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<div class="section" id="module-pickle">
<span id="pickle-python-object-serialization"></span><h1><a class="reference internal" href="#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> — Python object serialization<a class="headerlink" href="#module-pickle" title="Permalink to this headline">¶</a></h1>
<p><strong>Source code:</strong> <a class="reference external" href="https://github.com/python/cpython/tree/3.7/Lib/pickle.py">Lib/pickle.py</a></p>
<hr class="docutils" id="index-0" />
<p>The <a class="reference internal" href="#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 implements binary protocols for serializing and
de-serializing a Python object structure. <em>“Pickling”</em> is the process
whereby a Python object hierarchy is converted into a byte stream, and
<em>“unpickling”</em> is the inverse operation, whereby a byte stream
(from a <a class="reference internal" href="../glossary.html#term-binary-file"><span class="xref std std-term">binary file</span></a> or <a class="reference internal" href="../glossary.html#term-bytes-like-object"><span class="xref std std-term">bytes-like object</span></a>) is converted
back into an object hierarchy. Pickling (and unpickling) is alternatively
known as “serialization”, “marshalling,” <a class="footnote-reference brackets" href="#id6" id="id1">1</a> or “flattening”; however, to
avoid confusion, the terms used here are “pickling” and “unpickling”.</p>
<div class="admonition warning">
<p class="admonition-title">Warning</p>
<p>The <a class="reference internal" href="#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 is not secure against erroneous or maliciously
constructed data. Never unpickle data received from an untrusted or
unauthenticated source.</p>
</div>
<div class="section" id="relationship-to-other-python-modules">
<h2>Relationship to other Python modules<a class="headerlink" href="#relationship-to-other-python-modules" title="Permalink to this headline">¶</a></h2>
<div class="section" id="comparison-with-marshal">
<h3>Comparison with <code class="docutils literal notranslate"><span class="pre">marshal</span></code><a class="headerlink" href="#comparison-with-marshal" title="Permalink to this headline">¶</a></h3>
<p>Python has a more primitive serialization module called <a class="reference internal" href="marshal.html#module-marshal" title="marshal: Convert Python objects to streams of bytes and back (with different constraints)."><code class="xref py py-mod docutils literal notranslate"><span class="pre">marshal</span></code></a>, but in
general <a class="reference internal" href="#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> should always be the preferred way to serialize Python
objects. <a class="reference internal" href="marshal.html#module-marshal" title="marshal: Convert Python objects to streams of bytes and back (with different constraints)."><code class="xref py py-mod docutils literal notranslate"><span class="pre">marshal</span></code></a> exists primarily to support Python’s <code class="file docutils literal notranslate"><span class="pre">.pyc</span></code>
files.</p>
<p>The <a class="reference internal" href="#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 differs from <a class="reference internal" href="marshal.html#module-marshal" title="marshal: Convert Python objects to streams of bytes and back (with different constraints)."><code class="xref py py-mod docutils literal notranslate"><span class="pre">marshal</span></code></a> in several significant ways:</p>
<ul>
<li><p>The <a class="reference internal" href="#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 keeps track of the objects it has already serialized,
so that later references to the same object won’t be serialized again.
<a class="reference internal" href="marshal.html#module-marshal" title="marshal: Convert Python objects to streams of bytes and back (with different constraints)."><code class="xref py py-mod docutils literal notranslate"><span class="pre">marshal</span></code></a> doesn’t do this.</p>
<p>This has implications both for recursive objects and object sharing. Recursive
objects are objects that contain references to themselves. These are not
handled by marshal, and in fact, attempting to marshal recursive objects will
crash your Python interpreter. Object sharing happens when there are multiple
references to the same object in different places in the object hierarchy being
serialized. <a class="reference internal" href="#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> stores such objects only once, and ensures that all
other references point to the master copy. Shared objects remain shared, which
can be very important for mutable objects.</p>
</li>
<li><p><a class="reference internal" href="marshal.html#module-marshal" title="marshal: Convert Python objects to streams of bytes and back (with different constraints)."><code class="xref py py-mod docutils literal notranslate"><span class="pre">marshal</span></code></a> cannot be used to serialize user-defined classes and their
instances. <a class="reference internal" href="#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> can save and restore class instances transparently,
however the class definition must be importable and live in the same module as
when the object was stored.</p></li>
<li><p>The <a class="reference internal" href="marshal.html#module-marshal" title="marshal: Convert Python objects to streams of bytes and back (with different constraints)."><code class="xref py py-mod docutils literal notranslate"><span class="pre">marshal</span></code></a> serialization format is not guaranteed to be portable
across Python versions. Because its primary job in life is to support
<code class="file docutils literal notranslate"><span class="pre">.pyc</span></code> files, the Python implementers reserve the right to change the
serialization format in non-backwards compatible ways should the need arise.
The <a class="reference internal" href="#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> serialization format is guaranteed to be backwards compatible
across Python releases provided a compatible pickle protocol is chosen and
pickling and unpickling code deals with Python 2 to Python 3 type differences
if your data is crossing that unique breaking change language boundary.</p></li>
</ul>
</div>
<div class="section" id="comparison-with-json">
<h3>Comparison with <code class="docutils literal notranslate"><span class="pre">json</span></code><a class="headerlink" href="#comparison-with-json" title="Permalink to this headline">¶</a></h3>
<p>There are fundamental differences between the pickle protocols and
<a class="reference external" href="http://json.org">JSON (JavaScript Object Notation)</a>:</p>
<ul class="simple">
<li><p>JSON is a text serialization format (it outputs unicode text, although
most of the time it is then encoded to <code class="docutils literal notranslate"><span class="pre">utf-8</span></code>), while pickle is
a binary serialization format;</p></li>
<li><p>JSON is human-readable, while pickle is not;</p></li>
<li><p>JSON is interoperable and widely used outside of the Python ecosystem,
while pickle is Python-specific;</p></li>
<li><p>JSON, by default, can only represent a subset of the Python built-in
types, and no custom classes; pickle can represent an extremely large
number of Python types (many of them automatically, by clever usage
of Python’s introspection facilities; complex cases can be tackled by
implementing <a class="reference internal" href="#pickle-inst"><span class="std std-ref">specific object APIs</span></a>).</p></li>
</ul>
<div class="admonition seealso">
<p class="admonition-title">See also</p>
<p>The <a class="reference internal" href="json.html#module-json" title="json: Encode and decode the JSON format."><code class="xref py py-mod docutils literal notranslate"><span class="pre">json</span></code></a> module: a standard library module allowing JSON
serialization and deserialization.</p>
</div>
</div>
</div>
<div class="section" id="data-stream-format">
<span id="pickle-protocols"></span><h2>Data stream format<a class="headerlink" href="#data-stream-format" title="Permalink to this headline">¶</a></h2>
<p id="index-1">The data format used by <a class="reference internal" href="#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> is Python-specific. This has the
advantage that there are no restrictions imposed by external standards such as
JSON or XDR (which can’t represent pointer sharing); however it means that
non-Python programs may not be able to reconstruct pickled Python objects.</p>
<p>By default, the <a class="reference internal" href="#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> data format uses a relatively compact binary
representation. If you need optimal size characteristics, you can efficiently
<a class="reference internal" href="archiving.html"><span class="doc">compress</span></a> pickled data.</p>
<p>The module <a class="reference internal" href="pickletools.html#module-pickletools" title="pickletools: Contains extensive comments about the pickle protocols and pickle-machine opcodes, as well as some useful functions."><code class="xref py py-mod docutils literal notranslate"><span class="pre">pickletools</span></code></a> contains tools for analyzing data streams
generated by <a class="reference internal" href="#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>. <a class="reference internal" href="pickletools.html#module-pickletools" title="pickletools: Contains extensive comments about the pickle protocols and pickle-machine opcodes, as well as some useful functions."><code class="xref py py-mod docutils literal notranslate"><span class="pre">pickletools</span></code></a> source code has extensive
comments about opcodes used by pickle protocols.</p>
<p>There are currently 5 different protocols which can be used for pickling.
The higher the protocol used, the more recent the version of Python needed
to read the pickle produced.</p>
<ul class="simple">
<li><p>Protocol version 0 is the original “human-readable” protocol and is
backwards compatible with earlier versions of Python.</p></li>
<li><p>Protocol version 1 is an old binary format which is also compatible with
earlier versions of Python.</p></li>
<li><p>Protocol version 2 was introduced in Python 2.3. It provides much more
efficient pickling of <a class="reference internal" href="../glossary.html#term-new-style-class"><span class="xref std std-term">new-style class</span></a>es. Refer to <span class="target" id="index-2"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0307"><strong>PEP 307</strong></a> for
information about improvements brought by protocol 2.</p></li>
<li><p>Protocol version 3 was added in Python 3.0. It has explicit support for
<a class="reference internal" href="stdtypes.html#bytes" title="bytes"><code class="xref py py-class docutils literal notranslate"><span class="pre">bytes</span></code></a> objects and cannot be unpickled by Python 2.x. This is
the default protocol, and the recommended protocol when compatibility with
other Python 3 versions is required.</p></li>
<li><p>Protocol version 4 was added in Python 3.4. It adds support for very large
objects, pickling more kinds of objects, and some data format
optimizations. Refer to <span class="target" id="index-3"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-3154"><strong>PEP 3154</strong></a> for information about improvements
brought by protocol 4.</p></li>
</ul>
<div class="admonition note">
<p class="admonition-title">Note</p>
<p>Serialization is a more primitive notion than persistence; although
<a class="reference internal" href="#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> reads and writes file objects, it does not handle the issue of
naming persistent objects, nor the (even more complicated) issue of concurrent
access to persistent objects. The <a class="reference internal" href="#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 transform a complex
object into a byte stream and it can transform the byte stream into an object
with the same internal structure. Perhaps the most obvious thing to do with
these byte streams is to write them onto a file, but it is also conceivable to
send them across a network or store them in a database. The <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>
module provides a simple interface to pickle and unpickle objects on
DBM-style database files.</p>
</div>
</div>
<div class="section" id="module-interface">
<h2>Module Interface<a class="headerlink" href="#module-interface" title="Permalink to this headline">¶</a></h2>
<p>To serialize an object hierarchy, you simply call the <a class="reference internal" href="#pickle.dumps" title="pickle.dumps"><code class="xref py py-func docutils literal notranslate"><span class="pre">dumps()</span></code></a> function.
Similarly, to de-serialize a data stream, you call the <a class="reference internal" href="#pickle.loads" title="pickle.loads"><code class="xref py py-func docutils literal notranslate"><span class="pre">loads()</span></code></a> function.
