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<div class="section" id="module-dbm">
<span id="dbm-interfaces-to-unix-databases"></span><h1>12.5. <a class="reference internal" href="#module-dbm" title="dbm: Interfaces to various Unix "database" formats."><code class="xref py py-mod docutils literal notranslate"><span class="pre">dbm</span></code></a> — Interfaces to Unix “databases”<a class="headerlink" href="#module-dbm" title="Permalink to this headline">¶</a></h1>
<p><strong>Source code:</strong> <a class="reference external" href="https://github.com/python/cpython/tree/3.6/Lib/dbm/__init__.py">Lib/dbm/__init__.py</a></p>
<hr class="docutils" />
<p><a class="reference internal" href="#module-dbm" title="dbm: Interfaces to various Unix "database" formats."><code class="xref py py-mod docutils literal notranslate"><span class="pre">dbm</span></code></a> is a generic interface to variants of the DBM database —
<a class="reference internal" href="#module-dbm.gnu" title="dbm.gnu: GNU's reinterpretation of dbm. (Unix)"><code class="xref py py-mod docutils literal notranslate"><span class="pre">dbm.gnu</span></code></a> or <a class="reference internal" href="#module-dbm.ndbm" title="dbm.ndbm: The standard "database" interface, based on ndbm. (Unix)"><code class="xref py py-mod docutils literal notranslate"><span class="pre">dbm.ndbm</span></code></a>. If none of these modules is installed, the
slow-but-simple implementation in module <a class="reference internal" href="#module-dbm.dumb" title="dbm.dumb: Portable implementation of the simple DBM interface."><code class="xref py py-mod docutils literal notranslate"><span class="pre">dbm.dumb</span></code></a> will be used. There
is a <a class="reference external" href="https://www.jcea.es/programacion/pybsddb.htm">third party interface</a> to
the Oracle Berkeley DB.</p>
<dl class="exception">
<dt id="dbm.error">
<em class="property">exception </em><code class="descclassname">dbm.</code><code class="descname">error</code><a class="headerlink" href="#dbm.error" title="Permalink to this definition">¶</a></dt>
<dd><p>A tuple containing the exceptions that can be raised by each of the supported
modules, with a unique exception also named <a class="reference internal" href="#dbm.error" title="dbm.error"><code class="xref py py-exc docutils literal notranslate"><span class="pre">dbm.error</span></code></a> as the first
item — the latter is used when <a class="reference internal" href="#dbm.error" title="dbm.error"><code class="xref py py-exc docutils literal notranslate"><span class="pre">dbm.error</span></code></a> is raised.</p>
</dd></dl>
<dl class="function">
<dt id="dbm.whichdb">
<code class="descclassname">dbm.</code><code class="descname">whichdb</code><span class="sig-paren">(</span><em>filename</em><span class="sig-paren">)</span><a class="headerlink" href="#dbm.whichdb" title="Permalink to this definition">¶</a></dt>
<dd><p>This function attempts to guess which of the several simple database modules
available — <a class="reference internal" href="#module-dbm.gnu" title="dbm.gnu: GNU's reinterpretation of dbm. (Unix)"><code class="xref py py-mod docutils literal notranslate"><span class="pre">dbm.gnu</span></code></a>, <a class="reference internal" href="#module-dbm.ndbm" title="dbm.ndbm: The standard "database" interface, based on ndbm. (Unix)"><code class="xref py py-mod docutils literal notranslate"><span class="pre">dbm.ndbm</span></code></a> or <a class="reference internal" href="#module-dbm.dumb" title="dbm.dumb: Portable implementation of the simple DBM interface."><code class="xref py py-mod docutils literal notranslate"><span class="pre">dbm.dumb</span></code></a> — should
be used to open a given file.</p>
<p>Returns one of the following values: <code class="docutils literal notranslate"><span class="pre">None</span></code> if the file can’t be opened
because it’s unreadable or doesn’t exist; the empty string (<code class="docutils literal notranslate"><span class="pre">''</span></code>) if the
file’s format can’t be guessed; or a string containing the required module
name, such as <code class="docutils literal notranslate"><span class="pre">'dbm.ndbm'</span></code> or <code class="docutils literal notranslate"><span class="pre">'dbm.gnu'</span></code>.</p>
