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<div class="section" id="module-shelve">
<span id="shelve-python-object-persistence"></span><h1>11.4. <a class="reference internal" href="#module-shelve" title="shelve: Python object persistence."><code class="xref py py-mod docutils literal notranslate"><span class="pre">shelve</span></code></a> — Python object persistence<a class="headerlink" href="#module-shelve" title="Permalink to this headline">¶</a></h1>
<p id="index-0"><strong>Source code:</strong> <a class="reference external" href="https://github.com/python/cpython/tree/2.7/Lib/shelve.py">Lib/shelve.py</a></p>
<hr class="docutils" />
<p>A “shelf” is a persistent, dictionary-like object. The difference with “dbm”
databases is that the values (not the keys!) in a shelf can be essentially
arbitrary Python objects — anything that the <a class="reference internal" href="pickle.html#module-pickle" title="pickle: Convert Python objects to streams of bytes and back."><code class="xref py py-mod docutils literal notranslate"><span class="pre">pickle</span></code></a> module can handle.
This includes most class instances, recursive data types, and objects containing
lots of shared sub-objects. The keys are ordinary strings.</p>
<dl class="function">
<dt id="shelve.open">
<code class="descclassname">shelve.</code><code class="descname">open</code><span class="sig-paren">(</span><em>filename</em>, <em>flag='c'</em>, <em>protocol=None</em>, <em>writeback=False</em><span class="sig-paren">)</span><a class="headerlink" href="#shelve.open" title="Permalink to this definition">¶</a></dt>
<dd><p>Open a persistent dictionary. The filename specified is the base filename for
the underlying database. As a side-effect, an extension may be added to the
filename and more than one file may be created. By default, the underlying
database file is opened for reading and writing. The optional <em>flag</em> parameter
has the same interpretation as the <em>flag</em> parameter of <a class="reference internal" href="anydbm.html#anydbm.open" title="anydbm.open"><code class="xref py py-func docutils literal notranslate"><span class="pre">anydbm.open()</span></code></a>.</p>
<p>By default, version 0 pickles are used to serialize values. The version of the
pickle protocol can be specified with the <em>protocol</em> parameter.</p>
<div class="versionchanged">
<p><span class="versionmodified">Changed in version 2.3: </span>The <em>protocol</em> parameter was added.</p>
</div>
<p>Because of Python semantics, a shelf cannot know when a mutable
persistent-dictionary entry is modified. By default modified objects are
written <em>only</em> when assigned to the shelf (see <a class="reference internal" href="#shelve-example"><span class="std std-ref">Example</span></a>). If the
optional <em>writeback</em> parameter is set to <code class="docutils literal notranslate"><span class="pre">True</span></code>, all entries accessed are also
cached in memory, and written back on <a class="reference internal" href="#shelve.Shelf.sync" title="shelve.Shelf.sync"><code class="xref py py-meth docutils literal notranslate"><span class="pre">sync()</span></code></a> and
<a class="reference internal" href="#shelve.Shelf.close" title="shelve.Shelf.close"><code class="xref py py-meth docutils literal notranslate"><span class="pre">close()</span></code></a>; this can make it handier to mutate mutable entries in
the persistent dictionary, but, if many entries are accessed, it can consume
vast amounts of memory for the cache, and it can make the close operation
very slow since all accessed entries are written back (there is no way to
determine which accessed entries are mutable, nor which ones were actually
mutated).</p>
<p>Like file objects, shelve objects should be closed explicitly to ensure
that the persistent data is flushed to disk.</p>
</dd></dl>
<div class="admonition warning">
<p class="first admonition-title">Warning</p>
<p class="last">Because the <a class="reference internal" href="#module-shelve" title="shelve: Python object persistence."><code class="xref py py-mod docutils literal notranslate"><span class="pre">shelve</span></code></a> module is backed by <a class="reference internal" href="pickle.html#module-pickle" title="pickle: Convert Python objects to streams of bytes and back."><code class="xref py py-mod docutils literal notranslate"><span class="pre">pickle</span></code></a>, it is insecure