However, if you want more control over serialization and de-serialization,
you can create a <a class="reference internal" href="#pickle.Pickler" title="pickle.Pickler"><code class="xref py py-class docutils literal notranslate"><span class="pre">Pickler</span></code></a> or an <a class="reference internal" href="#pickle.Unpickler" title="pickle.Unpickler"><code class="xref py py-class docutils literal notranslate"><span class="pre">Unpickler</span></code></a> object, respectively.</p>
<p>The <a class="reference internal" href="#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 provides the following constants:</p>
<dl class="data">
<dt id="pickle.HIGHEST_PROTOCOL">
<code class="sig-prename descclassname">pickle.</code><code class="sig-name descname">HIGHEST_PROTOCOL</code><a class="headerlink" href="#pickle.HIGHEST_PROTOCOL" title="Permalink to this definition">¶</a></dt>
<dd><p>An integer, the highest <a class="reference internal" href="#pickle-protocols"><span class="std std-ref">protocol version</span></a>
available. This value can be passed as a <em>protocol</em> value to functions
<a class="reference internal" href="#pickle.dump" title="pickle.dump"><code class="xref py py-func docutils literal notranslate"><span class="pre">dump()</span></code></a> and <a class="reference internal" href="#pickle.dumps" title="pickle.dumps"><code class="xref py py-func docutils literal notranslate"><span class="pre">dumps()</span></code></a> as well as the <a class="reference internal" href="#pickle.Pickler" title="pickle.Pickler"><code class="xref py py-class docutils literal notranslate"><span class="pre">Pickler</span></code></a>
constructor.</p>
</dd></dl>
<dl class="data">
<dt id="pickle.DEFAULT_PROTOCOL">
<code class="sig-prename descclassname">pickle.</code><code class="sig-name descname">DEFAULT_PROTOCOL</code><a class="headerlink" href="#pickle.DEFAULT_PROTOCOL" title="Permalink to this definition">¶</a></dt>
<dd><p>An integer, the default <a class="reference internal" href="#pickle-protocols"><span class="std std-ref">protocol version</span></a> used
for pickling. May be less than <a class="reference internal" href="#pickle.HIGHEST_PROTOCOL" title="pickle.HIGHEST_PROTOCOL"><code class="xref py py-data docutils literal notranslate"><span class="pre">HIGHEST_PROTOCOL</span></code></a>. Currently the
default protocol is 3, a new protocol designed for Python 3.</p>
</dd></dl>
<p>The <a class="reference internal" href="#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 provides the following functions to make the pickling
process more convenient:</p>
<dl class="function">
<dt id="pickle.dump">
<code class="sig-prename descclassname">pickle.</code><code class="sig-name descname">dump</code><span class="sig-paren">(</span><em class="sig-param">obj</em>, <em class="sig-param">file</em>, <em class="sig-param">protocol=None</em>, <em class="sig-param">*</em>, <em class="sig-param">fix_imports=True</em><span class="sig-paren">)</span><a class="headerlink" href="#pickle.dump" title="Permalink to this definition">¶</a></dt>
<dd><p>Write the pickled representation of the object <em>obj</em> to the open
<a class="reference internal" href="../glossary.html#term-file-object"><span class="xref std std-term">file object</span></a> <em>file</em>. This is equivalent to
<code class="docutils literal notranslate"><span class="pre">Pickler(file,</span> <span class="pre">protocol).dump(obj)</span></code>.</p>
<p>The optional <em>protocol</em> argument, an integer, tells the pickler to use
the given protocol; supported protocols are 0 to <a class="reference internal" href="#pickle.HIGHEST_PROTOCOL" title="pickle.HIGHEST_PROTOCOL"><code class="xref py py-data docutils literal notranslate"><span class="pre">HIGHEST_PROTOCOL</span></code></a>.
If not specified, the default is <a class="reference internal" href="#pickle.DEFAULT_PROTOCOL" title="pickle.DEFAULT_PROTOCOL"><code class="xref py py-data docutils literal notranslate"><span class="pre">DEFAULT_PROTOCOL</span></code></a>. If a negative
number is specified, <a class="reference internal" href="#pickle.HIGHEST_PROTOCOL" title="pickle.HIGHEST_PROTOCOL"><code class="xref py py-data docutils literal notranslate"><span class="pre">HIGHEST_PROTOCOL</span></code></a> is selected.</p>
<p>The <em>file</em> argument must have a write() method that accepts a single bytes
argument. It can thus be an on-disk file opened for binary writing, an
<a class="reference internal" href="io.html#io.BytesIO" title="io.BytesIO"><code class="xref py py-class docutils literal notranslate"><span class="pre">io.BytesIO</span></code></a> instance, or any other custom object that meets this
interface.</p>
<p>If <em>fix_imports</em> is true and <em>protocol</em> is less than 3, pickle will try to
map the new Python 3 names to the old module names used in Python 2, so
that the pickle data stream is readable with Python 2.</p>
</dd></dl>
<dl class="function">
<dt id="pickle.dumps">
<code class="sig-prename descclassname">pickle.</code><code class="sig-name descname">dumps</code><span class="sig-paren">(</span><em class="sig-param">obj</em>, <em class="sig-param">protocol=None</em>, <em class="sig-param">*</em>, <em class="sig-param">fix_imports=True</em><span class="sig-paren">)</span><a class="headerlink" href="#pickle.dumps" title="Permalink to this definition">¶</a></dt>
<dd><p>Return the pickled representation of the object <em>obj</em> as a <a class="reference internal" href="stdtypes.html#bytes" title="bytes"><code class="xref py py-class docutils literal notranslate"><span class="pre">bytes</span></code></a> object,
instead of writing it to a file.</p>
<p>Arguments <em>protocol</em> and <em>fix_imports</em> have the same meaning as in
<a class="reference internal" href="#pickle.dump" title="pickle.dump"><code class="xref py py-func docutils literal notranslate"><span class="pre">dump()</span></code></a>.</p>
</dd></dl>
<dl class="function">
<dt id="pickle.load">
<code class="sig-prename descclassname">pickle.</code><code class="sig-name descname">load</code><span class="sig-paren">(</span><em class="sig-param">file</em>, <em class="sig-param">*</em>, <em class="sig-param">fix_imports=True</em>, <em class="sig-param">encoding="ASCII"</em>, <em class="sig-param">errors="strict"</em><span class="sig-paren">)</span><a class="headerlink" href="#pickle.load" title="Permalink to this definition">¶</a></dt>
<dd><p>Read the pickled representation of an object from the open <a class="reference internal" href="../glossary.html#term-file-object"><span class="xref std std-term">file object</span></a>
<em>file</em> and return the reconstituted object hierarchy specified therein.
This is equivalent to <code class="docutils literal notranslate"><span class="pre">Unpickler(file).load()</span></code>.</p>
<p>The protocol version of the pickle is detected automatically, so no
protocol argument is needed. Bytes past the pickled representation
of the object are ignored.</p>
<p>The argument <em>file</em> must have two methods, a read() method that takes an
integer argument, and a readline() method that requires no arguments. Both
methods should return bytes. Thus <em>file</em> can be an on-disk file opened for
binary reading, an <a class="reference internal" href="io.html#io.BytesIO" title="io.BytesIO"><code class="xref py py-class docutils literal notranslate"><span class="pre">io.BytesIO</span></code></a> object, or any other custom object
that meets this interface.</p>
<p>Optional keyword arguments are <em>fix_imports</em>, <em>encoding</em> and <em>errors</em>,
which are used to control compatibility support for pickle stream generated
by Python 2. If <em>fix_imports</em> is true, pickle will try to map the old
Python 2 names to the new names used in Python 3. The <em>encoding</em> and
<em>errors</em> tell pickle how to decode 8-bit string instances pickled by Python
2; these default to ‘ASCII’ and ‘strict’, respectively. The <em>encoding</em> can
be ‘bytes’ to read these 8-bit string instances as bytes objects.
Using <code class="docutils literal notranslate"><span class="pre">encoding='latin1'</span></code> is required for unpickling NumPy arrays and
instances of <a class="reference internal" href="datetime.html#datetime.datetime" title="datetime.datetime"><code class="xref py py-class docutils literal notranslate"><span class="pre">datetime</span></code></a>, <a class="reference internal" href="datetime.html#datetime.date" title="datetime.date"><code class="xref py py-class docutils literal notranslate"><span class="pre">date</span></code></a> and
<a class="reference internal" href="datetime.html#datetime.time" title="datetime.time"><code class="xref py py-class docutils literal notranslate"><span class="pre">time</span></code></a> pickled by Python 2.</p>
</dd></dl>
<dl class="function">
<dt id="pickle.loads">
<code class="sig-prename descclassname">pickle.</code><code class="sig-name descname">loads</code><span class="sig-paren">(</span><em class="sig-param">data</em>, <em class="sig-param">*</em>, <em class="sig-param">fix_imports=True</em>, <em class="sig-param">encoding="ASCII"</em>, <em class="sig-param">errors="strict"</em><span class="sig-paren">)</span><a class="headerlink" href="#pickle.loads" title="Permalink to this definition">¶</a></dt>
<dd><p>Return the reconstituted object hierarchy of the pickled representation
<em>data</em> of an object. <em>data</em> must be a <a class="reference internal" href="../glossary.html#term-bytes-like-object"><span class="xref std std-term">bytes-like object</span></a>.</p>
<p>The protocol version of the pickle is detected automatically, so no
protocol argument is needed. Bytes past the pickled representation
of the object are ignored.</p>
<p>Optional keyword arguments are <em>fix_imports</em>, <em>encoding</em> and <em>errors</em>,
which are used to control compatibility support for pickle stream generated
by Python 2. If <em>fix_imports</em> is true, pickle will try to map the old
Python 2 names to the new names used in Python 3. The <em>encoding</em> and
<em>errors</em> tell pickle how to decode 8-bit string instances pickled by Python
2; these default to ‘ASCII’ and ‘strict’, respectively. The <em>encoding</em> can
be ‘bytes’ to read these 8-bit string instances as bytes objects.
Using <code class="docutils literal notranslate"><span class="pre">encoding='latin1'</span></code> is required for unpickling NumPy arrays and
instances of <a class="reference internal" href="datetime.html#datetime.datetime" title="datetime.datetime"><code class="xref py py-class docutils literal notranslate"><span class="pre">datetime</span></code></a>, <a class="reference internal" href="datetime.html#datetime.date" title="datetime.date"><code class="xref py py-class docutils literal notranslate"><span class="pre">date</span></code></a> and
<a class="reference internal" href="datetime.html#datetime.time" title="datetime.time"><code class="xref py py-class docutils literal notranslate"><span class="pre">time</span></code></a> pickled by Python 2.</p>
</dd></dl>
<p>The <a class="reference internal" href="#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 defines three exceptions:</p>
<dl class="exception">
<dt id="pickle.PickleError">
<em class="property">exception </em><code class="sig-prename descclassname">pickle.</code><code class="sig-name descname">PickleError</code><a class="headerlink" href="#pickle.PickleError" title="Permalink to this definition">¶</a></dt>
<dd><p>Common base class for the other pickling exceptions. It inherits
<a class="reference internal" href="exceptions.html#Exception" title="Exception"><code class="xref py py-exc docutils literal notranslate"><span class="pre">Exception</span></code></a>.</p>
</dd></dl>
<dl class="exception">
<dt id="pickle.PicklingError">
<em class="property">exception </em><code class="sig-prename descclassname">pickle.</code><code class="sig-name descname">PicklingError</code><a class="headerlink" href="#pickle.PicklingError" title="Permalink to this definition">¶</a></dt>
<dd><p>Error raised when an unpicklable object is encountered by <a class="reference internal" href="#pickle.Pickler" title="pickle.Pickler"><code class="xref py py-class docutils literal notranslate"><span class="pre">Pickler</span></code></a>.