</dd></dl>
<dl class="function">
<dt id="dbm.open">
<code class="descclassname">dbm.</code><code class="descname">open</code><span class="sig-paren">(</span><em>file</em>, <em>flag='r'</em>, <em>mode=0o666</em><span class="sig-paren">)</span><a class="headerlink" href="#dbm.open" title="Permalink to this definition">¶</a></dt>
<dd><p>Open the database file <em>file</em> and return a corresponding object.</p>
<p>If the database file already exists, the <a class="reference internal" href="#dbm.whichdb" title="dbm.whichdb"><code class="xref py py-func docutils literal notranslate"><span class="pre">whichdb()</span></code></a> function is used to
determine its type and the appropriate module is used; if it does not exist,
the first module listed above that can be imported is used.</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><code class="docutils literal notranslate"><span class="pre">'r'</span></code></td>
<td>Open existing database for reading only
(default)</td>
</tr>
<tr class="row-odd"><td><code class="docutils literal notranslate"><span class="pre">'w'</span></code></td>
<td>Open existing database for reading and
writing</td>
</tr>
<tr class="row-even"><td><code class="docutils literal notranslate"><span class="pre">'c'</span></code></td>
<td>Open database for reading and writing,
creating it if it doesn’t exist</td>
</tr>
<tr class="row-odd"><td><code class="docutils literal notranslate"><span class="pre">'n'</span></code></td>
<td>Always create a new, empty database, open
for reading and writing</td>
</tr>
</tbody>
</table>
<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 <code class="docutils literal notranslate"><span class="pre">0o666</span></code> (and will be
modified by the prevailing umask).</p>
</dd></dl>
<p>The object returned by <a class="reference internal" href="#dbm.open" title="dbm.open"><code class="xref py py-func docutils literal notranslate"><span class="pre">open()</span></code></a> supports the same basic functionality as
dictionaries; keys and their corresponding values can be stored, retrieved, and
deleted, and the <a class="reference internal" href="../reference/expressions.html#in"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">in</span></code></a> operator and the <code class="xref py py-meth docutils literal notranslate"><span class="pre">keys()</span></code> method are
available, as well as <code class="xref py py-meth docutils literal notranslate"><span class="pre">get()</span></code> and <code class="xref py py-meth docutils literal notranslate"><span class="pre">setdefault()</span></code>.</p>
<div class="versionchanged">
<p><span class="versionmodified">Changed in version 3.2: </span><code class="xref py py-meth docutils literal notranslate"><span class="pre">get()</span></code> and <code class="xref py py-meth docutils literal notranslate"><span class="pre">setdefault()</span></code> are now available in all database modules.</p>
</div>
<p>Key and values are always stored as bytes. This means that when
strings are used they are implicitly converted to the default encoding before
being stored.</p>
<p>These objects also support being used in a <a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code></a> statement, which
will automatically close them when done.</p>
<div class="versionchanged">
<p><span class="versionmodified">Changed in version 3.4: </span>Added native support for the context management protocol to the objects
returned by <a class="reference internal" href="#dbm.open" title="dbm.open"><code class="xref py py-func docutils literal notranslate"><span class="pre">open()</span></code></a>.</p>
</div>
<p>The following example records some hostnames and a corresponding title, and
then prints out the contents of the database:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">dbm</span>
<span class="c1"># Open database, creating it if necessary.</span>
<span class="k">with</span> <span class="n">dbm</span><span class="o">.</span><span class="n">open</span><span class="p">(</span><span class="s1">'cache'</span><span class="p">,</span> <span class="s1">'c'</span><span class="p">)</span> <span class="k">as</span> <span class="n">db</span><span class="p">:</span>
<span class="c1"># Record some values</span>