to load a shelf from an untrusted source. Like with pickle, loading a shelf
can execute arbitrary code.</p>
</div>
<p>Shelf objects support most of the methods supported by dictionaries. This
eases the transition from dictionary based scripts to those requiring
persistent storage.</p>
<p>Note, the Python 3 transition methods (<a class="reference internal" href="stdtypes.html#dict.viewkeys" title="dict.viewkeys"><code class="xref py py-meth docutils literal notranslate"><span class="pre">viewkeys()</span></code></a>,
<a class="reference internal" href="stdtypes.html#dict.viewvalues" title="dict.viewvalues"><code class="xref py py-meth docutils literal notranslate"><span class="pre">viewvalues()</span></code></a>, and <a class="reference internal" href="stdtypes.html#dict.viewitems" title="dict.viewitems"><code class="xref py py-meth docutils literal notranslate"><span class="pre">viewitems()</span></code></a>) are not supported.</p>
<p>Two additional methods are supported:</p>
<dl class="method">
<dt id="shelve.Shelf.sync">
<code class="descclassname">Shelf.</code><code class="descname">sync</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#shelve.Shelf.sync" title="Permalink to this definition">¶</a></dt>
<dd><p>Write back all entries in the cache if the shelf was opened with <em>writeback</em>
set to <a class="reference internal" href="constants.html#True" title="True"><code class="xref py py-const docutils literal notranslate"><span class="pre">True</span></code></a>. Also empty the cache and synchronize the persistent
dictionary on disk, if feasible. This is called automatically when the shelf
is closed with <a class="reference internal" href="#shelve.Shelf.close" title="shelve.Shelf.close"><code class="xref py py-meth docutils literal notranslate"><span class="pre">close()</span></code></a>.</p>
</dd></dl>
<dl class="method">
<dt id="shelve.Shelf.close">
<code class="descclassname">Shelf.</code><code class="descname">close</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#shelve.Shelf.close" title="Permalink to this definition">¶</a></dt>
<dd><p>Synchronize and close the persistent <em>dict</em> object. Operations on a closed
shelf will fail with a <a class="reference internal" href="exceptions.html#exceptions.ValueError" title="exceptions.ValueError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">ValueError</span></code></a>.</p>
</dd></dl>
<div class="admonition seealso">
<p class="first admonition-title">See also</p>
<p class="last"><a class="reference external" href="https://code.activestate.com/recipes/576642/">Persistent dictionary recipe</a>
with widely supported storage formats and having the speed of native
dictionaries.</p>
</div>
<div class="section" id="restrictions">
<h2>11.4.1. Restrictions<a class="headerlink" href="#restrictions" title="Permalink to this headline">¶</a></h2>
<blockquote>
<div></div></blockquote>
<ul class="simple" id="index-1">
<li>The choice of which database package will be used (such as <a class="reference internal" href="dbm.html#module-dbm" title="dbm: The standard "database" interface, based on ndbm. (Unix)"><code class="xref py py-mod docutils literal notranslate"><span class="pre">dbm</span></code></a>,
<a class="reference internal" href="gdbm.html#module-gdbm" title="gdbm: GNU's reinterpretation of dbm. (Unix)"><code class="xref py py-mod docutils literal notranslate"><span class="pre">gdbm</span></code></a> or <a class="reference internal" href="bsddb.html#module-bsddb" title="bsddb: Interface to Berkeley DB database library"><code class="xref py py-mod docutils literal notranslate"><span class="pre">bsddb</span></code></a>) depends on which interface is available. Therefore
it is not safe to open the database directly using <a class="reference internal" href="dbm.html#module-dbm" title="dbm: The standard "database" interface, based on ndbm. (Unix)"><code class="xref py py-mod docutils literal notranslate"><span class="pre">dbm</span></code></a>. The database is
also (unfortunately) subject to the limitations of <a class="reference internal" href="dbm.html#module-dbm" title="dbm: The standard "database" interface, based on ndbm. (Unix)"><code class="xref py py-mod docutils literal notranslate"><span class="pre">dbm</span></code></a>, if it is used —