It inherits <a class="reference internal" href="#pickle.PickleError" title="pickle.PickleError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">PickleError</span></code></a>.</p>
<p>Refer to <a class="reference internal" href="#pickle-picklable"><span class="std std-ref">What can be pickled and unpickled?</span></a> to learn what kinds of objects can be
pickled.</p>
</dd></dl>
<dl class="exception">
<dt id="pickle.UnpicklingError">
<em class="property">exception </em><code class="sig-prename descclassname">pickle.</code><code class="sig-name descname">UnpicklingError</code><a class="headerlink" href="#pickle.UnpicklingError" title="Permalink to this definition">¶</a></dt>
<dd><p>Error raised when there is a problem unpickling an object, such as a data
corruption or a security violation. It inherits <a class="reference internal" href="#pickle.PickleError" title="pickle.PickleError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">PickleError</span></code></a>.</p>
<p>Note that other exceptions may also be raised during unpickling, including
(but not necessarily limited to) AttributeError, EOFError, ImportError, and
IndexError.</p>
</dd></dl>
<p>The <a class="reference internal" href="#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 exports two classes, <a class="reference internal" href="#pickle.Pickler" title="pickle.Pickler"><code class="xref py py-class docutils literal notranslate"><span class="pre">Pickler</span></code></a> and
<a class="reference internal" href="#pickle.Unpickler" title="pickle.Unpickler"><code class="xref py py-class docutils literal notranslate"><span class="pre">Unpickler</span></code></a>:</p>
<dl class="class">
<dt id="pickle.Pickler">
<em class="property">class </em><code class="sig-prename descclassname">pickle.</code><code class="sig-name descname">Pickler</code><span class="sig-paren">(</span><em class="sig-param">file</em>, <em class="sig-param">protocol=None</em>, <em class="sig-param">*</em>, <em class="sig-param">fix_imports=True</em><span class="sig-paren">)</span><a class="headerlink" href="#pickle.Pickler" title="Permalink to this definition">¶</a></dt>
<dd><p>This takes a binary file for writing a pickle data stream.</p>
<p>The optional <em>protocol</em> argument, an integer, tells the pickler to use
the given protocol; supported protocols are 0 to <a class="reference internal" href="#pickle.HIGHEST_PROTOCOL" title="pickle.HIGHEST_PROTOCOL"><code class="xref py py-data docutils literal notranslate"><span class="pre">HIGHEST_PROTOCOL</span></code></a>.
If not specified, the default is <a class="reference internal" href="#pickle.DEFAULT_PROTOCOL" title="pickle.DEFAULT_PROTOCOL"><code class="xref py py-data docutils literal notranslate"><span class="pre">DEFAULT_PROTOCOL</span></code></a>. If a negative
number is specified, <a class="reference internal" href="#pickle.HIGHEST_PROTOCOL" title="pickle.HIGHEST_PROTOCOL"><code class="xref py py-data docutils literal notranslate"><span class="pre">HIGHEST_PROTOCOL</span></code></a> is selected.</p>
<p>The <em>file</em> argument must have a write() method that accepts a single bytes
argument. It can thus be an on-disk file opened for binary writing, an
<a class="reference internal" href="io.html#io.BytesIO" title="io.BytesIO"><code class="xref py py-class docutils literal notranslate"><span class="pre">io.BytesIO</span></code></a> instance, or any other custom object that meets this
interface.</p>
<p>If <em>fix_imports</em> is true and <em>protocol</em> is less than 3, pickle will try to
map the new Python 3 names to the old module names used in Python 2, so
that the pickle data stream is readable with Python 2.</p>
<dl class="method">
<dt id="pickle.Pickler.dump">
<code class="sig-name descname">dump</code><span class="sig-paren">(</span><em class="sig-param">obj</em><span class="sig-paren">)</span><a class="headerlink" href="#pickle.Pickler.dump" title="Permalink to this definition">¶</a></dt>
<dd><p>Write the pickled representation of <em>obj</em> to the open file object given in
the constructor.</p>
</dd></dl>
<dl class="method">
<dt id="pickle.Pickler.persistent_id">
<code class="sig-name descname">persistent_id</code><span class="sig-paren">(</span><em class="sig-param">obj</em><span class="sig-paren">)</span><a class="headerlink" href="#pickle.Pickler.persistent_id" title="Permalink to this definition">¶</a></dt>
<dd><p>Do nothing by default. This exists so a subclass can override it.</p>
<p>If <a class="reference internal" href="#pickle.Pickler.persistent_id" title="pickle.Pickler.persistent_id"><code class="xref py py-meth docutils literal notranslate"><span class="pre">persistent_id()</span></code></a> returns <code class="docutils literal notranslate"><span class="pre">None</span></code>, <em>obj</em> is pickled as usual. Any
other value causes <a class="reference internal" href="#pickle.Pickler" title="pickle.Pickler"><code class="xref py py-class docutils literal notranslate"><span class="pre">Pickler</span></code></a> to emit the returned value as a
persistent ID for <em>obj</em>. The meaning of this persistent ID should be
defined by <a class="reference internal" href="#pickle.Unpickler.persistent_load" title="pickle.Unpickler.persistent_load"><code class="xref py py-meth docutils literal notranslate"><span class="pre">Unpickler.persistent_load()</span></code></a>. Note that the value
returned by <a class="reference internal" href="#pickle.Pickler.persistent_id" title="pickle.Pickler.persistent_id"><code class="xref py py-meth docutils literal notranslate"><span class="pre">persistent_id()</span></code></a> cannot itself have a persistent ID.</p>
<p>See <a class="reference internal" href="#pickle-persistent"><span class="std std-ref">Persistence of External Objects</span></a> for details and examples of uses.</p>
</dd></dl>
<dl class="attribute">
<dt id="pickle.Pickler.dispatch_table">
<code class="sig-name descname">dispatch_table</code><a class="headerlink" href="#pickle.Pickler.dispatch_table" title="Permalink to this definition">¶</a></dt>
<dd><p>A pickler object’s dispatch table is a registry of <em>reduction
functions</em> of the kind which can be declared using
<a class="reference internal" href="copyreg.html#copyreg.pickle" title="copyreg.pickle"><code class="xref py py-func docutils literal notranslate"><span class="pre">copyreg.pickle()</span></code></a>. It is a mapping whose keys are classes
and whose values are reduction functions. A reduction function
takes a single argument of the associated class and should
conform to the same interface as a <a class="reference internal" href="#object.__reduce__" title="object.__reduce__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__reduce__()</span></code></a>
method.</p>
<p>By default, a pickler object will not have a
<a class="reference internal" href="#pickle.Pickler.dispatch_table" title="pickle.Pickler.dispatch_table"><code class="xref py py-attr docutils literal notranslate"><span class="pre">dispatch_table</span></code></a> attribute, and it will instead use the
global dispatch table managed by the <a class="reference internal" href="copyreg.html#module-copyreg" title="copyreg: Register pickle support functions."><code class="xref py py-mod docutils literal notranslate"><span class="pre">copyreg</span></code></a> module.
However, to customize the pickling for a specific pickler object
one can set the <a class="reference internal" href="#pickle.Pickler.dispatch_table" title="pickle.Pickler.dispatch_table"><code class="xref py py-attr docutils literal notranslate"><span class="pre">dispatch_table</span></code></a> attribute to a dict-like
object. Alternatively, if a subclass of <a class="reference internal" href="#pickle.Pickler" title="pickle.Pickler"><code class="xref py py-class docutils literal notranslate"><span class="pre">Pickler</span></code></a> has a
<a class="reference internal" href="#pickle.Pickler.dispatch_table" title="pickle.Pickler.dispatch_table"><code class="xref py py-attr docutils literal notranslate"><span class="pre">dispatch_table</span></code></a> attribute then this will be used as the
default dispatch table for instances of that class.</p>
<p>See <a class="reference internal" href="#pickle-dispatch"><span class="std std-ref">Dispatch Tables</span></a> for usage examples.</p>
<div class="versionadded">
<p><span class="versionmodified added">New in version 3.3.</span></p>
</div>
</dd></dl>
<dl class="attribute">
<dt id="pickle.Pickler.fast">
<code class="sig-name descname">fast</code><a class="headerlink" href="#pickle.Pickler.fast" title="Permalink to this definition">¶</a></dt>
<dd><p>Deprecated. Enable fast mode if set to a true value. The fast mode
disables the usage of memo, therefore speeding the pickling process by not
generating superfluous PUT opcodes. It should not be used with
self-referential objects, doing otherwise will cause <a class="reference internal" href="#pickle.Pickler" title="pickle.Pickler"><code class="xref py py-class docutils literal notranslate"><span class="pre">Pickler</span></code></a> to
recurse infinitely.</p>
<p>Use <a class="reference internal" href="pickletools.html#pickletools.optimize" title="pickletools.optimize"><code class="xref py py-func docutils literal notranslate"><span class="pre">pickletools.optimize()</span></code></a> if you need more compact pickles.</p>
</dd></dl>
</dd></dl>
<dl class="class">
<dt id="pickle.Unpickler">
<em class="property">class </em><code class="sig-prename descclassname">pickle.</code><code class="sig-name descname">Unpickler</code><span class="sig-paren">(</span><em class="sig-param">file</em>, <em class="sig-param">*</em>, <em class="sig-param">fix_imports=True</em>, <em class="sig-param">encoding="ASCII"</em>, <em class="sig-param">errors="strict"</em><span class="sig-paren">)</span><a class="headerlink" href="#pickle.Unpickler" title="Permalink to this definition">¶</a></dt>
<dd><p>This takes a binary file for reading a pickle data stream.</p>
<p>The protocol version of the pickle is detected automatically, so no
protocol argument is needed.</p>
<p>The argument <em>file</em> must have two methods, a read() method that takes an
integer argument, and a readline() method that requires no arguments. Both
methods should return bytes. Thus <em>file</em> can be an on-disk file object
opened for binary reading, an <a class="reference internal" href="io.html#io.BytesIO" title="io.BytesIO"><code class="xref py py-class docutils literal notranslate"><span class="pre">io.BytesIO</span></code></a> object, or any other
custom object that meets this interface.</p>
<p>Optional keyword arguments are <em>fix_imports</em>, <em>encoding</em> and <em>errors</em>,
which are used to control compatibility support for pickle stream generated
by Python 2. If <em>fix_imports</em> is true, pickle will try to map the old
Python 2 names to the new names used in Python 3. The <em>encoding</em> and
<em>errors</em> tell pickle how to decode 8-bit string instances pickled by Python
2; these default to ‘ASCII’ and ‘strict’, respectively. The <em>encoding</em> can
be ‘bytes’ to read these 8-bit string instances as bytes objects.</p>
<dl class="method">
<dt id="pickle.Unpickler.load">
<code class="sig-name descname">load</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#pickle.Unpickler.load" title="Permalink to this definition">¶</a></dt>
<dd><p>Read the pickled representation of an object from the open file object
given in the constructor, and return the reconstituted object hierarchy
specified therein. Bytes past the pickled representation of the object
are ignored.</p>
</dd></dl>
<dl class="method">
<dt id="pickle.Unpickler.persistent_load">
<code class="sig-name descname">persistent_load</code><span class="sig-paren">(</span><em class="sig-param">pid</em><span class="sig-paren">)</span><a class="headerlink" href="#pickle.Unpickler.persistent_load" title="Permalink to this definition">¶</a></dt>
<dd><p>Raise an <a class="reference internal" href="#pickle.UnpicklingError" title="pickle.UnpicklingError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">UnpicklingError</span></code></a> by default.</p>
<p>If defined, <a class="reference internal" href="#pickle.Unpickler.persistent_load" title="pickle.Unpickler.persistent_load"><code class="xref py py-meth docutils literal notranslate"><span class="pre">persistent_load()</span></code></a> should return the object specified by
the persistent ID <em>pid</em>. If an invalid persistent ID is encountered, an
<a class="reference internal" href="#pickle.UnpicklingError" title="pickle.UnpicklingError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">UnpicklingError</span></code></a> should be raised.</p>