<span class="n">db</span><span class="p">[</span><span class="sa">b</span><span class="s1">'hello'</span><span class="p">]</span> <span class="o">=</span> <span class="sa">b</span><span class="s1">'there'</span>
<span class="n">db</span><span class="p">[</span><span class="s1">'www.python.org'</span><span class="p">]</span> <span class="o">=</span> <span class="s1">'Python Website'</span>
<span class="n">db</span><span class="p">[</span><span class="s1">'www.cnn.com'</span><span class="p">]</span> <span class="o">=</span> <span class="s1">'Cable News Network'</span>
<span class="c1"># Note that the keys are considered bytes now.</span>
<span class="k">assert</span> <span class="n">db</span><span class="p">[</span><span class="sa">b</span><span class="s1">'www.python.org'</span><span class="p">]</span> <span class="o">==</span> <span class="sa">b</span><span class="s1">'Python Website'</span>
<span class="c1"># Notice how the value is now in bytes.</span>
<span class="k">assert</span> <span class="n">db</span><span class="p">[</span><span class="s1">'www.cnn.com'</span><span class="p">]</span> <span class="o">==</span> <span class="sa">b</span><span class="s1">'Cable News Network'</span>
<span class="c1"># Often-used methods of the dict interface work too.</span>
<span class="nb">print</span><span class="p">(</span><span class="n">db</span><span class="o">.</span><span class="n">get</span><span class="p">(</span><span class="s1">'python.org'</span><span class="p">,</span> <span class="sa">b</span><span class="s1">'not present'</span><span class="p">))</span>
<span class="c1"># Storing a non-string key or value will raise an exception (most</span>
<span class="c1"># likely a TypeError).</span>
<span class="n">db</span><span class="p">[</span><span class="s1">'www.yahoo.com'</span><span class="p">]</span> <span class="o">=</span> <span class="mi">4</span>
<span class="c1"># db is automatically closed when leaving the with statement.</span>
</pre></div>
</div>
<div class="admonition seealso">
<p class="first admonition-title">See also</p>
<dl class="last docutils">
<dt>Module <a class="reference internal" href="shelve.html#module-shelve" title="shelve: Python object persistence."><code class="xref py py-mod docutils literal notranslate"><span class="pre">shelve</span></code></a></dt>
<dd>Persistence module which stores non-string data.</dd>
</dl>
</div>
<p>The individual submodules are described in the following sections.</p>
<div class="section" id="module-dbm.gnu">
<span id="dbm-gnu-gnu-s-reinterpretation-of-dbm"></span><h2>12.5.1. <a class="reference internal" href="#module-dbm.gnu" title="dbm.gnu: GNU's reinterpretation of dbm. (Unix)"><code class="xref py py-mod docutils literal notranslate"><span class="pre">dbm.gnu</span></code></a> — GNU’s reinterpretation of dbm<a class="headerlink" href="#module-dbm.gnu" title="Permalink to this headline">¶</a></h2>
<p><strong>Source code:</strong> <a class="reference external" href="https://github.com/python/cpython/tree/3.6/Lib/dbm/gnu.py">Lib/dbm/gnu.py</a></p>
<hr class="docutils" />
<p>This module is quite similar to the <a class="reference internal" href="#module-dbm" title="dbm: Interfaces to various Unix "database" formats."><code class="xref py py-mod docutils literal notranslate"><span class="pre">dbm</span></code></a> module, but uses the GNU library
<code class="docutils literal notranslate"><span class="pre">gdbm</span></code> instead to provide some additional functionality. Please note that the
file formats created by <a class="reference internal" href="#module-dbm.gnu" title="dbm.gnu: GNU's reinterpretation of dbm. (Unix)"><code class="xref py py-mod docutils literal notranslate"><span class="pre">dbm.gnu</span></code></a> and <a class="reference internal" href="#module-dbm.ndbm" title="dbm.ndbm: The standard "database" interface, based on ndbm. (Unix)"><code class="xref py py-mod docutils literal notranslate"><span class="pre">dbm.ndbm</span></code></a> are incompatible.</p>
<p>The <a class="reference internal" href="#module-dbm.gnu" title="dbm.gnu: GNU's reinterpretation of dbm. (Unix)"><code class="xref py py-mod docutils literal notranslate"><span class="pre">dbm.gnu</span></code></a> module provides an interface to the GNU DBM library.
