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<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>
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