<p>See <a class="reference internal" href="#pickle-persistent"><span class="std std-ref">Persistence of External Objects</span></a> for details and examples of uses.</p>
</dd></dl>
<dl class="method">
<dt id="pickle.Unpickler.find_class">
<code class="sig-name descname">find_class</code><span class="sig-paren">(</span><em class="sig-param">module</em>, <em class="sig-param">name</em><span class="sig-paren">)</span><a class="headerlink" href="#pickle.Unpickler.find_class" title="Permalink to this definition">¶</a></dt>
<dd><p>Import <em>module</em> if necessary and return the object called <em>name</em> from it,
where the <em>module</em> and <em>name</em> arguments are <a class="reference internal" href="stdtypes.html#str" title="str"><code class="xref py py-class docutils literal notranslate"><span class="pre">str</span></code></a> objects. Note,
unlike its name suggests, <a class="reference internal" href="#pickle.Unpickler.find_class" title="pickle.Unpickler.find_class"><code class="xref py py-meth docutils literal notranslate"><span class="pre">find_class()</span></code></a> is also used for finding
functions.</p>
<p>Subclasses may override this to gain control over what type of objects and
how they can be loaded, potentially reducing security risks. Refer to
<a class="reference internal" href="#pickle-restrict"><span class="std std-ref">Restricting Globals</span></a> for details.</p>
</dd></dl>
</dd></dl>
</div>
<div class="section" id="what-can-be-pickled-and-unpickled">
<span id="pickle-picklable"></span><h2>What can be pickled and unpickled?<a class="headerlink" href="#what-can-be-pickled-and-unpickled" title="Permalink to this headline">¶</a></h2>
<p>The following types can be pickled:</p>
<ul class="simple">
<li><p><code class="docutils literal notranslate"><span class="pre">None</span></code>, <code class="docutils literal notranslate"><span class="pre">True</span></code>, and <code class="docutils literal notranslate"><span class="pre">False</span></code></p></li>
<li><p>integers, floating point numbers, complex numbers</p></li>
<li><p>strings, bytes, bytearrays</p></li>
<li><p>tuples, lists, sets, and dictionaries containing only picklable objects</p></li>
<li><p>functions defined at the top level of a module (using <a class="reference internal" href="../reference/compound_stmts.html#def"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">def</span></code></a>, not
<a class="reference internal" href="../reference/expressions.html#lambda"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">lambda</span></code></a>)</p></li>
<li><p>built-in functions defined at the top level of a module</p></li>
<li><p>classes that are defined at the top level of a module</p></li>
<li><p>instances of such classes whose <a class="reference internal" href="stdtypes.html#object.__dict__" title="object.__dict__"><code class="xref py py-attr docutils literal notranslate"><span class="pre">__dict__</span></code></a> or the result of
calling <a class="reference internal" href="#object.__getstate__" title="object.__getstate__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__getstate__()</span></code></a> is picklable (see section <a class="reference internal" href="#pickle-inst"><span class="std std-ref">Pickling Class Instances</span></a> for
details).</p></li>
</ul>
<p>Attempts to pickle unpicklable objects will raise the <a class="reference internal" href="#pickle.PicklingError" title="pickle.PicklingError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">PicklingError</span></code></a>
exception; when this happens, an unspecified number of bytes may have already
been written to the underlying file. Trying to pickle a highly recursive data
structure may exceed the maximum recursion depth, a <a class="reference internal" href="exceptions.html#RecursionError" title="RecursionError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">RecursionError</span></code></a> will be
raised in this case. You can carefully raise this limit with
<a class="reference internal" href="sys.html#sys.setrecursionlimit" title="sys.setrecursionlimit"><code class="xref py py-func docutils literal notranslate"><span class="pre">sys.setrecursionlimit()</span></code></a>.</p>
<p>Note that functions (built-in and user-defined) are pickled by “fully qualified”
name reference, not by value. <a class="footnote-reference brackets" href="#id7" id="id2">2</a> This means that only the function name is
pickled, along with the name of the module the function is defined in. Neither
the function’s code, nor any of its function attributes are pickled. Thus the
defining module must be importable in the unpickling environment, and the module
must contain the named object, otherwise an exception will be raised. <a class="footnote-reference brackets" href="#id8" id="id3">3</a></p>
<p>Similarly, classes are pickled by named reference, so the same restrictions in
the unpickling environment apply. Note that none of the class’s code or data is
pickled, so in the following example the class attribute <code class="docutils literal notranslate"><span class="pre">attr</span></code> is not
restored in the unpickling environment:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">Foo</span><span class="p">:</span>
<span class="n">attr</span> <span class="o">=</span> <span class="s1">'A class attribute'</span>
<span class="n">picklestring</span> <span class="o">=</span> <span class="n">pickle</span><span class="o">.</span><span class="n">dumps</span><span class="p">(</span><span class="n">Foo</span><span class="p">)</span>
</pre></div>
</div>
<p>These restrictions are why picklable functions and classes must be defined in
the top level of a module.</p>
<p>Similarly, when class instances are pickled, their class’s code and data are not
pickled along with them. Only the instance data are pickled. This is done on
purpose, so you can fix bugs in a class or add methods to the class and still
load objects that were created with an earlier version of the class. If you
plan to have long-lived objects that will see many versions of a class, it may
be worthwhile to put a version number in the objects so that suitable
conversions can be made by the class’s <a class="reference internal" href="#object.__setstate__" title="object.__setstate__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__setstate__()</span></code></a> method.</p>
</div>
<div class="section" id="pickling-class-instances">
<span id="pickle-inst"></span><h2>Pickling Class Instances<a class="headerlink" href="#pickling-class-instances" title="Permalink to this headline">¶</a></h2>
<p>In this section, we describe the general mechanisms available to you to define,
customize, and control how class instances are pickled and unpickled.</p>
<p>In most cases, no additional code is needed to make instances picklable. By
default, pickle will retrieve the class and the attributes of an instance via
introspection. When a class instance is unpickled, its <a class="reference internal" href="../reference/datamodel.html#object.__init__" title="object.__init__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__init__()</span></code></a> method
is usually <em>not</em> invoked. The default behaviour first creates an uninitialized
instance and then restores the saved attributes. The following code shows an
implementation of this behaviour:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">save</span><span class="p">(</span><span class="n">obj</span><span class="p">):</span>
<span class="k">return</span> <span class="p">(</span><span class="n">obj</span><span class="o">.</span><span class="vm">__class__</span><span class="p">,</span> <span class="n">obj</span><span class="o">.</span><span class="vm">__dict__</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">load</span><span class="p">(</span><span class="bp">cls</span><span class="p">,</span> <span class="n">attributes</span><span class="p">):</span>
<span class="n">obj</span> <span class="o">=</span> <span class="bp">cls</span><span class="o">.</span><span class="fm">__new__</span><span class="p">(</span><span class="bp">cls</span><span class="p">)</span>
<span class="n">obj</span><span class="o">.</span><span class="vm">__dict__</span><span class="o">.</span><span class="n">update</span><span class="p">(</span><span class="n">attributes</span><span class="p">)</span>
<span class="k">return</span> <span class="n">obj</span>
</pre></div>
</div>
<p>Classes can alter the default behaviour by providing one or several special
methods:</p>
<dl class="method">
<dt id="object.__getnewargs_ex__">
<code class="sig-prename descclassname">object.</code><code class="sig-name descname">__getnewargs_ex__</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#object.__getnewargs_ex__" title="Permalink to this definition">¶</a></dt>
<dd><p>In protocols 2 and newer, classes that implements the
<a class="reference internal" href="#object.__getnewargs_ex__" title="object.__getnewargs_ex__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__getnewargs_ex__()</span></code></a> method can dictate the values passed to the
<a class="reference internal" href="../reference/datamodel.html#object.__new__" title="object.__new__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__new__()</span></code></a> method upon unpickling. The method must return a pair
<code class="docutils literal notranslate"><span class="pre">(args,</span> <span class="pre">kwargs)</span></code> where <em>args</em> is a tuple of positional arguments
and <em>kwargs</em> a dictionary of named arguments for constructing the
object. Those will be passed to the <a class="reference internal" href="../reference/datamodel.html#object.__new__" title="object.__new__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__new__()</span></code></a> method upon
unpickling.</p>
<p>You should implement this method if the <a class="reference internal" href="../reference/datamodel.html#object.__new__" title="object.__new__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__new__()</span></code></a> method of your
class requires keyword-only arguments. Otherwise, it is recommended for
compatibility to implement <a class="reference internal" href="#object.__getnewargs__" title="object.__getnewargs__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__getnewargs__()</span></code></a>.</p>
<div class="versionchanged">
<p><span class="versionmodified changed">Changed in version 3.6: </span><a class="reference internal" href="#object.__getnewargs_ex__" title="object.__getnewargs_ex__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__getnewargs_ex__()</span></code></a> is now used in protocols 2 and 3.</p>
</div>
</dd></dl>
<dl class="method">
<dt id="object.__getnewargs__">
<code class="sig-prename descclassname">object.</code><code class="sig-name descname">__getnewargs__</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#object.__getnewargs__" title="Permalink to this definition">¶</a></dt>
<dd><p>This method serves a similar purpose as <a class="reference internal" href="#object.__getnewargs_ex__" title="object.__getnewargs_ex__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__getnewargs_ex__()</span></code></a>, but
supports only positional arguments. It must return a tuple of arguments
<code class="docutils literal notranslate"><span class="pre">args</span></code> which will be passed to the <a class="reference internal" href="../reference/datamodel.html#object.__new__" title="object.__new__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__new__()</span></code></a> method upon unpickling.</p>
<p><a class="reference internal" href="#object.__getnewargs__" title="object.__getnewargs__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__getnewargs__()</span></code></a> will not be called if <a class="reference internal" href="#object.__getnewargs_ex__" title="object.__getnewargs_ex__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__getnewargs_ex__()</span></code></a> is
defined.</p>
<div class="versionchanged">
<p><span class="versionmodified changed">Changed in version 3.6: </span>Before Python 3.6, <a class="reference internal" href="#object.__getnewargs__" title="object.__getnewargs__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__getnewargs__()</span></code></a> was called instead of
<a class="reference internal" href="#object.__getnewargs_ex__" title="object.__getnewargs_ex__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__getnewargs_ex__()</span></code></a> in protocols 2 and 3.</p>
</div>
</dd></dl>
<dl class="method">
<dt id="object.__getstate__">
<code class="sig-prename descclassname">object.</code><code class="sig-name descname">__getstate__</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#object.__getstate__" title="Permalink to this definition">¶</a></dt>