<code class="docutils literal notranslate"><span class="pre">dbm.gnu.gdbm</span></code> objects behave like mappings (dictionaries), except that keys and
values are always converted to bytes before storing. Printing a <code class="docutils literal notranslate"><span class="pre">gdbm</span></code>
object doesn’t print the
keys and values, and the <code class="xref py py-meth docutils literal notranslate"><span class="pre">items()</span></code> and <code class="xref py py-meth docutils literal notranslate"><span class="pre">values()</span></code> methods are not
supported.</p>
<dl class="exception">
<dt id="dbm.gnu.error">
<em class="property">exception </em><code class="descclassname">dbm.gnu.</code><code class="descname">error</code><a class="headerlink" href="#dbm.gnu.error" title="Permalink to this definition">¶</a></dt>
<dd><p>Raised on <a class="reference internal" href="#module-dbm.gnu" title="dbm.gnu: GNU's reinterpretation of dbm. (Unix)"><code class="xref py py-mod docutils literal notranslate"><span class="pre">dbm.gnu</span></code></a>-specific errors, such as I/O errors. <a class="reference internal" href="exceptions.html#KeyError" title="KeyError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">KeyError</span></code></a> is
raised for general mapping errors like specifying an incorrect key.</p>
</dd></dl>
<dl class="function">
<dt id="dbm.gnu.open">
<code class="descclassname">dbm.gnu.</code><code class="descname">open</code><span class="sig-paren">(</span><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><span class="sig-paren">)</span><a class="headerlink" href="#dbm.gnu.open" title="Permalink to this definition">¶</a></dt>
<dd><p>Open a <code class="docutils literal notranslate"><span class="pre">gdbm</span></code> database and return a <code class="xref py py-class docutils literal notranslate"><span class="pre">gdbm</span></code> 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><code class="docutils literal notranslate"><span class="pre">'r'</span></code></td>
<td>Open existing database for reading only
(default)</td>
</tr>
<tr class="row-odd"><td><code class="docutils literal notranslate"><span class="pre">'w'</span></code></td>
<td>Open existing database for reading and
writing</td>
</tr>
<tr class="row-even"><td><code class="docutils literal notranslate"><span class="pre">'c'</span></code></td>
<td>Open database for reading and writing,
creating it if it doesn’t exist</td>
</tr>
<tr class="row-odd"><td><code class="docutils literal notranslate"><span class="pre">'n'</span></code></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><code class="docutils literal notranslate"><span class="pre">'f'</span></code></td>
<td>Open the database in fast mode. Writes
to the database will not be synchronized.</td>
</tr>
<tr class="row-odd"><td><code class="docutils literal notranslate"><span class="pre">'s'</span></code></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><code class="docutils literal notranslate"><span class="pre">'u'</span></code></td>
<td>Do not lock database.</td>
</tr>
</tbody>
</table>
<p>Not all flags are valid for all versions of <code class="docutils literal notranslate"><span class="pre">gdbm</span></code>. The module constant
<code class="xref py py-const docutils literal notranslate"><span class="pre">open_flags</span></code> is a string of supported flag characters. The exception
<a class="reference internal" href="#dbm.gnu.error" title="dbm.gnu.error"><code class="xref py py-exc docutils literal notranslate"><span class="pre">error</span></code></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 <code class="docutils literal notranslate"><span class="pre">0o666</span></code>.</p>
<p>In addition to the dictionary-like methods, <code class="docutils literal notranslate"><span class="pre">gdbm</span></code> objects have the
following methods:</p>
<dl class="method">
<dt id="dbm.gnu.gdbm.firstkey">
<code class="descclassname">gdbm.</code><code class="descname">firstkey</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#dbm.gnu.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="#dbm.gnu.gdbm.nextkey" title="dbm.gnu.gdbm.nextkey"><code class="xref py py-meth docutils literal notranslate"><span class="pre">nextkey()</span></code></a> method. The traversal is ordered by <code class="docutils literal notranslate"><span class="pre">gdbm</span></code>’s internal
hash values, and won’t be sorted by the key values. This method returns
the starting key.</p>
</dd></dl>
<dl class="method">
<dt id="dbm.gnu.gdbm.nextkey">
<code class="descclassname">gdbm.</code><code class="descname">nextkey</code><span class="sig-paren">(</span><em>key</em><span class="sig-paren">)</span><a class="headerlink" href="#dbm.gnu.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 <code class="docutils literal notranslate"><span class="pre">db</span></code>, without having to create a list in memory that
