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<div class="section" id="module-marshal">
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<span id="marshal-internal-python-object-serialization"></span><h1>12.4. <a class="reference internal" href="#module-marshal" title="marshal: Convert Python objects to streams of bytes and back (with different constraints)."><code class="xref py py-mod docutils literal"><span class="pre">marshal</span></code></a> — Internal Python object serialization<a class="headerlink" href="#module-marshal" title="Permalink to this headline">¶</a></h1>
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<p>This module contains functions that can read and write Python values in a binary
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format. The format is specific to Python, but independent of machine
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architecture issues (e.g., you can write a Python value to a file on a PC,
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transport the file to a Sun, and read it back there). Details of the format are
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undocumented on purpose; it may change between Python versions (although it
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rarely does). <a class="footnote-reference" href="#id2" id="id1">[1]</a></p>
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<p id="index-0">This is not a general “persistence” module. For general persistence and
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transfer of Python objects through RPC calls, see the modules <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"><span class="pre">pickle</span></code></a> and
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<a class="reference internal" href="shelve.html#module-shelve" title="shelve: Python object persistence."><code class="xref py py-mod docutils literal"><span class="pre">shelve</span></code></a>. The <a class="reference internal" href="#module-marshal" title="marshal: Convert Python objects to streams of bytes and back (with different constraints)."><code class="xref py py-mod docutils literal"><span class="pre">marshal</span></code></a> module exists mainly to support reading and
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writing the “pseudo-compiled” code for Python modules of <code class="file docutils literal"><span class="pre">.pyc</span></code> files.
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Therefore, the Python maintainers reserve the right to modify the marshal format
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in backward incompatible ways should the need arise. If you’re serializing and
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de-serializing Python objects, use 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"><span class="pre">pickle</span></code></a> module instead – the
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performance is comparable, version independence is guaranteed, and pickle
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supports a substantially wider range of objects than marshal.</p>
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<div class="admonition warning">
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<p class="first admonition-title">Warning</p>
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<p class="last">The <a class="reference internal" href="#module-marshal" title="marshal: Convert Python objects to streams of bytes and back (with different constraints)."><code class="xref py py-mod docutils literal"><span class="pre">marshal</span></code></a> module is not intended to be secure against erroneous or
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maliciously constructed data. Never unmarshal data received from an
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untrusted or unauthenticated source.</p>
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<p>Not all Python object types are supported; in general, only objects whose value
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is independent from a particular invocation of Python can be written and read by
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this module. The following types are supported: booleans, integers, floating
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point numbers, complex numbers, strings, bytes, bytearrays, tuples, lists, sets,
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frozensets, dictionaries, and code objects, where it should be understood that
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tuples, lists, sets, frozensets and dictionaries are only supported as long as
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the values contained therein are themselves supported.
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singletons <a class="reference internal" href="constants.html#None" title="None"><code class="xref py py-const docutils literal"><span class="pre">None</span></code></a>, <a class="reference internal" href="constants.html#Ellipsis" title="Ellipsis"><code class="xref py py-const docutils literal"><span class="pre">Ellipsis</span></code></a> and <a class="reference internal" href="exceptions.html#StopIteration" title="StopIteration"><code class="xref py py-exc docutils literal"><span class="pre">StopIteration</span></code></a> can also be
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marshalled and unmarshalled.
