbm</span></tt>-style databases.</dd> <dt>Module <a class="reference internal" href="whichdb.html#module-whichdb" title="whichdb: Guess which DBM-style module created a given database."><tt class="xref py py-mod docutils literal"><span class="pre">whichdb</span></tt></a></dt> <dd>Utility module used to determine the type of an existing database.</dd> </dl> </div> </div> </div> </div> </div> <div class="sphinxsidebar"> <div class="sphinxsidebarwrapper"> <h4>Previous topic</h4> <p class="topless"><a href="dbm.html" title="previous chapter">11.8. <tt class="docutils literal"><span class="pre">dbm</span></tt> — Simple “database” interface</a></p> <h4>Next topic</h4> <p class="topless"><a href="dbhash.html" title="next chapter">11.10. <tt class="docutils literal"><span class="pre">dbhash</span></tt> — DBM-style interface to the BSD database library</a></p> <h3>This Page</h3> <ul class="this-page-menu"> <li><a href="../bugs.html">Report a Bug</a></li> <li><a href="../_sources/library/gdbm.txt" rel="nofollow">Show Source</a></li> </ul> <div id="searchbox" style="display: none"> <h3>Quick search</h3> <form class="search" action="../search.html" method="get"> <input type="text" name="q" /> <input type="submit" value="Go" /> <input type="hidden" name="check_keywords" value="yes" /> <input type="hidden" name="area" value="default" /> </form> <p class="searchtip" style="font-size: 90%"> Enter search terms or a module, class or function name. </p> </div> <script type="text/javascript">$('#searchbox').show(0);</script> </div> </div> <div class="clearer"></div> </div> <div class="related"> <h3>Navigation</h3> <ul> <li class="right" style="margin-right: 10px"> <a href="../genindex.html" title="General Index" >index</a></li> <li class="right" > <a href="../py-modindex.html" title="Python Module Index" >modules</a> |</li> <li class="right" > <a href="dbhash.html" title="11.10. dbhash — DBM-style interface to the BSD database library" >next</a> |</li> <li class="right" > <a href="dbm.html" title="11.8. dbm — Simple “database” interface" >previous</a> |</li> <li><img src="../_static/py.png" alt="" style="vertical-align: middle; margin-top: -1px"/></li> <li><a href="http://www.python.org/">Python</a> »</li> <li> <a href="../index.html">Python 2.7.5 documentation</a> » </li> <li><a href="index.html" >The Python Standard Library</a> »</li> <li><a href="persistence.html" >11. 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