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 <div class="section" id="pickle-python-object-serialization">
<h1>11.1. <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">pickle</code></a> — Python object serialization<a class="headerlink" href="#pickle-python-object-serialization" title="Permalink to this headline">¶</a></h1>
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">pickle</code></a> module implements a fundamental, but powerful algorithm for
serializing and de-serializing a Python object structure. “Pickling” is the
process whereby a Python object hierarchy is converted into a byte stream, and
“unpickling” is the inverse operation, whereby a byte stream is converted back
into an object hierarchy. Pickling (and unpickling) is alternatively known as
“serialization”, “marshalling,” <a class="footnote-reference" href="#id11" id="id1">[1]</a> or “flattening”, however, to avoid
confusion, the terms used here are “pickling” and “unpickling”.
This documentation describes both 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">pickle</code></a> module and the
<a class="reference internal" href="#module-cPickle" title="cPickle: Faster version of pickle, but not subclassable."><code class="xref py py-mod docutils literal notranslate">cPickle</code></a> module.
<div class="admonition warning">
Warning
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">pickle</code></a> module is not secure against erroneous or maliciously
constructed data. Never unpickle data received from an untrusted or
unauthenticated source.
</div>
<div class="section" id="relationship-to-other-python-modules">
<h2>11.1.1. Relationship to other Python modules<a class="headerlink" href="#relationship-to-other-python-modules" title="Permalink to this headline">¶</a></h2>
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">pickle</code></a> module has an optimized cousin called the <a class="reference internal" href="#module-cPickle" title="cPickle: Faster version of pickle, but not subclassable."><code class="xref py py-mod docutils literal notranslate">cPickle</code></a>
module. As its name implies, <a class="reference internal" href="#module-cPickle" title="cPickle: Faster version of pickle, but not subclassable."><code class="xref py py-mod docutils literal notranslate">cPickle</code></a> is written in C, so it can be up to
1000 times faster than <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">pickle</code></a>. However it does not support subclassing
of the <a class="reference internal" href="#pickle.Pickler" title="pickle.Pickler"><code class="xref py py-func docutils literal notranslate">Pickler()</code></a> and <a class="reference internal" href="#pickle.Unpickler" title="pickle.Unpickler"><code class="xref py py-func docutils literal notranslate">Unpickler()</code></a> classes, because in <a class="reference internal" href="#module-cPickle" title="cPickle: Faster version of pickle, but not subclassable."><code class="xref py py-mod docutils literal notranslate">cPickle</code></a>
these are functions, not classes. Most applications have no need for this
functionality, and can benefit from the improved performance of <a class="reference internal" href="#module-cPickle" title="cPickle: Faster version of pickle, but not subclassable."><code class="xref py py-mod docutils literal notranslate">cPickle</code></a>.
Other than that, the interfaces of the two modules are nearly identical; the
common interface is described in this manual and differences are pointed out
where necessary. In the following discussions, we use the term “pickle” to
collectively describe 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">pickle</code></a> and <a class="reference internal" href="#module-cPickle" title="cPickle: Faster version of pickle, but not subclassable."><code class="xref py py-mod docutils literal notranslate">cPickle</code></a> modules.
The data streams the two modules produce are guaranteed to be interchangeable.
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">marshal</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">pickle</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">marshal</code></a> exists primarily to support Python’s <code class="file docutils literal notranslate">.pyc</code>
files.
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">pickle</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">marshal</code></a> in several significant ways:
<ul>
<li>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">pickle</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">marshal</code></a> doesn’t do this.
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">pickle</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.
</li>
<li><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">marshal</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">pickle</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.
</li>
<li>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">marshal</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">.pyc</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">pickle</code></a> serialization format is guaranteed to be backwards compatible
across Python releases.
</li>
</ul>
Note that 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">pickle</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">pickle</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 module
<a class="reference internal" href="shelve.html#module-shelve" title="shelve: Python object persistence."><code class="xref py py-mod docutils literal notranslate">shelve</code></a> provides a simple interface to pickle and unpickle objects on
DBM-style database files.
</div>
<div class="section" id="data-stream-format">
<h2>11.1.2. Data stream format<a class="headerlink" href="#data-stream-format" title="Permalink to this headline">¶</a></h2>
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">pickle</code></a> is Python-specific. This has the
advantage that there are no restrictions imposed by external standards such as
XDR (which can’t represent pointer sharing); however it means that non-Python
programs may not be able to reconstruct pickled Python objects.
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">pickle</code></a> data format uses a printable ASCII representation.
This is slightly more voluminous than a binary representation. The big
advantage of using printable ASCII (and of some other characteristics of
<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">pickle</code></a>’s representation) is that for debugging or recovery purposes it is
possible for a human to read the pickled file with a standard text editor.
There are currently 3 different protocols which can be used for pickling.
<ul class="simple">
<li>Protocol version 0 is the original ASCII protocol and is backwards compatible
with earlier versions of Python.</li>
<li>Protocol version 1 is the old binary format which is also compatible with
earlier versions of Python.</li>
<li>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">new-style class</a>es.</li>
</ul>
Refer to <a class="pep reference external" href="https://www.python.org/dev/peps/pep-0307">PEP 307</a> for more information.
If a protocol is not specified, protocol 0 is used. If protocol is specified
as a negative value or <a class="reference internal" href="#pickle.HIGHEST_PROTOCOL" title="pickle.HIGHEST_PROTOCOL"><code class="xref py py-const docutils literal notranslate">HIGHEST_PROTOCOL</code></a>, the highest protocol version
available will be used.
<div class="versionchanged">
Changed in version 2.3: Introduced the protocol parameter.
</div>
A binary format, which is slightly more efficient, can be chosen by specifying a
protocol version &gt;= 1.
</div>
<div class="section" id="usage">
<h2>11.1.3. Usage<a class="headerlink" href="#usage" title="Permalink to this headline">¶</a></h2>
To serialize an object hierarchy, you first create a pickler, then you call the
pickler’s <a class="reference internal" href="#pickle.dump" title="pickle.dump"><code class="xref py py-meth docutils literal notranslate">dump()</code></a> method. To de-serialize a data stream, you first create
an unpickler, then you call the unpickler’s <a class="reference internal" href="#pickle.load" title="pickle.load"><code class="xref py py-meth docutils literal notranslate">load()</code></a> method. 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">pickle</code></a> module provides the following constant:
<dl class="data">
<dt id="pickle.HIGHEST_PROTOCOL">
<code class="descclassname">pickle.</code><code class="descname">HIGHEST_PROTOCOL</code><a class="headerlink" href="#pickle.HIGHEST_PROTOCOL" title="Permalink to this definition">¶</a></dt>
<dd>The highest protocol version available. This value can be passed as a
protocol value.
<div class="versionadded">
New in version 2.3.
</div>
</dd></dl>