<dd><p>Classes can further influence how their instances are pickled; if the class
defines the method <a class="reference internal" href="#object.__getstate__" title="object.__getstate__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__getstate__()</span></code></a>, it is called and the returned object
is pickled as the contents for the instance, instead of the contents of the
instance’s dictionary. If the <a class="reference internal" href="#object.__getstate__" title="object.__getstate__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__getstate__()</span></code></a> method is absent, the
instance’s <a class="reference internal" href="stdtypes.html#object.__dict__" title="object.__dict__"><code class="xref py py-attr docutils literal notranslate"><span class="pre">__dict__</span></code></a> is pickled as usual.</p>
</dd></dl>
<dl class="method">
<dt id="object.__setstate__">
<code class="sig-prename descclassname">object.</code><code class="sig-name descname">__setstate__</code><span class="sig-paren">(</span><em class="sig-param">state</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__setstate__" title="Permalink to this definition">¶</a></dt>
<dd><p>Upon unpickling, if the class defines <a class="reference internal" href="#object.__setstate__" title="object.__setstate__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__setstate__()</span></code></a>, it is called with
the unpickled state. In that case, there is no requirement for the state
object to be a dictionary. Otherwise, the pickled state must be a dictionary
and its items are assigned to the new instance’s dictionary.</p>
<div class="admonition note">
<p class="admonition-title">Note</p>
<p>If <a class="reference internal" href="#object.__getstate__" title="object.__getstate__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__getstate__()</span></code></a> returns a false value, the <a class="reference internal" href="#object.__setstate__" title="object.__setstate__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__setstate__()</span></code></a>
method will not be called upon unpickling.</p>
</div>
</dd></dl>
<p>Refer to the section <a class="reference internal" href="#pickle-state"><span class="std std-ref">Handling Stateful Objects</span></a> for more information about how to use
the methods <a class="reference internal" href="#object.__getstate__" title="object.__getstate__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__getstate__()</span></code></a> and <a class="reference internal" href="#object.__setstate__" title="object.__setstate__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__setstate__()</span></code></a>.</p>
<div class="admonition note">
<p class="admonition-title">Note</p>
<p>At unpickling time, some methods like <a class="reference internal" href="../reference/datamodel.html#object.__getattr__" title="object.__getattr__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__getattr__()</span></code></a>,
<a class="reference internal" href="../reference/datamodel.html#object.__getattribute__" title="object.__getattribute__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__getattribute__()</span></code></a>, or <a class="reference internal" href="../reference/datamodel.html#object.__setattr__" title="object.__setattr__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__setattr__()</span></code></a> may be called upon the
instance. In case those methods rely on some internal invariant being
true, the type should implement <a class="reference internal" href="../reference/datamodel.html#object.__new__" title="object.__new__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__new__()</span></code></a> to establish such an
invariant, as <a class="reference internal" href="../reference/datamodel.html#object.__init__" title="object.__init__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__init__()</span></code></a> is not called when unpickling an
instance.</p>
</div>
<p id="index-4">As we shall see, pickle does not use directly the methods described above. In
fact, these methods are part of the copy protocol which implements the
<a class="reference internal" href="#object.__reduce__" title="object.__reduce__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__reduce__()</span></code></a> special method. The copy protocol provides a unified
interface for retrieving the data necessary for pickling and copying
objects. <a class="footnote-reference brackets" href="#id9" id="id4">4</a></p>
<p>Although powerful, implementing <a class="reference internal" href="#object.__reduce__" title="object.__reduce__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__reduce__()</span></code></a> directly in your classes is
error prone. For this reason, class designers should use the high-level
interface (i.e., <a class="reference internal" href="#object.__getnewargs_ex__" title="object.__getnewargs_ex__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__getnewargs_ex__()</span></code></a>, <a class="reference internal" href="#object.__getstate__" title="object.__getstate__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__getstate__()</span></code></a> and
<a class="reference internal" href="#object.__setstate__" title="object.__setstate__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__setstate__()</span></code></a>) whenever possible. We will show, however, cases where
using <a class="reference internal" href="#object.__reduce__" title="object.__reduce__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__reduce__()</span></code></a> is the only option or leads to more efficient pickling
or both.</p>
<dl class="method">
<dt id="object.__reduce__">
<code class="sig-prename descclassname">object.</code><code class="sig-name descname">__reduce__</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#object.__reduce__" title="Permalink to this definition">¶</a></dt>
<dd><p>The interface is currently defined as follows. The <a class="reference internal" href="#object.__reduce__" title="object.__reduce__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__reduce__()</span></code></a> method
takes no argument and shall return either a string or preferably a tuple (the
returned object is often referred to as the “reduce value”).</p>
<p>If a string is returned, the string should be interpreted as the name of a
global variable. It should be the object’s local name relative to its
module; the pickle module searches the module namespace to determine the
object’s module. This behaviour is typically useful for singletons.</p>
<p>When a tuple is returned, it must be between two and five items long.
Optional items can either be omitted, or <code class="docutils literal notranslate"><span class="pre">None</span></code> can be provided as their
value. The semantics of each item are in order:</p>
<ul class="simple">
<li><p>A callable object that will be called to create the initial version of the
object.</p></li>
<li><p>A tuple of arguments for the callable object. An empty tuple must be given
if the callable does not accept any argument.</p></li>
<li><p>Optionally, the object’s state, which will be passed to the object’s
<a class="reference internal" href="#object.__setstate__" title="object.__setstate__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__setstate__()</span></code></a> method as previously described. If the object has no
such method then, the value must be a dictionary and it will be added to
the object’s <a class="reference internal" href="stdtypes.html#object.__dict__" title="object.__dict__"><code class="xref py py-attr docutils literal notranslate"><span class="pre">__dict__</span></code></a> attribute.</p></li>
<li><p>Optionally, an iterator (and not a sequence) yielding successive items.
These items will be appended to the object either using
<code class="docutils literal notranslate"><span class="pre">obj.append(item)</span></code> or, in batch, using <code class="docutils literal notranslate"><span class="pre">obj.extend(list_of_items)</span></code>.
This is primarily used for list subclasses, but may be used by other
classes as long as they have <code class="xref py py-meth docutils literal notranslate"><span class="pre">append()</span></code> and <code class="xref py py-meth docutils literal notranslate"><span class="pre">extend()</span></code> methods with
the appropriate signature. (Whether <code class="xref py py-meth docutils literal notranslate"><span class="pre">append()</span></code> or <code class="xref py py-meth docutils literal notranslate"><span class="pre">extend()</span></code> is
used depends on which pickle protocol version is used as well as the number
of items to append, so both must be supported.)</p></li>
<li><p>Optionally, an iterator (not a sequence) yielding successive key-value
pairs. These items will be stored to the object using <code class="docutils literal notranslate"><span class="pre">obj[key]</span> <span class="pre">=</span>
<span class="pre">value</span></code>. This is primarily used for dictionary subclasses, but may be used
by other classes as long as they implement <a class="reference internal" href="../reference/datamodel.html#object.__setitem__" title="object.__setitem__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__setitem__()</span></code></a>.</p></li>
</ul>
</dd></dl>
<dl class="method">
<dt id="object.__reduce_ex__">
<code class="sig-prename descclassname">object.</code><code class="sig-name descname">__reduce_ex__</code><span class="sig-paren">(</span><em class="sig-param">protocol</em><span class="sig-paren">)</span><a class="headerlink" href="#object.__reduce_ex__" title="Permalink to this definition">¶</a></dt>
<dd><p>Alternatively, a <a class="reference internal" href="#object.__reduce_ex__" title="object.__reduce_ex__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__reduce_ex__()</span></code></a> method may be defined. The only
difference is this method should take a single integer argument, the protocol
version. When defined, pickle will prefer it over the <a class="reference internal" href="#object.__reduce__" title="object.__reduce__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__reduce__()</span></code></a>
method. In addition, <a class="reference internal" href="#object.__reduce__" title="object.__reduce__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__reduce__()</span></code></a> automatically becomes a synonym for
the extended version. The main use for this method is to provide
backwards-compatible reduce values for older Python releases.</p>
</dd></dl>
<div class="section" id="persistence-of-external-objects">
<span id="pickle-persistent"></span><h3>Persistence of External Objects<a class="headerlink" href="#persistence-of-external-objects" title="Permalink to this headline">¶</a></h3>
<p id="index-5">For the benefit of object persistence, the <a class="reference internal" href="#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 supports the
notion of a reference to an object outside the pickled data stream. Such
objects are referenced by a persistent ID, which should be either a string of
alphanumeric characters (for protocol 0) <a class="footnote-reference brackets" href="#id10" id="id5">5</a> or just an arbitrary object (for
any newer protocol).</p>
<p>The resolution of such persistent IDs is not defined by the <a class="reference internal" href="#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; it will delegate this resolution to the user-defined methods on the
pickler and unpickler, <a class="reference internal" href="#pickle.Pickler.persistent_id" title="pickle.Pickler.persistent_id"><code class="xref py py-meth docutils literal notranslate"><span class="pre">persistent_id()</span></code></a> and
<a class="reference internal" href="#pickle.Unpickler.persistent_load" title="pickle.Unpickler.persistent_load"><code class="xref py py-meth docutils literal notranslate"><span class="pre">persistent_load()</span></code></a> respectively.</p>
<p>To pickle objects that have an external persistent ID, the pickler must have a
custom <a class="reference internal" href="#pickle.Pickler.persistent_id" title="pickle.Pickler.persistent_id"><code class="xref py py-meth docutils literal notranslate"><span class="pre">persistent_id()</span></code></a> method that takes an object as an
argument and returns either <code class="docutils literal notranslate"><span class="pre">None</span></code> or the persistent ID for that object.
When <code class="docutils literal notranslate"><span class="pre">None</span></code> is returned, the pickler simply pickles the object as normal.