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For format <em>version</em> lower than 3, recursive lists, sets and dictionaries cannot
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be written (see below).</p>
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<p>There are functions that read/write files as well as functions operating on
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<p>The module defines these functions:</p>
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<dl class="function">
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<dt id="marshal.dump">
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<code class="descclassname">marshal.</code><code class="descname">dump</code><span class="sig-paren">(</span><em>value</em>, <em>file</em><span class="optional">[</span>, <em>version</em><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#marshal.dump" title="Permalink to this definition">¶</a></dt>
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<dd><p>Write the value on the open file. The value must be a supported type. The
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file must be an open file object such as <code class="docutils literal"><span class="pre">sys.stdout</span></code> or returned by
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<a class="reference internal" href="functions.html#open" title="open"><code class="xref py py-func docutils literal"><span class="pre">open()</span></code></a> or <a class="reference internal" href="os.html#os.popen" title="os.popen"><code class="xref py py-func docutils literal"><span class="pre">os.popen()</span></code></a>. It must be opened in binary mode (<code class="docutils literal"><span class="pre">'wb'</span></code>
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or <code class="docutils literal"><span class="pre">'w+b'</span></code>).</p>
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<p>If the value has (or contains an object that has) an unsupported type, a
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<a class="reference internal" href="exceptions.html#ValueError" title="ValueError"><code class="xref py py-exc docutils literal"><span class="pre">ValueError</span></code></a> exception is raised — but garbage data will also be written
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to the file. The object will not be properly read back by <a class="reference internal" href="#marshal.load" title="marshal.load"><code class="xref py py-func docutils literal"><span class="pre">load()</span></code></a>.</p>
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<p>The <em>version</em> argument indicates the data format that <code class="docutils literal"><span class="pre">dump</span></code> should use
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<dl class="function">
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<dt id="marshal.load">
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<code class="descclassname">marshal.</code><code class="descname">load</code><span class="sig-paren">(</span><em>file</em><span class="sig-paren">)</span><a class="headerlink" href="#marshal.load" title="Permalink to this definition">¶</a></dt>
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<dd><p>Read one value from the open file and return it. If no valid value is read
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(e.g. because the data has a different Python version’s incompatible marshal
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format), raise <a class="reference internal" href="exceptions.html#EOFError" title="EOFError"><code class="xref py py-exc docutils literal"><span class="pre">EOFError</span></code></a>, <a class="reference internal" href="exceptions.html#ValueError" title="ValueError"><code class="xref py py-exc docutils literal"><span class="pre">ValueError</span></code></a> or <a class="reference internal" href="exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal"><span class="pre">TypeError</span></code></a>. The
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file must be an open file object opened in binary mode (<code class="docutils literal"><span class="pre">'rb'</span></code> or
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<code class="docutils literal"><span class="pre">'r+b'</span></code>).</p>
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<div class="admonition note">
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<p class="first admonition-title">Note</p>
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<p class="last">If an object containing an unsupported type was marshalled with <a class="reference internal" href="#marshal.dump" title="marshal.dump"><code class="xref py py-func docutils literal"><span class="pre">dump()</span></code></a>,
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<a class="reference internal" href="#marshal.load" title="marshal.load"><code class="xref py py-func docutils literal"><span class="pre">load()</span></code></a> will substitute <code class="docutils literal"><span class="pre">None</span></code> for the unmarshallable type.</p>
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<dl class="function">
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<dt id="marshal.dumps">
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<code class="descclassname">marshal.</code><code class="descname">dumps</code><span class="sig-paren">(</span><em>value</em><span class="optional">[</span>, <em>version</em><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#marshal.dumps" title="Permalink to this definition">¶</a></dt>
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<dd><p>Return the string that would be written to a file by <code class="docutils literal"><span class="pre">dump(value,</span> <span class="pre">file)</span></code>. The
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value must be a supported type. Raise a <a class="reference internal" href="exceptions.html#ValueError" title="ValueError"><code class="xref py py-exc docutils literal"><span class="pre">ValueError</span></code></a> exception if value
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has (or contains an object that has) an unsupported type.</p>
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<p>The <em>version</em> argument indicates the data format that <code class="docutils literal"><span class="pre">dumps</span></code> should use
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<dl class="function">
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<dt id="marshal.loads">
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<code class="descclassname">marshal.</code><code class="descname">loads</code><span class="sig-paren">(</span><em>string</em><span class="sig-paren">)</span><a class="headerlink" href="#marshal.loads" title="Permalink to this definition">¶</a></dt>
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<dd><p>Convert the string to a value. If no valid value is found, raise
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<a class="reference internal" href="exceptions.html#EOFError" title="EOFError"><code class="xref py py-exc docutils literal"><span class="pre">EOFError</span></code></a>, <a class="reference internal" href="exceptions.html#ValueError" title="ValueError"><code class="xref py py-exc docutils literal"><span class="pre">ValueError</span></code></a> or <a class="reference internal" href="exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal"><span class="pre">TypeError</span></code></a>. Extra characters in the
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string are ignored.</p>
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<p>In addition, the following constants are defined:</p>
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<dt id="marshal.version">
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<code class="descclassname">marshal.</code><code class="descname">version</code><a class="headerlink" href="#marshal.version" title="Permalink to this definition">¶</a></dt>
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<dd><p>Indicates the format that the module uses. Version 0 is the historical
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format, version 1 shares interned strings and version 2 uses a binary format
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for floating point numbers.
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Version 3 adds support for object instancing and recursion.
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The current version is 4.</p>
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<p class="rubric">Footnotes</p>
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<table class="docutils footnote" frame="void" id="id2" rules="none">
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<colgroup><col class="label" /><col /></colgroup>
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<tr><td class="label"><a class="fn-backref" href="#id1">[1]</a></td><td>The name of this module stems from a bit of terminology used by the designers of
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Modula-3 (amongst others), who use the term “marshalling” for shipping of data
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around in a self-contained form. Strictly speaking, “to marshal” means to
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convert some data from internal to external form (in an RPC buffer for instance)
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and “unmarshalling” for the reverse process.</td></tr>
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