this means that (the pickled representation of) the objects stored in the
database should be fairly small, and in rare cases key collisions may cause the
database to refuse updates.</li>
<li>The <a class="reference internal" href="#module-shelve" title="shelve: Python object persistence."><code class="xref py py-mod docutils literal notranslate"><span class="pre">shelve</span></code></a> module does not support <em>concurrent</em> read/write access to
shelved objects. (Multiple simultaneous read accesses are safe.) When a
program has a shelf open for writing, no other program should have it open for
reading or writing. Unix file locking can be used to solve this, but this
differs across Unix versions and requires knowledge about the database
implementation used.</li>
</ul>
<dl class="class">
<dt id="shelve.Shelf">
<em class="property">class </em><code class="descclassname">shelve.</code><code class="descname">Shelf</code><span class="sig-paren">(</span><em>dict</em>, <em>protocol=None</em>, <em>writeback=False</em><span class="sig-paren">)</span><a class="headerlink" href="#shelve.Shelf" title="Permalink to this definition">¶</a></dt>
<dd><p>A subclass of <a class="reference internal" href="userdict.html#UserDict.DictMixin" title="UserDict.DictMixin"><code class="xref py py-class docutils literal notranslate"><span class="pre">UserDict.DictMixin</span></code></a> which stores pickled values in the
<em>dict</em> object.</p>
<p>By default, version 0 pickles are used to serialize values. The version of the
pickle protocol can be specified with the <em>protocol</em> parameter. See the
<a class="reference internal" href="pickle.html#module-pickle" title="pickle: Convert Python objects to streams of bytes and back."><code class="xref py py-mod docutils literal notranslate"><span class="pre">pickle</span></code></a> documentation for a discussion of the pickle protocols.</p>
<div class="versionchanged">
<p><span class="versionmodified">Changed in version 2.3: </span>The <em>protocol</em> parameter was added.</p>
</div>
<p>If the <em>writeback</em> parameter is <code class="docutils literal notranslate"><span class="pre">True</span></code>, the object will hold a cache of all
entries accessed and write them back to the <em>dict</em> at sync and close times.
This allows natural operations on mutable entries, but can consume much more
memory and make sync and close take a long time.</p>
</dd></dl>
<dl class="class">
<dt id="shelve.BsdDbShelf">
<em class="property">class </em><code class="descclassname">shelve.</code><code class="descname">BsdDbShelf</code><span class="sig-paren">(</span><em>dict</em>, <em>protocol=None</em>, <em>writeback=False</em><span class="sig-paren">)</span><a class="headerlink" href="#shelve.BsdDbShelf" title="Permalink to this definition">¶</a></dt>
<dd><p>A subclass of <a class="reference internal" href="#shelve.Shelf" title="shelve.Shelf"><code class="xref py py-class docutils literal notranslate"><span class="pre">Shelf</span></code></a> which exposes <code class="xref py py-meth docutils literal notranslate"><span class="pre">first()</span></code>, <code class="xref py py-meth docutils literal notranslate"><span class="pre">next()</span></code>,
<code class="xref py py-meth docutils literal notranslate"><span class="pre">previous()</span></code>, <code class="xref py py-meth docutils literal notranslate"><span class="pre">last()</span></code> and <code class="xref py py-meth docutils literal notranslate"><span class="pre">set_location()</span></code> which are available in
the <a class="reference internal" href="bsddb.html#module-bsddb" title="bsddb: Interface to Berkeley DB database library"><code class="xref py py-mod docutils literal notranslate"><span class="pre">bsddb</span></code></a> module but not in other database modules. The <em>dict</em> object
passed to the constructor must support those methods. This is generally
accomplished by calling one of <a class="reference internal" href="bsddb.html#bsddb.hashopen" title="bsddb.hashopen"><code class="xref py py-func docutils literal notranslate"><span class="pre">bsddb.hashopen()</span></code></a>, <a class="reference internal" href="bsddb.html#bsddb.btopen" title="bsddb.btopen"><code class="xref py py-func docutils literal notranslate"><span class="pre">bsddb.btopen()</span></code></a> or