contains them all:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><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="kc">None</span><span class="p">:</span>
<span class="nb">print</span><span class="p">(</span><span class="n">k</span><span class="p">)</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="method">
<dt id="dbm.gnu.gdbm.reorganize">
<code class="descclassname">gdbm.</code><code class="descname">reorganize</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#dbm.gnu.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 <code class="docutils literal notranslate"><span class="pre">gdbm</span></code> file, this routine will reorganize the database. <code class="docutils literal notranslate"><span class="pre">gdbm</span></code>
objects 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="method">
<dt id="dbm.gnu.gdbm.sync">
<code class="descclassname">gdbm.</code><code class="descname">sync</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#dbm.gnu.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>
<dl class="method">
<dt id="dbm.gnu.gdbm.close">
<code class="descclassname">gdbm.</code><code class="descname">close</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#dbm.gnu.gdbm.close" title="Permalink to this definition">¶</a></dt>
<dd><p>Close the <code class="docutils literal notranslate"><span class="pre">gdbm</span></code> database.</p>
</dd></dl>
</dd></dl>
</div>
<div class="section" id="module-dbm.ndbm">
<span id="dbm-ndbm-interface-based-on-ndbm"></span><h2>12.5.2. <a class="reference internal" href="#module-dbm.ndbm" title="dbm.ndbm: The standard "database" interface, based on ndbm. (Unix)"><code class="xref py py-mod docutils literal notranslate"><span class="pre">dbm.ndbm</span></code></a> — Interface based on ndbm<a class="headerlink" href="#module-dbm.ndbm" title="Permalink to this headline">¶</a></h2>
<p><strong>Source code:</strong> <a class="reference external" href="https://github.com/python/cpython/tree/3.6/Lib/dbm/ndbm.py">Lib/dbm/ndbm.py</a></p>
<hr class="docutils" />
<p>The <a class="reference internal" href="#module-dbm.ndbm" title="dbm.ndbm: The standard "database" interface, based on ndbm. (Unix)"><code class="xref py py-mod docutils literal notranslate"><span class="pre">dbm.ndbm</span></code></a> module provides an interface to the Unix “(n)dbm” library.
Dbm objects behave like mappings (dictionaries), except that keys and values are
always stored as bytes. Printing a <code class="docutils literal notranslate"><span class="pre">dbm</span></code> object doesn’t print the keys and
values, and the <code class="xref py py-meth docutils literal notranslate"><span class="pre">items()</span></code> and <code class="xref py py-meth docutils literal notranslate"><span class="pre">values()</span></code> methods are not supported.</p>
<p>This module can be used with the “classic” ndbm interface or the GNU GDBM
compatibility interface. On Unix, the <strong class="program">configure</strong> script will attempt
to locate the appropriate header file to simplify building this module.</p>
<dl class="exception">
<dt id="dbm.ndbm.error">
<em class="property">exception </em><code class="descclassname">dbm.ndbm.</code><code class="descname">error</code><a class="headerlink" href="#dbm.ndbm.error" title="Permalink to this definition">¶</a></dt>
<dd><p>Raised on <a class="reference internal" href="#module-dbm.ndbm" title="dbm.ndbm: The standard "database" interface, based on ndbm. (Unix)"><code class="xref py py-mod docutils literal notranslate"><span class="pre">dbm.ndbm</span></code></a>-specific errors, such as I/O errors. <a class="reference internal" href="exceptions.html#KeyError" title="KeyError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">KeyError</span></code></a> is raised
for general mapping errors like specifying an incorrect key.</p>
</dd></dl>
<dl class="data">
<dt id="dbm.ndbm.library">
<code class="descclassname">dbm.ndbm.</code><code class="descname">library</code><a class="headerlink" href="#dbm.ndbm.library" title="Permalink to this definition">¶</a></dt>
<dd><p>Name of the <code class="docutils literal notranslate"><span class="pre">ndbm</span></code> implementation library used.</p>
</dd></dl>
<dl class="function">
<dt id="dbm.ndbm.open">
<code class="descclassname">dbm.ndbm.</code><code class="descname">open</code><span class="sig-paren">(</span><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><span class="sig-paren">)</span><a class="headerlink" href="#dbm.ndbm.open" title="Permalink to this definition">¶</a></dt>