<div class="admonition note">
Note
Be sure to always open pickle files created with protocols &gt;= 1 in binary mode.
For the old ASCII-based pickle protocol 0 you can use either text mode or binary
mode as long as you stay consistent.
A pickle file written with protocol 0 in binary mode will contain lone linefeeds
as line terminators and therefore will look “funny” when viewed in Notepad or
other editors which do not support this format.
</div>
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">pickle</code></a> module provides the following functions to make the pickling
process more convenient:
<dl class="function">
<dt id="pickle.dump">
<code class="descclassname">pickle.</code><code class="descname">dump</code>(obj, file[, protocol])<a class="headerlink" href="#pickle.dump" title="Permalink to this definition">¶</a></dt>
<dd>Write a pickled representation of obj to the open file object file. This is
equivalent to <code class="docutils literal notranslate">Pickler(file, protocol).dump(obj)</code>.
If the protocol parameter is omitted, protocol 0 is used. If protocol is
specified as a negative value or <a class="reference internal" href="#pickle.HIGHEST_PROTOCOL" title="pickle.HIGHEST_PROTOCOL"><code class="xref py py-const docutils literal notranslate">HIGHEST_PROTOCOL</code></a>, the highest protocol
version will be used.
<div class="versionchanged">
Changed in version 2.3: Introduced the protocol parameter.
</div>
file must have a <code class="xref py py-meth docutils literal notranslate">write()</code> method that accepts a single string argument.
It can thus be a file object opened for writing, a <a class="reference internal" href="stringio.html#module-StringIO" title="StringIO: Read and write strings as if they were files."><code class="xref py py-mod docutils literal notranslate">StringIO</code></a> object, or
any other custom object that meets this interface.
</dd></dl>

<dl class="function">
<dt id="pickle.load">
<code class="descclassname">pickle.</code><code class="descname">load</code>(file)<a class="headerlink" href="#pickle.load" title="Permalink to this definition">¶</a></dt>
<dd>Read a string from the open file object file and interpret it as a pickle data
stream, reconstructing and returning the original object hierarchy. This is
equivalent to <code class="docutils literal notranslate">Unpickler(file).load()</code>.
file must have two methods, a <code class="xref py py-meth docutils literal notranslate">read()</code> method that takes an integer
argument, and a <a class="reference internal" href="readline.html#module-readline" title="readline: GNU readline support for Python. (Unix)"><code class="xref py py-meth docutils literal notranslate">readline()</code></a> method that requires no arguments. Both
methods should return a string. Thus file can be a file object opened for
reading, a <a class="reference internal" href="stringio.html#module-StringIO" title="StringIO: Read and write strings as if they were files."><code class="xref py py-mod docutils literal notranslate">StringIO</code></a> object, or any other custom object that meets this
interface.
This function automatically determines whether the data stream was written in
binary mode or not.
</dd></dl>

<dl class="function">
<dt id="pickle.dumps">
<code class="descclassname">pickle.</code><code class="descname">dumps</code>(obj[, protocol])<a class="headerlink" href="#pickle.dumps" title="Permalink to this definition">¶</a></dt>
<dd>Return the pickled representation of the object as a string, instead of writing
it to a file.
If the protocol parameter is omitted, protocol 0 is used. If protocol is
specified as a negative value or <a class="reference internal" href="#pickle.HIGHEST_PROTOCOL" title="pickle.HIGHEST_PROTOCOL"><code class="xref py py-const docutils literal notranslate">HIGHEST_PROTOCOL</code></a>, the highest protocol
version will be used.
<div class="versionchanged">
Changed in version 2.3: The protocol parameter was added.
</div>
</dd></dl>