When a persistent ID string is returned, the pickler will pickle that object,
along with a marker so that the unpickler will recognize it as a persistent ID.</p>
<p>To unpickle external objects, the unpickler must have a custom
<a class="reference internal" href="#pickle.Unpickler.persistent_load" title="pickle.Unpickler.persistent_load"><code class="xref py py-meth docutils literal notranslate"><span class="pre">persistent_load()</span></code></a> method that takes a persistent ID object and
returns the referenced object.</p>
<p>Here is a comprehensive example presenting how persistent ID can be used to
pickle external objects by reference.</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="c1"># Simple example presenting how persistent ID can be used to pickle</span>
<span class="c1"># external objects by reference.</span>
<span class="kn">import</span> <span class="nn">pickle</span>
<span class="kn">import</span> <span class="nn">sqlite3</span>
<span class="kn">from</span> <span class="nn">collections</span> <span class="kn">import</span> <span class="n">namedtuple</span>
<span class="c1"># Simple class representing a record in our database.</span>
<span class="n">MemoRecord</span> <span class="o">=</span> <span class="n">namedtuple</span><span class="p">(</span><span class="s2">"MemoRecord"</span><span class="p">,</span> <span class="s2">"key, task"</span><span class="p">)</span>
<span class="k">class</span> <span class="nc">DBPickler</span><span class="p">(</span><span class="n">pickle</span><span class="o">.</span><span class="n">Pickler</span><span class="p">):</span>
<span class="k">def</span> <span class="nf">persistent_id</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">obj</span><span class="p">):</span>
<span class="c1"># Instead of pickling MemoRecord as a regular class instance, we emit a</span>
<span class="c1"># persistent ID.</span>
<span class="k">if</span> <span class="nb">isinstance</span><span class="p">(</span><span class="n">obj</span><span class="p">,</span> <span class="n">MemoRecord</span><span class="p">):</span>
<span class="c1"># Here, our persistent ID is simply a tuple, containing a tag and a</span>
<span class="c1"># key, which refers to a specific record in the database.</span>
<span class="k">return</span> <span class="p">(</span><span class="s2">"MemoRecord"</span><span class="p">,</span> <span class="n">obj</span><span class="o">.</span><span class="n">key</span><span class="p">)</span>
<span class="k">else</span><span class="p">:</span>
<span class="c1"># If obj does not have a persistent ID, return None. This means obj</span>
<span class="c1"># needs to be pickled as usual.</span>
<span class="k">return</span> <span class="kc">None</span>
<span class="k">class</span> <span class="nc">DBUnpickler</span><span class="p">(</span><span class="n">pickle</span><span class="o">.</span><span class="n">Unpickler</span><span class="p">):</span>
<span class="k">def</span> <span class="fm">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">file</span><span class="p">,</span> <span class="n">connection</span><span class="p">):</span>
<span class="nb">super</span><span class="p">()</span><span class="o">.</span><span class="fm">__init__</span><span class="p">(</span><span class="n">file</span><span class="p">)</span>
<span class="bp">self</span><span class="o">.</span><span class="n">connection</span> <span class="o">=</span> <span class="n">connection</span>
<span class="k">def</span> <span class="nf">persistent_load</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">pid</span><span class="p">):</span>
<span class="c1"># This method is invoked whenever a persistent ID is encountered.</span>
<span class="c1"># Here, pid is the tuple returned by DBPickler.</span>
<span class="n">cursor</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">connection</span><span class="o">.</span><span class="n">cursor</span><span class="p">()</span>
<span class="n">type_tag</span><span class="p">,</span> <span class="n">key_id</span> <span class="o">=</span> <span class="n">pid</span>
<span class="k">if</span> <span class="n">type_tag</span> <span class="o">==</span> <span class="s2">"MemoRecord"</span><span class="p">:</span>
<span class="c1"># Fetch the referenced record from the database and return it.</span>
<span class="n">cursor</span><span class="o">.</span><span class="n">execute</span><span class="p">(</span><span class="s2">"SELECT * FROM memos WHERE key=?"</span><span class="p">,</span> <span class="p">(</span><span class="nb">str</span><span class="p">(</span><span class="n">key_id</span><span class="p">),))</span>
<span class="n">key</span><span class="p">,</span> <span class="n">task</span> <span class="o">=</span> <span class="n">cursor</span><span class="o">.</span><span class="n">fetchone</span><span class="p">()</span>
<span class="k">return</span> <span class="n">MemoRecord</span><span class="p">(</span><span class="n">key</span><span class="p">,</span> <span class="n">task</span><span class="p">)</span>
<span class="k">else</span><span class="p">:</span>
<span class="c1"># Always raises an error if you cannot return the correct object.</span>
<span class="c1"># Otherwise, the unpickler will think None is the object referenced</span>
<span class="c1"># by the persistent ID.</span>
<span class="k">raise</span> <span class="n">pickle</span><span class="o">.</span><span class="n">UnpicklingError</span><span class="p">(</span><span class="s2">"unsupported persistent object"</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">main</span><span class="p">():</span>
<span class="kn">import</span> <span class="nn">io</span>
<span class="kn">import</span> <span class="nn">pprint</span>
<span class="c1"># Initialize and populate our database.</span>
<span class="n">conn</span> <span class="o">=</span> <span class="n">sqlite3</span><span class="o">.</span><span class="n">connect</span><span class="p">(</span><span class="s2">":memory:"</span><span class="p">)</span>
<span class="n">cursor</span> <span class="o">=</span> <span class="n">conn</span><span class="o">.</span><span class="n">cursor</span><span class="p">()</span>
<span class="n">cursor</span><span class="o">.</span><span class="n">execute</span><span class="p">(</span><span class="s2">"CREATE TABLE memos(key INTEGER PRIMARY KEY, task TEXT)"</span><span class="p">)</span>
<span class="n">tasks</span> <span class="o">=</span> <span class="p">(</span>
<span class="s1">'give food to fish'</span><span class="p">,</span>
<span class="s1">'prepare group meeting'</span><span class="p">,</span>
<span class="s1">'fight with a zebra'</span><span class="p">,</span>
<span class="p">)</span>
<span class="k">for</span> <span class="n">task</span> <span class="ow">in</span> <span class="n">tasks</span><span class="p">:</span>
<span class="n">cursor</span><span class="o">.</span><span class="n">execute</span><span class="p">(</span><span class="s2">"INSERT INTO memos VALUES(NULL, ?)"</span><span class="p">,</span> <span class="p">(</span><span class="n">task</span><span class="p">,))</span>
<span class="c1"># Fetch the records to be pickled.</span>
<span class="n">cursor</span><span class="o">.</span><span class="n">execute</span><span class="p">(</span><span class="s2">"SELECT * FROM memos"</span><span class="p">)</span>
<span class="n">memos</span> <span class="o">=</span> <span class="p">[</span><span class="n">MemoRecord</span><span class="p">(</span><span class="n">key</span><span class="p">,</span> <span class="n">task</span><span class="p">)</span> <span class="k">for</span> <span class="n">key</span><span class="p">,</span> <span class="n">task</span> <span class="ow">in</span> <span class="n">cursor</span><span class="p">]</span>
<span class="c1"># Save the records using our custom DBPickler.</span>
<span class="n">file</span> <span class="o">=</span> <span class="n">io</span><span class="o">.</span><span class="n">BytesIO</span><span class="p">()</span>
<span class="n">DBPickler</span><span class="p">(</span><span class="n">file</span><span class="p">)</span><span class="o">.</span><span class="n">dump</span><span class="p">(</span><span class="n">memos</span><span class="p">)</span>
<span class="nb">print</span><span class="p">(</span><span class="s2">"Pickled records:"</span><span class="p">)</span>
<span class="n">pprint</span><span class="o">.</span><span class="n">pprint</span><span class="p">(</span><span class="n">memos</span><span class="p">)</span>
<span class="c1"># Update a record, just for good measure.</span>
<span class="n">cursor</span><span class="o">.</span><span class="n">execute</span><span class="p">(</span><span class="s2">"UPDATE memos SET task='learn italian' WHERE key=1"</span><span class="p">)</span>
<span class="c1"># Load the records from the pickle data stream.</span>
<span class="n">file</span><span class="o">.</span><span class="n">seek</span><span class="p">(</span><span class="mi">0</span><span class="p">)</span>
<span class="n">memos</span> <span class="o">=</span> <span class="n">DBUnpickler</span><span class="p">(</span><span class="n">file</span><span class="p">,</span> <span class="n">conn</span><span class="p">)</span><span class="o">.</span><span class="n">load</span><span class="p">()</span>
<span class="nb">print</span><span class="p">(</span><span class="s2">"Unpickled records:"</span><span class="p">)</span>
<span class="n">pprint</span><span class="o">.</span><span class="n">pprint</span><span class="p">(</span><span class="n">memos</span><span class="p">)</span>
<span class="k">if</span> <span class="vm">__name__</span> <span class="o">==</span> <span class="s1">'__main__'</span><span class="p">:</span>
<span class="n">main</span><span class="p">()</span>
</pre></div>
</div>
</div>
<div class="section" id="dispatch-tables">
<span id="pickle-dispatch"></span><h3>Dispatch Tables<a class="headerlink" href="#dispatch-tables" title="Permalink to this headline">¶</a></h3>
<p>If one wants to customize pickling of some classes without disturbing
any other code which depends on pickling, then one can create a
pickler with a private dispatch table.</p>
<p>The global dispatch table managed by the <a class="reference internal" href="copyreg.html#module-copyreg" title="copyreg: Register pickle support functions."><code class="xref py py-mod docutils literal notranslate"><span class="pre">copyreg</span></code></a> module is
available as <code class="xref py py-data docutils literal notranslate"><span class="pre">copyreg.dispatch_table</span></code>. Therefore, one may
choose to use a modified copy of <code class="xref py py-data docutils literal notranslate"><span class="pre">copyreg.dispatch_table</span></code> as a
private dispatch table.</p>
<p>For example</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">f</span> <span class="o">=</span> <span class="n">io</span><span class="o">.</span><span class="n">BytesIO</span><span class="p">()</span>
<span class="n">p</span> <span class="o">=</span> <span class="n">pickle</span><span class="o">.</span><span class="n">Pickler</span><span class="p">(</span><span class="n">f</span><span class="p">)</span>
<span class="n">p</span><span class="o">.</span><span class="n">dispatch_table</span> <span class="o">=</span> <span class="n">copyreg</span><span class="o">.</span><span class="n">dispatch_table</span><span class="o">.</span><span class="n">copy</span><span class="p">()</span>
<span class="n">p</span><span class="o">.</span><span class="n">dispatch_table</span><span class="p">[</span><span class="n">SomeClass</span><span class="p">]</span> <span class="o">=</span> <span class="n">reduce_SomeClass</span>
</pre></div>
</div>
<p>creates an instance of <a class="reference internal" href="#pickle.Pickler" title="pickle.Pickler"><code class="xref py py-class docutils literal notranslate"><span class="pre">pickle.Pickler</span></code></a> with a private dispatch
table which handles the <code class="docutils literal notranslate"><span class="pre">SomeClass</span></code> class specially. Alternatively,
the code</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">MyPickler</span><span class="p">(</span><span class="n">pickle</span><span class="o">.</span><span class="n">Pickler</span><span class="p">):</span>
<span class="n">dispatch_table</span> <span class="o">=</span> <span class="n">copyreg</span><span class="o">.</span><span class="n">dispatch_table</span><span class="o">.</span><span class="n">copy</span><span class="p">()</span>
<span class="n">dispatch_table</span><span class="p">[</span><span class="n">SomeClass</span><span class="p">]</span> <span class="o">=</span> <span class="n">reduce_SomeClass</span>
<span class="n">f</span> <span class="o">=</span> <span class="n">io</span><span class="o">.</span><span class="n">BytesIO</span><span class="p">()</span>
<span class="n">p</span> <span class="o">=</span> <span class="n">MyPickler</span><span class="p">(</span><span class="n">f</span><span class="p">)</span>
</pre></div>
</div>
<p>does the same, but all instances of <code class="docutils literal notranslate"><span class="pre">MyPickler</span></code> will by default
share the same dispatch table. The equivalent code using the
<a class="reference internal" href="copyreg.html#module-copyreg" title="copyreg: Register pickle support functions."><code class="xref py py-mod docutils literal notranslate"><span class="pre">copyreg</span></code></a> module is</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">copyreg</span><span class="o">.</span><span class="n">pickle</span><span class="p">(</span><span class="n">SomeClass</span><span class="p">,</span> <span class="n">reduce_SomeClass</span><span class="p">)</span>
<span class="n">f</span> <span class="o">=</span> <span class="n">io</span><span class="o">.</span><span class="n">BytesIO</span><span class="p">()</span>
<span class="n">p</span> <span class="o">=</span> <span class="n">pickle</span><span class="o">.</span><span class="n">Pickler</span><span class="p">(</span><span class="n">f</span><span class="p">)</span>
</pre></div>
</div>
</div>
<div class="section" id="handling-stateful-objects">
<span id="pickle-state"></span><h3>Handling Stateful Objects<a class="headerlink" href="#handling-stateful-objects" title="Permalink to this headline">¶</a></h3>
<p id="index-6">Here’s an example that shows how to modify pickling behavior for a class.