<dd><p>Open a dbm database and return a <code class="docutils literal notranslate"><span class="pre">ndbm</span></code> object. The <em>filename</em> argument is the
name of the database file (without the <code class="file docutils literal notranslate"><span class="pre">.dir</span></code> or <code class="file docutils literal notranslate"><span class="pre">.pag</span></code> extensions).</p>
<p>The optional <em>flag</em> argument must be one of these values:</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><code class="docutils literal notranslate"><span class="pre">'r'</span></code></td>
<td>Open existing database for reading only
(default)</td>
</tr>
<tr class="row-odd"><td><code class="docutils literal notranslate"><span class="pre">'w'</span></code></td>
<td>Open existing database for reading and
writing</td>
</tr>
<tr class="row-even"><td><code class="docutils literal notranslate"><span class="pre">'c'</span></code></td>
<td>Open database for reading and writing,
creating it if it doesn’t exist</td>
</tr>
<tr class="row-odd"><td><code class="docutils literal notranslate"><span class="pre">'n'</span></code></td>
<td>Always create a new, empty database, open
for reading and writing</td>
</tr>
</tbody>
</table>
<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 <code class="docutils literal notranslate"><span class="pre">0o666</span></code> (and will be
modified by the prevailing umask).</p>
<p>In addition to the dictionary-like methods, <code class="docutils literal notranslate"><span class="pre">ndbm</span></code> objects
provide the following method:</p>
<dl class="method">
<dt id="dbm.ndbm.ndbm.close">
<code class="descclassname">ndbm.</code><code class="descname">close</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#dbm.ndbm.ndbm.close" title="Permalink to this definition">¶</a></dt>
<dd><p>Close the <code class="docutils literal notranslate"><span class="pre">ndbm</span></code> database.</p>
</dd></dl>
</dd></dl>
</div>
<div class="section" id="module-dbm.dumb">
<span id="dbm-dumb-portable-dbm-implementation"></span><h2>12.5.3. <a class="reference internal" href="#module-dbm.dumb" title="dbm.dumb: Portable implementation of the simple DBM interface."><code class="xref py py-mod docutils literal notranslate"><span class="pre">dbm.dumb</span></code></a> — Portable DBM implementation<a class="headerlink" href="#module-dbm.dumb" title="Permalink to this headline">¶</a></h2>
<p><strong>Source code:</strong> <a class="reference external" href="https://github.com/python/cpython/tree/3.6/Lib/dbm/dumb.py">Lib/dbm/dumb.py</a></p>
<div class="admonition note" id="index-0">
<p class="first admonition-title">Note</p>
<p class="last">The <a class="reference internal" href="#module-dbm.dumb" title="dbm.dumb: Portable implementation of the simple DBM interface."><code class="xref py py-mod docutils literal notranslate"><span class="pre">dbm.dumb</span></code></a> module is intended as a last resort fallback for the
<a class="reference internal" href="#module-dbm" title="dbm: Interfaces to various Unix "database" formats."><code class="xref py py-mod docutils literal notranslate"><span class="pre">dbm</span></code></a> module when a more robust module is not available. The <a class="reference internal" href="#module-dbm.dumb" title="dbm.dumb: Portable implementation of the simple DBM interface."><code class="xref py py-mod docutils literal notranslate"><span class="pre">dbm.dumb</span></code></a>
module is not written for speed and is not nearly as heavily used as the other
database modules.</p>
</div>
<hr class="docutils" />
<p>The <a class="reference internal" href="#module-dbm.dumb" title="dbm.dumb: Portable implementation of the simple DBM interface."><code class="xref py py-mod docutils literal notranslate"><span class="pre">dbm.dumb</span></code></a> module provides a persistent dictionary-like interface which
is written entirely in Python. Unlike other modules such as <a class="reference internal" href="#module-dbm.gnu" title="dbm.gnu: GNU's reinterpretation of dbm. (Unix)"><code class="xref py py-mod docutils literal notranslate"><span class="pre">dbm.gnu</span></code></a> no
external library is required. As with other persistent mappings, the keys and
values are always stored as bytes.</p>
<p>The module defines the following:</p>
<dl class="exception">
<dt id="dbm.dumb.error">
<em class="property">exception </em><code class="descclassname">dbm.dumb.</code><code class="descname">error</code><a class="headerlink" href="#dbm.dumb.error" title="Permalink to this definition">¶</a></dt>