<dl class="function">
<dt id="pickle.loads">
<code class="descclassname">pickle.</code><code class="descname">loads</code>(string)<a class="headerlink" href="#pickle.loads" title="Permalink to this definition">¶</a></dt>
<dd>Read a pickled object hierarchy from a string. Characters in the string past
the pickled object’s representation are ignored.
</dd></dl>

<dl class="exception">
<dt id="pickle.PicklingError">
exception <code class="descclassname">pickle.</code><code class="descname">PicklingError</code><a class="headerlink" href="#pickle.PicklingError" title="Permalink to this definition">¶</a></dt>
<dd>This exception is raised when an unpicklable object is passed to the
<a class="reference internal" href="#pickle.dump" title="pickle.dump"><code class="xref py py-meth docutils literal notranslate">dump()</code></a> method.
</dd></dl>

<dl class="exception">
<dt id="pickle.UnpicklingError">
exception <code class="descclassname">pickle.</code><code class="descname">UnpicklingError</code><a class="headerlink" href="#pickle.UnpicklingError" title="Permalink to this definition">¶</a></dt>
<dd>This exception is raised when there is a problem unpickling an object. Note that
other exceptions may also be raised during unpickling, including (but not
necessarily limited to) <a class="reference internal" href="exceptions.html#exceptions.AttributeError" title="exceptions.AttributeError"><code class="xref py py-exc docutils literal notranslate">AttributeError</code></a>, <a class="reference internal" href="exceptions.html#exceptions.EOFError" title="exceptions.EOFError"><code class="xref py py-exc docutils literal notranslate">EOFError</code></a>,
<a class="reference internal" href="exceptions.html#exceptions.ImportError" title="exceptions.ImportError"><code class="xref py py-exc docutils literal notranslate">ImportError</code></a>, and <a class="reference internal" href="exceptions.html#exceptions.IndexError" title="exceptions.IndexError"><code class="xref py py-exc docutils literal notranslate">IndexError</code></a>.
</dd></dl>

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">pickle</code></a> module also exports two callables <a class="footnote-reference" href="#id12" id="id2">[2]</a>, <a class="reference internal" href="#pickle.Pickler" title="pickle.Pickler"><code class="xref py py-class docutils literal notranslate">Pickler</code></a> and
<a class="reference internal" href="#pickle.Unpickler" title="pickle.Unpickler"><code class="xref py py-class docutils literal notranslate">Unpickler</code></a>:
<dl class="class">
<dt id="pickle.Pickler">
class <code class="descclassname">pickle.</code><code class="descname">Pickler</code>(file[, protocol])<a class="headerlink" href="#pickle.Pickler" title="Permalink to this definition">¶</a></dt>
<dd>This takes a file-like object to which it will write a pickle data stream.
If the protocol parameter is omitted, protocol 0 is used. If protocol is
specified as a negative value or <a class="reference internal" href="#pickle.HIGHEST_PROTOCOL" title="pickle.HIGHEST_PROTOCOL"><code class="xref py py-const docutils literal notranslate">HIGHEST_PROTOCOL</code></a>, the highest
protocol version will be used.
<div class="versionchanged">
Changed in version 2.3: Introduced the protocol parameter.
</div>
file must have a <code class="xref py py-meth docutils literal notranslate">write()</code> method that accepts a single string argument.
It can thus be an open file object, a <a class="reference internal" href="stringio.html#module-StringIO" title="StringIO: Read and write strings as if they were files."><code class="xref py py-mod docutils literal notranslate">StringIO</code></a> object, or any other
custom object that meets this interface.
<a class="reference internal" href="#pickle.Pickler" title="pickle.Pickler"><code class="xref py py-class docutils literal notranslate">Pickler</code></a> objects define one (or two) public methods:
<dl class="method">
<dt id="pickle.Pickler.dump">
<code class="descname">dump</code>(obj)<a class="headerlink" href="#pickle.Pickler.dump" title="Permalink to this definition">¶</a></dt>
<dd>Write a pickled representation of obj to the open file object given in the
constructor. Either the binary or ASCII format will be used, depending on the
value of the protocol argument passed to the constructor.
</dd></dl>

<dl class="method">
<dt id="pickle.Pickler.clear_memo">
<code class="descname">clear_memo</code>()<a class="headerlink" href="#pickle.Pickler.clear_memo" title="Permalink to this definition">¶</a></dt>
<dd>Clears the pickler’s “memo”. The memo is the data structure that remembers
which objects the pickler has already seen, so that shared or recursive objects
pickled by reference and not by value. This method is useful when re-using
picklers.
<div class="admonition note">
Note
Prior to Python 2.3, <a class="reference internal" href="#pickle.Pickler.clear_memo" title="pickle.Pickler.clear_memo"><code class="xref py py-meth docutils literal notranslate">clear_memo()</code></a> was only available on the picklers
created by <a class="reference internal" href="#module-cPickle" title="cPickle: Faster version of pickle, but not subclassable."><code class="xref py py-mod docutils literal notranslate">cPickle</code></a>. In 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">pickle</code></a> module, picklers have an
instance variable called <code class="xref py py-attr docutils literal notranslate">memo</code> which is a Python dictionary. So to clear
the memo for a <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">pickle</code></a> module pickler, you could do the following:
<div class="highlight-default notranslate"><div class="highlight"><pre>mypickler.memo.clear()
</pre></div>
</div>
Code that does not need to support older versions of Python should simply use
<a class="reference internal" href="#pickle.Pickler.clear_memo" title="pickle.Pickler.clear_memo"><code class="xref py py-meth docutils literal notranslate">clear_memo()</code></a>.
</div>
</dd></dl>