<a class="reference internal" href="bsddb.html#bsddb.rnopen" title="bsddb.rnopen"><code class="xref py py-func docutils literal notranslate"><span class="pre">bsddb.rnopen()</span></code></a>. The optional <em>protocol</em> and <em>writeback</em> parameters have
the same interpretation as for the <a class="reference internal" href="#shelve.Shelf" title="shelve.Shelf"><code class="xref py py-class docutils literal notranslate"><span class="pre">Shelf</span></code></a> class.</p>
</dd></dl>
<dl class="class">
<dt id="shelve.DbfilenameShelf">
<em class="property">class </em><code class="descclassname">shelve.</code><code class="descname">DbfilenameShelf</code><span class="sig-paren">(</span><em>filename</em>, <em>flag='c'</em>, <em>protocol=None</em>, <em>writeback=False</em><span class="sig-paren">)</span><a class="headerlink" href="#shelve.DbfilenameShelf" title="Permalink to this definition">¶</a></dt>
<dd><p>A subclass of <a class="reference internal" href="#shelve.Shelf" title="shelve.Shelf"><code class="xref py py-class docutils literal notranslate"><span class="pre">Shelf</span></code></a> which accepts a <em>filename</em> instead of a dict-like
object. The underlying file will be opened using <a class="reference internal" href="anydbm.html#anydbm.open" title="anydbm.open"><code class="xref py py-func docutils literal notranslate"><span class="pre">anydbm.open()</span></code></a>. By
default, the file will be created and opened for both read and write. The
optional <em>flag</em> parameter has the same interpretation as for the <a class="reference internal" href="#shelve.open" title="shelve.open"><code class="xref py py-func docutils literal notranslate"><span class="pre">open()</span></code></a>
function. The optional <em>protocol</em> and <em>writeback</em> parameters have the same
interpretation as for the <a class="reference internal" href="#shelve.Shelf" title="shelve.Shelf"><code class="xref py py-class docutils literal notranslate"><span class="pre">Shelf</span></code></a> class.</p>
</dd></dl>
</div>
<div class="section" id="example">
<span id="shelve-example"></span><h2>11.4.2. Example<a class="headerlink" href="#example" title="Permalink to this headline">¶</a></h2>
<p>To summarize the interface (<code class="docutils literal notranslate"><span class="pre">key</span></code> is a string, <code class="docutils literal notranslate"><span class="pre">data</span></code> is an arbitrary
object):</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">shelve</span>
<span class="n">d</span> <span class="o">=</span> <span class="n">shelve</span><span class="o">.</span><span class="n">open</span><span class="p">(</span><span class="n">filename</span><span class="p">)</span> <span class="c1"># open -- file may get suffix added by low-level</span>
<span class="c1"># library</span>
<span class="n">d</span><span class="p">[</span><span class="n">key</span><span class="p">]</span> <span class="o">=</span> <span class="n">data</span> <span class="c1"># store data at key (overwrites old data if</span>
<span class="c1"># using an existing key)</span>
<span class="n">data</span> <span class="o">=</span> <span class="n">d</span><span class="p">[</span><span class="n">key</span><span class="p">]</span> <span class="c1"># retrieve a COPY of data at key (raise KeyError if no</span>
<span class="c1"># such key)</span>
<span class="k">del</span> <span class="n">d</span><span class="p">[</span><span class="n">key</span><span class="p">]</span> <span class="c1"># delete data stored at key (raises KeyError</span>
<span class="c1"># if no such key)</span>
<span class="n">flag</span> <span class="o">=</span> <span class="n">d</span><span class="o">.</span><span class="n">has_key</span><span class="p">(</span><span class="n">key</span><span class="p">)</span> <span class="c1"># true if the key exists</span>
<span class="n">klist</span> <span class="o">=</span> <span class="n">d</span><span class="o">.</span><span class="n">keys</span><span class="p">()</span> <span class="c1"># a list of all existing keys (slow!)</span>
<span class="c1"># as d was opened WITHOUT writeback=True, beware:</span>