The <code class="xref py py-class docutils literal notranslate"><span class="pre">TextReader</span></code> class opens a text file, and returns the line number and
line contents each time its <code class="xref py py-meth docutils literal notranslate"><span class="pre">readline()</span></code> method is called. If a
<code class="xref py py-class docutils literal notranslate"><span class="pre">TextReader</span></code> instance is pickled, all attributes <em>except</em> the file object
member are saved. When the instance is unpickled, the file is reopened, and
reading resumes from the last location. The <a class="reference internal" href="#object.__setstate__" title="object.__setstate__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__setstate__()</span></code></a> and
<a class="reference internal" href="#object.__getstate__" title="object.__getstate__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__getstate__()</span></code></a> methods are used to implement this behavior.</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">TextReader</span><span class="p">:</span>
<span class="sd">"""Print and number lines in a text file."""</span>
<span class="k">def</span> <span class="fm">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">filename</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">filename</span> <span class="o">=</span> <span class="n">filename</span>
<span class="bp">self</span><span class="o">.</span><span class="n">file</span> <span class="o">=</span> <span class="nb">open</span><span class="p">(</span><span class="n">filename</span><span class="p">)</span>
<span class="bp">self</span><span class="o">.</span><span class="n">lineno</span> <span class="o">=</span> <span class="mi">0</span>
<span class="k">def</span> <span class="nf">readline</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">lineno</span> <span class="o">+=</span> <span class="mi">1</span>
<span class="n">line</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">file</span><span class="o">.</span><span class="n">readline</span><span class="p">()</span>
<span class="k">if</span> <span class="ow">not</span> <span class="n">line</span><span class="p">:</span>
<span class="k">return</span> <span class="kc">None</span>
<span class="k">if</span> <span class="n">line</span><span class="o">.</span><span class="n">endswith</span><span class="p">(</span><span class="s1">'</span><span class="se">\n</span><span class="s1">'</span><span class="p">):</span>
<span class="n">line</span> <span class="o">=</span> <span class="n">line</span><span class="p">[:</span><span class="o">-</span><span class="mi">1</span><span class="p">]</span>
<span class="k">return</span> <span class="s2">"</span><span class="si">%i</span><span class="s2">: </span><span class="si">%s</span><span class="s2">"</span> <span class="o">%</span> <span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">lineno</span><span class="p">,</span> <span class="n">line</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">__getstate__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="c1"># Copy the object's state from self.__dict__ which contains</span>
<span class="c1"># all our instance attributes. Always use the dict.copy()</span>
<span class="c1"># method to avoid modifying the original state.</span>
<span class="n">state</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="vm">__dict__</span><span class="o">.</span><span class="n">copy</span><span class="p">()</span>
<span class="c1"># Remove the unpicklable entries.</span>
<span class="k">del</span> <span class="n">state</span><span class="p">[</span><span class="s1">'file'</span><span class="p">]</span>
<span class="k">return</span> <span class="n">state</span>
<span class="k">def</span> <span class="nf">__setstate__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">state</span><span class="p">):</span>
<span class="c1"># Restore instance attributes (i.e., filename and lineno).</span>
<span class="bp">self</span><span class="o">.</span><span class="vm">__dict__</span><span class="o">.</span><span class="n">update</span><span class="p">(</span><span class="n">state</span><span class="p">)</span>
<span class="c1"># Restore the previously opened file's state. To do so, we need to</span>
<span class="c1"># reopen it and read from it until the line count is restored.</span>
<span class="n">file</span> <span class="o">=</span> <span class="nb">open</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">filename</span><span class="p">)</span>
<span class="k">for</span> <span class="n">_</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">lineno</span><span class="p">):</span>
<span class="n">file</span><span class="o">.</span><span class="n">readline</span><span class="p">()</span>
<span class="c1"># Finally, save the file.</span>
<span class="bp">self</span><span class="o">.</span><span class="n">file</span> <span class="o">=</span> <span class="n">file</span>
</pre></div>
</div>
<p>A sample usage might be something like this:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">reader</span> <span class="o">=</span> <span class="n">TextReader</span><span class="p">(</span><span class="s2">"hello.txt"</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">reader</span><span class="o">.</span><span class="n">readline</span><span class="p">()</span>
<span class="go">'1: Hello world!'</span>
<span class="gp">>>> </span><span class="n">reader</span><span class="o">.</span><span class="n">readline</span><span class="p">()</span>
<span class="go">'2: I am line number two.'</span>
<span class="gp">>>> </span><span class="n">new_reader</span> <span class="o">=</span> <span class="n">pickle</span><span class="o">.</span><span class="n">loads</span><span class="p">(</span><span class="n">pickle</span><span class="o">.</span><span class="n">dumps</span><span class="p">(</span><span class="n">reader</span><span class="p">))</span>
<span class="gp">>>> </span><span class="n">new_reader</span><span class="o">.</span><span class="n">readline</span><span class="p">()</span>
<span class="go">'3: Goodbye!'</span>
</pre></div>
</div>
</div>
</div>
<div class="section" id="restricting-globals">
<span id="pickle-restrict"></span><h2>Restricting Globals<a class="headerlink" href="#restricting-globals" title="Permalink to this headline">¶</a></h2>
<p id="index-7">By default, unpickling will import any class or function that it finds in the
pickle data. For many applications, this behaviour is unacceptable as it
permits the unpickler to import and invoke arbitrary code. Just consider what
this hand-crafted pickle data stream does when loaded:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="kn">import</span> <span class="nn">pickle</span>
<span class="gp">>>> </span><span class="n">pickle</span><span class="o">.</span><span class="n">loads</span><span class="p">(</span><span class="sa">b</span><span class="s2">"cos</span><span class="se">\n</span><span class="s2">system</span><span class="se">\n</span><span class="s2">(S'echo hello world'</span><span class="se">\n</span><span class="s2">tR."</span><span class="p">)</span>
<span class="go">hello world</span>
<span class="go">0</span>
</pre></div>
</div>
<p>In this example, the unpickler imports the <a class="reference internal" href="os.html#os.system" title="os.system"><code class="xref py py-func docutils literal notranslate"><span class="pre">os.system()</span></code></a> function and then
apply the string argument “echo hello world”. Although this example is
inoffensive, it is not difficult to imagine one that could damage your system.</p>
<p>For this reason, you may want to control what gets unpickled by customizing
<a class="reference internal" href="#pickle.Unpickler.find_class" title="pickle.Unpickler.find_class"><code class="xref py py-meth docutils literal notranslate"><span class="pre">Unpickler.find_class()</span></code></a>. Unlike its name suggests,
<a class="reference internal" href="#pickle.Unpickler.find_class" title="pickle.Unpickler.find_class"><code class="xref py py-meth docutils literal notranslate"><span class="pre">Unpickler.find_class()</span></code></a> is called whenever a global (i.e., a class or
a function) is requested. Thus it is possible to either completely forbid
globals or restrict them to a safe subset.</p>
<p>Here is an example of an unpickler allowing only few safe classes from the
<a class="reference internal" href="builtins.html#module-builtins" title="builtins: The module that provides the built-in namespace."><code class="xref py py-mod docutils literal notranslate"><span class="pre">builtins</span></code></a> module to be loaded:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">builtins</span>
<span class="kn">import</span> <span class="nn">io</span>
<span class="kn">import</span> <span class="nn">pickle</span>
<span class="n">safe_builtins</span> <span class="o">=</span> <span class="p">{</span>
<span class="s1">'range'</span><span class="p">,</span>
<span class="s1">'complex'</span><span class="p">,</span>
<span class="s1">'set'</span><span class="p">,</span>
<span class="s1">'frozenset'</span><span class="p">,</span>
<span class="s1">'slice'</span><span class="p">,</span>
<span class="p">}</span>
<span class="k">class</span> <span class="nc">RestrictedUnpickler</span><span class="p">(</span><span class="n">pickle</span><span class="o">.</span><span class="n">Unpickler</span><span class="p">):</span>
<span class="k">def</span> <span class="nf">find_class</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">module</span><span class="p">,</span> <span class="n">name</span><span class="p">):</span>
<span class="c1"># Only allow safe classes from builtins.</span>
<span class="k">if</span> <span class="n">module</span> <span class="o">==</span> <span class="s2">"builtins"</span> <span class="ow">and</span> <span class="n">name</span> <span class="ow">in</span> <span class="n">safe_builtins</span><span class="p">:</span>
<span class="k">return</span> <span class="nb">getattr</span><span class="p">(</span><span class="n">builtins</span><span class="p">,</span> <span class="n">name</span><span class="p">)</span>
<span class="c1"># Forbid everything else.</span>
<span class="k">raise</span> <span class="n">pickle</span><span class="o">.</span><span class="n">UnpicklingError</span><span class="p">(</span><span class="s2">"global '</span><span class="si">%s</span><span class="s2">.</span><span class="si">%s</span><span class="s2">' is forbidden"</span> <span class="o">%</span>
<span class="p">(</span><span class="n">module</span><span class="p">,</span> <span class="n">name</span><span class="p">))</span>
<span class="k">def</span> <span class="nf">restricted_loads</span><span class="p">(</span><span class="n">s</span><span class="p">):</span>
<span class="sd">"""Helper function analogous to pickle.loads()."""</span>
<span class="k">return</span> <span class="n">RestrictedUnpickler</span><span class="p">(</span><span class="n">io</span><span class="o">.</span><span class="n">BytesIO</span><span class="p">(</span><span class="n">s</span><span class="p">))</span><span class="o">.</span><span class="n">load</span><span class="p">()</span>
</pre></div>
</div>
<p>A sample usage of our unpickler working has intended:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">restricted_loads</span><span class="p">(</span><span class="n">pickle</span><span class="o">.</span><span class="n">dumps</span><span class="p">([</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="nb">range</span><span class="p">(</span><span class="mi">15</span><span class="p">)]))</span>
<span class="go">[1, 2, range(0, 15)]</span>
<span class="gp">>>> </span><span class="n">restricted_loads</span><span class="p">(</span><span class="sa">b</span><span class="s2">"cos</span><span class="se">\n</span><span class="s2">system</span><span class="se">\n</span><span class="s2">(S'echo hello world'</span><span class="se">\n</span><span class="s2">tR."</span><span class="p">)</span>
<span class="gt">Traceback (most recent call last):</span>
<span class="c">...</span>
<span class="gr">pickle.UnpicklingError</span>: <span class="n">global 'os.system' is forbidden</span>
<span class="gp">>>> </span><span class="n">restricted_loads</span><span class="p">(</span><span class="sa">b</span><span class="s1">'cbuiltins</span><span class="se">\n</span><span class="s1">eval</span><span class="se">\n</span><span class="s1">'</span>
<span class="gp">... </span> <span class="sa">b</span><span class="s1">'(S</span><span class="se">\'</span><span class="s1">getattr(__import__("os"), "system")'</span>
<span class="gp">... </span> <span class="sa">b</span><span class="s1">'("echo hello world")</span><span class="se">\'\n</span><span class="s1">tR.'</span><span class="p">)</span>
<span class="gt">Traceback (most recent call last):</span>
<span class="c">...</span>
<span class="gr">pickle.UnpicklingError</span>: <span class="n">global 'builtins.eval' is forbidden</span>
</pre></div>
</div>
<p>As our examples shows, you have to be careful with what you allow to be
unpickled. Therefore if security is a concern, you may want to consider
alternatives such as the marshalling API in <a class="reference internal" href="xmlrpc.client.html#module-xmlrpc.client" title="xmlrpc.client: XML-RPC client access."><code class="xref py py-mod docutils literal notranslate"><span class="pre">xmlrpc.client</span></code></a> or
third-party solutions.</p>
</div>
<div class="section" id="performance">
<h2>Performance<a class="headerlink" href="#performance" title="Permalink to this headline">¶</a></h2>
<p>Recent versions of the pickle protocol (from protocol 2 and upwards) feature
efficient binary encodings for several common features and built-in types.