</dd></dl>

It is possible to make multiple calls to the <a class="reference internal" href="#pickle.dump" title="pickle.dump"><code class="xref py py-meth docutils literal notranslate">dump()</code></a> method of the same
<a class="reference internal" href="#pickle.Pickler" title="pickle.Pickler"><code class="xref py py-class docutils literal notranslate">Pickler</code></a> instance. These must then be matched to the same number of
calls to the <a class="reference internal" href="#pickle.load" title="pickle.load"><code class="xref py py-meth docutils literal notranslate">load()</code></a> method of the corresponding <a class="reference internal" href="#pickle.Unpickler" title="pickle.Unpickler"><code class="xref py py-class docutils literal notranslate">Unpickler</code></a>
instance. If the same object is pickled by multiple <a class="reference internal" href="#pickle.dump" title="pickle.dump"><code class="xref py py-meth docutils literal notranslate">dump()</code></a> calls, the
<a class="reference internal" href="#pickle.load" title="pickle.load"><code class="xref py py-meth docutils literal notranslate">load()</code></a> will all yield references to the same object. <a class="footnote-reference" href="#id13" id="id3">[3]</a>
<a class="reference internal" href="#pickle.Unpickler" title="pickle.Unpickler"><code class="xref py py-class docutils literal notranslate">Unpickler</code></a> objects are defined as:
<dl class="class">
<dt id="pickle.Unpickler">
class <code class="descclassname">pickle.</code><code class="descname">Unpickler</code>(file)<a class="headerlink" href="#pickle.Unpickler" title="Permalink to this definition">¶</a></dt>
<dd>This takes a file-like object from which it will read a pickle data stream.
This class automatically determines whether the data stream was written in
binary mode or not, so it does not need a flag as in the <a class="reference internal" href="#pickle.Pickler" title="pickle.Pickler"><code class="xref py py-class docutils literal notranslate">Pickler</code></a>
factory.
file must have two methods, a <code class="xref py py-meth docutils literal notranslate">read()</code> method that takes an integer
argument, and a <a class="reference internal" href="readline.html#module-readline" title="readline: GNU readline support for Python. (Unix)"><code class="xref py py-meth docutils literal notranslate">readline()</code></a> method that requires no arguments. Both
methods should return a string. Thus file can be a file object opened for
reading, a <a class="reference internal" href="stringio.html#module-StringIO" title="StringIO: Read and write strings as if they were files."><code class="xref py py-mod docutils literal notranslate">StringIO</code></a> object, or any other custom object that meets this
interface.
<a class="reference internal" href="#pickle.Unpickler" title="pickle.Unpickler"><code class="xref py py-class docutils literal notranslate">Unpickler</code></a> objects have one (or two) public methods:
<dl class="method">
<dt id="pickle.Unpickler.load">
<code class="descname">load</code>()<a class="headerlink" href="#pickle.Unpickler.load" title="Permalink to this definition">¶</a></dt>
<dd>Read a pickled object representation from the open file object given in
the constructor, and return the reconstituted object hierarchy specified
therein.
This method automatically determines whether the data stream was written
in binary mode or not.
</dd></dl>

<dl class="method">
<dt id="pickle.Unpickler.noload">
<code class="descname">noload</code>()<a class="headerlink" href="#pickle.Unpickler.noload" title="Permalink to this definition">¶</a></dt>
<dd>This is just like <a class="reference internal" href="#pickle.load" title="pickle.load"><code class="xref py py-meth docutils literal notranslate">load()</code></a> except that it doesn’t actually create any
objects. This is useful primarily for finding what’s called “persistent
ids” that may be referenced in a pickle data stream. See section
<a class="reference internal" href="#pickle-protocol">The pickle protocol</a> below for more details.
Note: the <a class="reference internal" href="#pickle.Unpickler.noload" title="pickle.Unpickler.noload"><code class="xref py py-meth docutils literal notranslate">noload()</code></a> method is currently only available on
<a class="reference internal" href="#pickle.Unpickler" title="pickle.Unpickler"><code class="xref py py-class docutils literal notranslate">Unpickler</code></a> objects created with the <a class="reference internal" href="#module-cPickle" title="cPickle: Faster version of pickle, but not subclassable."><code class="xref py py-mod docutils literal notranslate">cPickle</code></a> module.
<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">pickle</code></a> module <a class="reference internal" href="#pickle.Unpickler" title="pickle.Unpickler"><code class="xref py py-class docutils literal notranslate">Unpickler</code></a>s do not have the <a class="reference internal" href="#pickle.Unpickler.noload" title="pickle.Unpickler.noload"><code class="xref py py-meth docutils literal notranslate">noload()</code></a>
method.
</dd></dl>