<span class="n">d</span><span class="p">[</span><span class="s1">'xx'</span><span class="p">]</span> <span class="o">=</span> <span class="nb">range</span><span class="p">(</span><span class="mi">4</span><span class="p">)</span> <span class="c1"># this works as expected, but...</span>
<span class="n">d</span><span class="p">[</span><span class="s1">'xx'</span><span class="p">]</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="mi">5</span><span class="p">)</span> <span class="c1"># *this doesn't!* -- d['xx'] is STILL range(4)!</span>
<span class="c1"># having opened d without writeback=True, you need to code carefully:</span>
<span class="n">temp</span> <span class="o">=</span> <span class="n">d</span><span class="p">[</span><span class="s1">'xx'</span><span class="p">]</span> <span class="c1"># extracts the copy</span>
<span class="n">temp</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="mi">5</span><span class="p">)</span> <span class="c1"># mutates the copy</span>
<span class="n">d</span><span class="p">[</span><span class="s1">'xx'</span><span class="p">]</span> <span class="o">=</span> <span class="n">temp</span> <span class="c1"># stores the copy right back, to persist it</span>
<span class="c1"># or, d=shelve.open(filename,writeback=True) would let you just code</span>
<span class="c1"># d['xx'].append(5) and have it work as expected, BUT it would also</span>
<span class="c1"># consume more memory and make the d.close() operation slower.</span>
<span class="n">d</span><span class="o">.</span><span class="n">close</span><span class="p">()</span> <span class="c1"># close it</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="anydbm.html#module-anydbm" title="anydbm: Generic interface to DBM-style database modules."><code class="xref py py-mod docutils literal notranslate"><span class="pre">anydbm</span></code></a></dt>
<dd>Generic interface to <code class="docutils literal notranslate"><span class="pre">dbm</span></code>-style databases.</dd>
<dt>Module <a class="reference internal" href="bsddb.html#module-bsddb" title="bsddb: Interface to Berkeley DB database library"><code class="xref py py-mod docutils literal notranslate"><span class="pre">bsddb</span></code></a></dt>
<dd>BSD <code class="docutils literal notranslate"><span class="pre">db</span></code> database interface.</dd>
<dt>Module <a class="reference internal" href="dbhash.html#module-dbhash" title="dbhash: DBM-style interface to the BSD database library."><code class="xref py py-mod docutils literal notranslate"><span class="pre">dbhash</span></code></a></dt>
<dd>Thin layer around the <a class="reference internal" href="bsddb.html#module-bsddb" title="bsddb: Interface to Berkeley DB database library"><code class="xref py py-mod docutils literal notranslate"><span class="pre">bsddb</span></code></a> which provides an <a class="reference internal" href="dbhash.html#dbhash.open" title="dbhash.open"><code class="xref py py-func docutils literal notranslate"><span class="pre">open()</span></code></a>
function like the other database modules.</dd>
<dt>Module <a class="reference internal" href="dbm.html#module-dbm" title="dbm: The standard "database" interface, based on ndbm. (Unix)"><code class="xref py py-mod docutils literal notranslate"><span class="pre">dbm</span></code></a></dt>
<dd>Standard Unix database interface.</dd>
<dt>Module <a class="reference internal" href="dumbdbm.html#module-dumbdbm" title="dumbdbm: Portable implementation of the simple DBM interface."><code class="xref py py-mod docutils literal notranslate"><span class="pre">dumbdbm</span></code></a></dt>
<dd>Portable implementation of the <code class="docutils literal notranslate"><span class="pre">dbm</span></code> interface.</dd>
<dt>Module <a class="reference internal" href="gdbm.html#module-gdbm" title="gdbm: GNU's reinterpretation of dbm. (Unix)"><code class="xref py py-mod docutils literal notranslate"><span class="pre">gdbm</span></code></a></dt>
<dd>GNU database interface, based on the <code class="docutils literal notranslate"><span class="pre">dbm</span></code> interface.</dd>
<dt>Module <a class="reference internal" href="pickle.html#module-pickle" title="pickle: Convert Python objects to streams of bytes and back."><code class="xref py py-mod docutils literal notranslate"><span class="pre">pickle</span></code></a></dt>
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