<dd><p>Raised on <a class="reference internal" href="#module-dbm.dumb" title="dbm.dumb: Portable implementation of the simple DBM interface."><code class="xref py py-mod docutils literal notranslate"><span class="pre">dbm.dumb</span></code></a>-specific errors, such as I/O errors. <a class="reference internal" href="exceptions.html#KeyError" title="KeyError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">KeyError</span></code></a> is
raised for general mapping errors like specifying an incorrect key.</p>
</dd></dl>
<dl class="function">
<dt id="dbm.dumb.open">
<code class="descclassname">dbm.dumb.</code><code class="descname">open</code><span class="sig-paren">(</span><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><span class="sig-paren">)</span><a class="headerlink" href="#dbm.dumb.open" title="Permalink to this definition">¶</a></dt>
<dd><p>Open a <code class="docutils literal notranslate"><span class="pre">dumbdbm</span></code> database and return a dumbdbm object. The <em>filename</em> argument is
the basename of the database file (without any specific extensions). When a
dumbdbm database is created, files with <code class="file docutils literal notranslate"><span class="pre">.dat</span></code> and <code class="file docutils literal notranslate"><span class="pre">.dir</span></code> extensions
are created.</p>
<p>The optional <em>flag</em> argument supports only the semantics of <code class="docutils literal notranslate"><span class="pre">'c'</span></code>
and <code class="docutils literal notranslate"><span class="pre">'n'</span></code> values. Other values will default to database being always
opened for update, and will be created if it does not exist.</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 <code class="docutils literal notranslate"><span class="pre">0o666</span></code> (and will be modified
by the prevailing umask).</p>
<div class="versionchanged">
<p><span class="versionmodified">Changed in version 3.5: </span><a class="reference internal" href="#dbm.dumb.open" title="dbm.dumb.open"><code class="xref py py-func docutils literal notranslate"><span class="pre">open()</span></code></a> always creates a new database when the flag has the value
<code class="docutils literal notranslate"><span class="pre">'n'</span></code>.</p>
</div>
<div class="deprecated-removed">
<p><span class="versionmodified">Deprecated since version 3.6, will be removed in version 3.8: </span>Creating database in <code class="docutils literal notranslate"><span class="pre">'r'</span></code> and <code class="docutils literal notranslate"><span class="pre">'w'</span></code> modes. Modifying database in
<code class="docutils literal notranslate"><span class="pre">'r'</span></code> mode.</p>
</div>
<p>In addition to the methods provided by the
<a class="reference internal" href="collections.abc.html#collections.abc.MutableMapping" title="collections.abc.MutableMapping"><code class="xref py py-class docutils literal notranslate"><span class="pre">collections.abc.MutableMapping</span></code></a> class, <code class="xref py py-class docutils literal notranslate"><span class="pre">dumbdbm</span></code> objects
provide the following methods:</p>
<dl class="method">
<dt id="dbm.dumb.dumbdbm.sync">
<code class="descclassname">dumbdbm.</code><code class="descname">sync</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#dbm.dumb.dumbdbm.sync" title="Permalink to this definition">¶</a></dt>
<dd><p>Synchronize the on-disk directory and data files. This method is called
by the <code class="xref py py-meth docutils literal notranslate"><span class="pre">Shelve.sync()</span></code> method.</p>
</dd></dl>
<dl class="method">
<dt id="dbm.dumb.dumbdbm.close">
<code class="descclassname">dumbdbm.</code><code class="descname">close</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#dbm.dumb.dumbdbm.close" title="Permalink to this definition">¶</a></dt>
<dd><p>Close the <code class="docutils literal notranslate"><span class="pre">dumbdbm</span></code> database.</p>
</dd></dl>
</dd></dl>
</div>
</div>
</div>
</div>
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<h3><a href="../contents.html">Table Of Contents</a></h3>
<ul>
<li><a class="reference internal" href="#">12.5. <code class="docutils literal notranslate"><span class="pre">dbm</span></code> — Interfaces to Unix “databases”</a><ul>
<li><a class="reference internal" href="#module-dbm.gnu">12.5.1. <code class="docutils literal notranslate"><span class="pre">dbm.gnu</span></code> — GNU’s reinterpretation of dbm</a></li>
<li><a class="reference internal" href="#module-dbm.ndbm">12.5.2. <code class="docutils literal notranslate"><span class="pre">dbm.ndbm</span></code> — Interface based on ndbm</a></li>
<li><a class="reference internal" href="#module-dbm.dumb">12.5.3. <code class="docutils literal notranslate"><span class="pre">dbm.dumb</span></code> — Portable DBM implementation</a></li>
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