</dd></dl>

</div>
<div class="section" id="what-can-be-pickled-and-unpickled">
<h2>11.1.4. What can be pickled and unpickled?<a class="headerlink" href="#what-can-be-pickled-and-unpickled" title="Permalink to this headline">¶</a></h2>
The following types can be pickled:
<ul class="simple">
<li><code class="docutils literal notranslate">None</code>, <code class="docutils literal notranslate">True</code>, and <code class="docutils literal notranslate">False</code></li>
<li>integers, long integers, floating point numbers, complex numbers</li>
<li>normal and Unicode strings</li>
<li>tuples, lists, sets, and dictionaries containing only picklable objects</li>
<li>functions defined at the top level of a module</li>
<li>built-in functions defined at the top level of a module</li>
<li>classes that are defined at the top level of a module</li>
<li>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">__dict__</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">__getstate__()</code></a> is picklable (see section <a class="reference internal" href="#pickle-protocol">The pickle protocol</a>
for details).</li>
</ul>
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">PicklingError</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#exceptions.RuntimeError" title="exceptions.RuntimeError"><code class="xref py py-exc docutils literal notranslate">RuntimeError</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">sys.setrecursionlimit()</code></a>.
Note that functions (built-in and user-defined) are pickled by “fully qualified”
name reference, not by value. 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" href="#id14" id="id4">[4]</a>
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">attr</code> is not
restored in the unpickling environment:
<div class="highlight-default notranslate"><div class="highlight"><pre>class Foo:
 attr = &#39;a class attr&#39;

picklestring = pickle.dumps(Foo)
</pre></div>
</div>
These restrictions are why picklable functions and classes must be defined in
the top level of a module.
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">__setstate__()</code></a> method.
</div>
<div class="section" id="the-pickle-protocol">
<h2>11.1.5. The pickle protocol<a class="headerlink" href="#the-pickle-protocol" title="Permalink to this headline">¶</a></h2>
This section describes the “pickling protocol” that defines the interface
between the pickler/unpickler and the objects that are being serialized. This
protocol provides a standard way for you to define, customize, and control how
your objects are serialized and de-serialized. The description in this section
doesn’t cover specific customizations that you can employ to make the unpickling
environment slightly safer from untrusted pickle data streams; see section
<a class="reference internal" href="#pickle-sub">Subclassing Unpicklers</a> for more details.
<div class="section" id="pickling-and-unpickling-normal-class-instances">
<h3>11.1.5.1. Pickling and unpickling normal class instances<a class="headerlink" href="#pickling-and-unpickling-normal-class-instances" title="Permalink to this headline">¶</a></h3>
<dl class="method">
<dt id="object.__getinitargs__">
<code class="descclassname">object.</code><code class="descname">__getinitargs__</code>()<a class="headerlink" href="#object.__getinitargs__" title="Permalink to this definition">¶</a></dt>
<dd>When a pickled 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">__init__()</code></a> method is
normally not invoked. If it is desirable that the <a class="reference internal" href="../reference/datamodel.html#object.__init__" title="object.__init__"><code class="xref py py-meth docutils literal notranslate">__init__()</code></a> method
be called on unpickling, an old-style class can define a method
<a class="reference internal" href="#object.__getinitargs__" title="object.__getinitargs__"><code class="xref py py-meth docutils literal notranslate">__getinitargs__()</code></a>, which should return a tuple of positional
arguments to be passed to the class constructor (<a class="reference internal" href="../reference/datamodel.html#object.__init__" title="object.__init__"><code class="xref py py-meth docutils literal notranslate">__init__()</code></a> for
example). Keyword arguments are not supported. The <a class="reference internal" href="#object.__getinitargs__" title="object.__getinitargs__"><code class="xref py py-meth docutils literal notranslate">__getinitargs__()</code></a>
method is called at pickle time; the tuple it returns is incorporated in the
pickle for the instance.
</dd></dl>

<dl class="method">
<dt id="object.__getnewargs__">
<code class="descclassname">object.</code><code class="descname">__getnewargs__</code>()<a class="headerlink" href="#object.__getnewargs__" title="Permalink to this definition">¶</a></dt>
<dd>New-style types can provide a <a class="reference internal" href="#object.__getnewargs__" title="object.__getnewargs__"><code class="xref py py-meth docutils literal notranslate">__getnewargs__()</code></a> method that is used for
protocol 2. Implementing this method is needed if the type establishes some
internal invariants when the instance is created, or if the memory allocation
is affected by 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">__new__()</code></a> method for the type
(as it is for tuples and strings). Instances of a <a class="reference internal" href="../glossary.html#term-new-style-class">new-style class</a>
<code class="docutils literal notranslate">C</code> are created using
<div class="highlight-default notranslate"><div class="highlight"><pre>obj = C.__new__(C, *args)
</pre></div>
</div>
where args is the result of calling <a class="reference internal" href="#object.__getnewargs__" title="object.__getnewargs__"><code class="xref py py-meth docutils literal notranslate">__getnewargs__()</code></a> on the original
object; if there is no <a class="reference internal" href="#object.__getnewargs__" title="object.__getnewargs__"><code class="xref py py-meth docutils literal notranslate">__getnewargs__()</code></a>, an empty tuple is assumed.
</dd></dl>