Also, the <a class="reference internal" href="#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 has a transparent optimizer written in C.</p>
</div>
<div class="section" id="examples">
<span id="pickle-example"></span><h2>Examples<a class="headerlink" href="#examples" title="Permalink to this headline">¶</a></h2>
<p>For the simplest code, use the <a class="reference internal" href="#pickle.dump" title="pickle.dump"><code class="xref py py-func docutils literal notranslate"><span class="pre">dump()</span></code></a> and <a class="reference internal" href="#pickle.load" title="pickle.load"><code class="xref py py-func docutils literal notranslate"><span class="pre">load()</span></code></a> functions.</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">pickle</span>
<span class="c1"># An arbitrary collection of objects supported by pickle.</span>
<span class="n">data</span> <span class="o">=</span> <span class="p">{</span>
<span class="s1">'a'</span><span class="p">:</span> <span class="p">[</span><span class="mi">1</span><span class="p">,</span> <span class="mf">2.0</span><span class="p">,</span> <span class="mi">3</span><span class="p">,</span> <span class="mi">4</span><span class="o">+</span><span class="mi">6</span><span class="n">j</span><span class="p">],</span>
<span class="s1">'b'</span><span class="p">:</span> <span class="p">(</span><span class="s2">"character string"</span><span class="p">,</span> <span class="sa">b</span><span class="s2">"byte string"</span><span class="p">),</span>
<span class="s1">'c'</span><span class="p">:</span> <span class="p">{</span><span class="kc">None</span><span class="p">,</span> <span class="kc">True</span><span class="p">,</span> <span class="kc">False</span><span class="p">}</span>
<span class="p">}</span>
<span class="k">with</span> <span class="nb">open</span><span class="p">(</span><span class="s1">'data.pickle'</span><span class="p">,</span> <span class="s1">'wb'</span><span class="p">)</span> <span class="k">as</span> <span class="n">f</span><span class="p">:</span>
<span class="c1"># Pickle the 'data' dictionary using the highest protocol available.</span>
<span class="n">pickle</span><span class="o">.</span><span class="n">dump</span><span class="p">(</span><span class="n">data</span><span class="p">,</span> <span class="n">f</span><span class="p">,</span> <span class="n">pickle</span><span class="o">.</span><span class="n">HIGHEST_PROTOCOL</span><span class="p">)</span>
</pre></div>
</div>
<p>The following example reads the resulting pickled data.</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">pickle</span>
<span class="k">with</span> <span class="nb">open</span><span class="p">(</span><span class="s1">'data.pickle'</span><span class="p">,</span> <span class="s1">'rb'</span><span class="p">)</span> <span class="k">as</span> <span class="n">f</span><span class="p">:</span>
<span class="c1"># The protocol version used is detected automatically, so we do not</span>
<span class="c1"># have to specify it.</span>
<span class="n">data</span> <span class="o">=</span> <span class="n">pickle</span><span class="o">.</span><span class="n">load</span><span class="p">(</span><span class="n">f</span><span class="p">)</span>
</pre></div>
</div>
<div class="admonition seealso">
<p class="admonition-title">See also</p>
<dl class="simple">
<dt>Module <a class="reference internal" href="copyreg.html#module-copyreg" title="copyreg: Register pickle support functions."><code class="xref py py-mod docutils literal notranslate"><span class="pre">copyreg</span></code></a></dt><dd><p>Pickle interface constructor registration for extension types.</p>
</dd>
<dt>Module <a class="reference internal" href="pickletools.html#module-pickletools" title="pickletools: Contains extensive comments about the pickle protocols and pickle-machine opcodes, as well as some useful functions."><code class="xref py py-mod docutils literal notranslate"><span class="pre">pickletools</span></code></a></dt><dd><p>Tools for working with and analyzing pickled data.</p>
</dd>
<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><p>Indexed databases of objects; uses <a class="reference internal" href="#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>.</p>
</dd>
<dt>Module <a class="reference internal" href="copy.html#module-copy" title="copy: Shallow and deep copy operations."><code class="xref py py-mod docutils literal notranslate"><span class="pre">copy</span></code></a></dt><dd><p>Shallow and deep object copying.</p>
</dd>
<dt>Module <a class="reference internal" href="marshal.html#module-marshal" title="marshal: Convert Python objects to streams of bytes and back (with different constraints)."><code class="xref py py-mod docutils literal notranslate"><span class="pre">marshal</span></code></a></dt><dd><p>High-performance serialization of built-in types.</p>
</dd>
</dl>
</div>
<p class="rubric">Footnotes</p>
<dl class="footnote brackets">
<dt class="label" id="id6"><span class="brackets"><a class="fn-backref" href="#id1">1</a></span></dt>
<dd><p>Don’t confuse this with the <a class="reference internal" href="marshal.html#module-marshal" title="marshal: Convert Python objects to streams of bytes and back (with different constraints)."><code class="xref py py-mod docutils literal notranslate"><span class="pre">marshal</span></code></a> module</p>
</dd>
<dt class="label" id="id7"><span class="brackets"><a class="fn-backref" href="#id2">2</a></span></dt>
<dd><p>This is why <a class="reference internal" href="../reference/expressions.html#lambda"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">lambda</span></code></a> functions cannot be pickled: all
<code class="xref std std-keyword docutils literal notranslate"><span class="pre">lambda</span></code> functions share the same name: <code class="docutils literal notranslate"><span class="pre"><lambda></span></code>.</p>
</dd>
<dt class="label" id="id8"><span class="brackets"><a class="fn-backref" href="#id3">3</a></span></dt>
<dd><p>The exception raised will likely be an <a class="reference internal" href="exceptions.html#ImportError" title="ImportError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">ImportError</span></code></a> or an
<a class="reference internal" href="exceptions.html#AttributeError" title="AttributeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">AttributeError</span></code></a> but it could be something else.</p>
</dd>
<dt class="label" id="id9"><span class="brackets"><a class="fn-backref" href="#id4">4</a></span></dt>
<dd><p>The <a class="reference internal" href="copy.html#module-copy" title="copy: Shallow and deep copy operations."><code class="xref py py-mod docutils literal notranslate"><span class="pre">copy</span></code></a> module uses this protocol for shallow and deep copying
operations.</p>
</dd>
<dt class="label" id="id10"><span class="brackets"><a class="fn-backref" href="#id5">5</a></span></dt>
<dd><p>The limitation on alphanumeric characters is due to the fact
the persistent IDs, in protocol 0, are delimited by the newline
character. Therefore if any kind of newline characters occurs in
persistent IDs, the resulting pickle will become unreadable.</p>
</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="#"><code class="xref py py-mod docutils literal notranslate"><span class="pre">pickle</span></code> — Python object serialization</a><ul>
<li><a class="reference internal" href="#relationship-to-other-python-modules">Relationship to other Python modules</a><ul>
<li><a class="reference internal" href="#comparison-with-marshal">Comparison with <code class="docutils literal notranslate"><span class="pre">marshal</span></code></a></li>
<li><a class="reference internal" href="#comparison-with-json">Comparison with <code class="docutils literal notranslate"><span class="pre">json</span></code></a></li>
</ul>
</li>
<li><a class="reference internal" href="#data-stream-format">Data stream format</a></li>
<li><a class="reference internal" href="#module-interface">Module Interface</a></li>
<li><a class="reference internal" href="#what-can-be-pickled-and-unpickled">What can be pickled and unpickled?</a></li>
<li><a class="reference internal" href="#pickling-class-instances">Pickling Class Instances</a><ul>
<li><a class="reference internal" href="#persistence-of-external-objects">Persistence of External Objects</a></li>
<li><a class="reference internal" href="#dispatch-tables">Dispatch Tables</a></li>
<li><a class="reference internal" href="#handling-stateful-objects">Handling Stateful Objects</a></li>
</ul>
</li>
<li><a class="reference internal" href="#restricting-globals">Restricting Globals</a></li>
<li><a class="reference internal" href="#performance">Performance</a></li>
<li><a class="reference internal" href="#examples">Examples</a></li>
</ul>
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</ul>
<h4>Previous topic</h4>
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title="next chapter"><code class="xref py py-mod docutils literal notranslate"><span class="pre">copyreg</span></code> — Register <code class="xref py py-mod docutils literal notranslate"><span class="pre">pickle</span></code> support functions</a></p>
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