<dl class="method">
<dt id="object.__getstate__">
<code class="descclassname">object.</code><code class="descname">__getstate__</code>()<a class="headerlink" href="#object.__getstate__" title="Permalink to this definition">¶</a></dt>
<dd>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">__getstate__()</code></a>, it is called and the return state is
pickled as the contents for the instance, instead of the contents of the
instance’s dictionary. If there is no <a class="reference internal" href="#object.__getstate__" title="object.__getstate__"><code class="xref py py-meth docutils literal notranslate">__getstate__()</code></a> method, the
instance’s <a class="reference internal" href="stdtypes.html#object.__dict__" title="object.__dict__"><code class="xref py py-attr docutils literal notranslate">__dict__</code></a> is pickled.
</dd></dl>

<dl class="method">
<dt id="object.__setstate__">
<code class="descclassname">object.</code><code class="descname">__setstate__</code>(state)<a class="headerlink" href="#object.__setstate__" title="Permalink to this definition">¶</a></dt>
<dd>Upon unpickling, if the class also defines the method <a class="reference internal" href="#object.__setstate__" title="object.__setstate__"><code class="xref py py-meth docutils literal notranslate">__setstate__()</code></a>,
it is called with the unpickled state. <a class="footnote-reference" href="#id15" id="id5">[5]</a> If there is no
<a class="reference internal" href="#object.__setstate__" title="object.__setstate__"><code class="xref py py-meth docutils literal notranslate">__setstate__()</code></a> method, the pickled state must be a dictionary and its
items are assigned to the new instance’s dictionary. If a class defines both
<a class="reference internal" href="#object.__getstate__" title="object.__getstate__"><code class="xref py py-meth docutils literal notranslate">__getstate__()</code></a> and <a class="reference internal" href="#object.__setstate__" title="object.__setstate__"><code class="xref py py-meth docutils literal notranslate">__setstate__()</code></a>, the state object needn’t be a
dictionary and these methods can do what they want. <a class="footnote-reference" href="#id16" id="id6">[6]</a>
<div class="admonition note">
Note
For <a class="reference internal" href="../glossary.html#term-new-style-class">new-style class</a>es, if <a class="reference internal" href="#object.__getstate__" title="object.__getstate__"><code class="xref py py-meth docutils literal notranslate">__getstate__()</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">__setstate__()</code></a> method will not be called.
</div>
</dd></dl>

<div class="admonition note">
Note
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">__getattr__()</code></a>,
<a class="reference internal" href="../reference/datamodel.html#object.__getattribute__" title="object.__getattribute__"><code class="xref py py-meth docutils literal notranslate">__getattribute__()</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">__setattr__()</code></a> may be called upon the
instance. In case those methods rely on some internal invariant being
true, the type should implement either <a class="reference internal" href="#object.__getinitargs__" title="object.__getinitargs__"><code class="xref py py-meth docutils literal notranslate">__getinitargs__()</code></a> or
<a class="reference internal" href="#object.__getnewargs__" title="object.__getnewargs__"><code class="xref py py-meth docutils literal notranslate">__getnewargs__()</code></a> to establish such an invariant; otherwise, neither
<a class="reference internal" href="../reference/datamodel.html#object.__new__" title="object.__new__"><code class="xref py py-meth docutils literal notranslate">__new__()</code></a> nor <a class="reference internal" href="../reference/datamodel.html#object.__init__" title="object.__init__"><code class="xref py py-meth docutils literal notranslate">__init__()</code></a> will be called.
</div>
</div>
<div class="section" id="pickling-and-unpickling-extension-types">
<h3>11.1.5.2. Pickling and unpickling extension types<a class="headerlink" href="#pickling-and-unpickling-extension-types" title="Permalink to this headline">¶</a></h3>
<dl class="method">
<dt id="object.__reduce__">
<code class="descclassname">object.</code><code class="descname">__reduce__</code>()<a class="headerlink" href="#object.__reduce__" title="Permalink to this definition">¶</a></dt>
<dd>When the <code class="xref py py-class docutils literal notranslate">Pickler</code> encounters an object of a type it knows nothing
about — such as an extension type — it looks in two places for a hint of
how to pickle it. One alternative is for the object to implement a
<a class="reference internal" href="#object.__reduce__" title="object.__reduce__"><code class="xref py py-meth docutils literal notranslate">__reduce__()</code></a> method. If provided, at pickling time <a class="reference internal" href="#object.__reduce__" title="object.__reduce__"><code class="xref py py-meth docutils literal notranslate">__reduce__()</code></a>
will be called with no arguments, and it must return either a string or a
tuple.
If a string is returned, it names a global variable whose contents are
pickled as normal. The string returned by <a class="reference internal" href="#object.__reduce__" title="object.__reduce__"><code class="xref py py-meth docutils literal notranslate">__reduce__()</code></a> should be the
object’s local name relative to its module; the pickle module searches the
module namespace to determine the object’s module.
When a tuple is returned, it must be between two and five elements long.
Optional elements can either be omitted, or <code class="docutils literal notranslate">None</code> can be provided as their
value. The contents of this tuple are pickled as normal and used to
reconstruct the object at unpickling time. The semantics of each element
are:
<ul>
<li>A callable object that will be called to create the initial version of the
object. The next element of the tuple will provide arguments for this
callable, and later elements provide additional state information that will
subsequently be used to fully reconstruct the pickled data.
In the unpickling environment this object must be either a class, a
callable registered as a “safe constructor” (see below), or it must have an
attribute <code class="xref py py-attr docutils literal notranslate">__safe_for_unpickling__</code> with a true value. Otherwise, an
<code class="xref py py-exc docutils literal notranslate">UnpicklingError</code> will be raised in the unpickling environment. Note
that as usual, the callable itself is pickled by name.
</li>
<li>A tuple of arguments for the callable object.
<div class="versionchanged">
Changed in version 2.5: Formerly, this argument could also be <code class="docutils literal notranslate">None</code>.
</div>
</li>
<li>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">__setstate__()</code></a> method as described in section <a class="reference internal" href="#pickle-inst">Pickling and unpickling normal class instances</a>. If
the object has no <a class="reference internal" href="#object.__setstate__" title="object.__setstate__"><code class="xref py py-meth docutils literal notranslate">__setstate__()</code></a> method, then, as above, 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">__dict__</code></a>.
</li>
<li>Optionally, an iterator (and not a sequence) yielding successive list
items. These list items will be pickled, and appended to the object using
either <code class="docutils literal notranslate">obj.append(item)</code> or <code class="docutils literal notranslate">obj.extend(list_of_items)</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">append()</code> and <code class="xref py py-meth docutils literal notranslate">extend()</code> methods with the
appropriate signature. (Whether <code class="xref py py-meth docutils literal notranslate">append()</code> or <code class="xref py py-meth docutils literal notranslate">extend()</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.)
</li>
<li>Optionally, an iterator (not a sequence) yielding successive dictionary
items, which should be tuples of the form <code class="docutils literal notranslate">(key, value)</code>. These items
will be pickled and stored to the object using <code class="docutils literal notranslate">obj[key] = value</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">__setitem__()</code></a>.
</li>
</ul>
</dd></dl>

<dl class="method">
<dt id="object.__reduce_ex__">
<code class="descclassname">object.</code><code class="descname">__reduce_ex__</code>(protocol)<a class="headerlink" href="#object.__reduce_ex__" title="Permalink to this definition">¶</a></dt>
<dd>It is sometimes useful to know the protocol version when implementing
<a class="reference internal" href="#object.__reduce__" title="object.__reduce__"><code class="xref py py-meth docutils literal notranslate">__reduce__()</code></a>. This can be done by implementing a method named
<a class="reference internal" href="#object.__reduce_ex__" title="object.__reduce_ex__"><code class="xref py py-meth docutils literal notranslate">__reduce_ex__()</code></a> instead of <a class="reference internal" href="#object.__reduce__" title="object.__reduce__"><code class="xref py py-meth docutils literal notranslate">__reduce__()</code></a>. <a class="reference internal" href="#object.__reduce_ex__" title="object.__reduce_ex__"><code class="xref py py-meth docutils literal notranslate">__reduce_ex__()</code></a>,
when it exists, is called in preference over <a class="reference internal" href="#object.__reduce__" title="object.__reduce__"><code class="xref py py-meth docutils literal notranslate">__reduce__()</code></a> (you may
still provide <a class="reference internal" href="#object.__reduce__" title="object.__reduce__"><code class="xref py py-meth docutils literal notranslate">__reduce__()</code></a> for backwards compatibility). The
<a class="reference internal" href="#object.__reduce_ex__" title="object.__reduce_ex__"><code class="xref py py-meth docutils literal notranslate">__reduce_ex__()</code></a> method will be called with a single integer argument,
the protocol version.
The <a class="reference internal" href="functions.html#object" title="object"><code class="xref py py-class docutils literal notranslate">object</code></a> class implements both <a class="reference internal" href="#object.__reduce__" title="object.__reduce__"><code class="xref py py-meth docutils literal notranslate">__reduce__()</code></a> and
<a class="reference internal" href="#object.__reduce_ex__" title="object.__reduce_ex__"><code class="xref py py-meth docutils literal notranslate">__reduce_ex__()</code></a>; however, if a subclass overrides <a class="reference internal" href="#object.__reduce__" title="object.__reduce__"><code class="xref py py-meth docutils literal notranslate">__reduce__()</code></a>
but not <a class="reference internal" href="#object.__reduce_ex__" title="object.__reduce_ex__"><code class="xref py py-meth docutils literal notranslate">__reduce_ex__()</code></a>, the <a class="reference internal" href="#object.__reduce_ex__" title="object.__reduce_ex__"><code class="xref py py-meth docutils literal notranslate">__reduce_ex__()</code></a> implementation
detects this and calls <a class="reference internal" href="#object.__reduce__" title="object.__reduce__"><code class="xref py py-meth docutils literal notranslate">__reduce__()</code></a>.
</dd></dl>

An alternative to implementing a <a class="reference internal" href="#object.__reduce__" title="object.__reduce__"><code class="xref py py-meth docutils literal notranslate">__reduce__()</code></a> method on the object to be
pickled, is to register the callable with the <a class="reference internal" href="copy_reg.html#module-copy_reg" title="copy_reg: Register pickle support functions."><code class="xref py py-mod docutils literal notranslate">copy_reg</code></a> module. This
module provides a way for programs to register “reduction functions” and
constructors for user-defined types. Reduction functions have the same
semantics and interface as the <a class="reference internal" href="#object.__reduce__" title="object.__reduce__"><code class="xref py py-meth docutils literal notranslate">__reduce__()</code></a> method described above, except
that they are called with a single argument, the object to be pickled.
The registered constructor is deemed a “safe constructor” for purposes of
unpickling as described above.
</div>
<div class="section" id="pickling-and-unpickling-external-objects">
<h3>11.1.5.3. Pickling and unpickling external objects<a class="headerlink" href="#pickling-and-unpickling-external-objects" title="Permalink to this headline">¶</a></h3>
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">pickle</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 is just an arbitrary string
of printable ASCII characters. The resolution of such names 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">pickle</code></a> module; it will delegate this resolution to user defined
functions on the pickler and unpickler. <a class="footnote-reference" href="#id17" id="id7">[7]</a>
To define external persistent id resolution, you need to set the
<code class="xref py py-attr docutils literal notranslate">persistent_id</code> attribute of the pickler object and the
<code class="xref py py-attr docutils literal notranslate">persistent_load</code> attribute of the unpickler object.
To pickle objects that have an external persistent id, the pickler must have a
custom <code class="xref py py-func docutils literal notranslate">persistent_id()</code> method that takes an object as an
argument and returns either <code class="docutils literal notranslate">None</code> or the persistent id for that object.
When <code class="docutils literal notranslate">None</code> is returned, the pickler simply pickles the object as normal.
When a persistent id string is returned, the pickler will pickle that string,
along with a marker so that the unpickler will recognize the string as a
persistent id.
To unpickle external objects, the unpickler must have a custom
<code class="xref py py-func docutils literal notranslate">persistent_load()</code> function that takes a persistent id string
and returns the referenced object.
Here’s a silly example that might shed more light:
<div class="highlight-default notranslate"><div class="highlight"><pre>import pickle
from cStringIO import StringIO

src = StringIO()
p = pickle.Pickler(src)

def persistent_id(obj):
 if hasattr(obj, &#39;x&#39;):
 return &#39;the value %d&#39; % obj.x
 else:
 return None

p.persistent_id = persistent_id

class Integer:
 def __init__(self, x):
 self.x = x
 def __str__(self):
 return &#39;My name is integer %d&#39; % self.x

i = Integer(7)
print i
p.dump(i)

datastream = src.getvalue()
print repr(datastream)
dst = StringIO(datastream)

up = pickle.Unpickler(dst)

class FancyInteger(Integer):
 def __str__(self):
 return &#39;I am the integer %d&#39; % self.x

def persistent_load(persid):
 if persid.startswith(&#39;the value &#39;):
 value = int(persid.split()[2])
 return FancyInteger(value)
 else:
 raise pickle.UnpicklingError, &#39;Invalid persistent id&#39;

up.persistent_load = persistent_load

j = up.load()
print j
</pre></div>
</div>
In the <a class="reference internal" href="#module-cPickle" title="cPickle: Faster version of pickle, but not subclassable."><code class="xref py py-mod docutils literal notranslate">cPickle</code></a> module, the unpickler’s <code class="xref py py-attr docutils literal notranslate">persistent_load</code>
attribute can also be set to a Python list, in which case, when the unpickler
reaches a persistent id, the persistent id string will simply be appended to
this list. This functionality exists so that a pickle data stream can be
“sniffed” for object references without actually instantiating all the objects
in a pickle.
<a class="footnote-reference" href="#id18" id="id8">[8]</a> Setting <code class="xref py py-attr docutils literal notranslate">persistent_load</code> to a list is usually used in
conjunction with the <code class="xref py py-meth docutils literal notranslate">noload()</code> method on the Unpickler.
</div>
</div>
<div class="section" id="subclassing-unpicklers">
<h2>11.1.6. Subclassing Unpicklers<a class="headerlink" href="#subclassing-unpicklers" title="Permalink to this headline">¶</a></h2>
By default, unpickling will import any class that it finds in the pickle data.
You can control exactly what gets unpickled and what gets called by customizing
your unpickler. Unfortunately, exactly how you do this is different depending
on whether you’re using <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">pickle</code></a> or <a class="reference internal" href="#module-cPickle" title="cPickle: Faster version of pickle, but not subclassable."><code class="xref py py-mod docutils literal notranslate">cPickle</code></a>. <a class="footnote-reference" href="#id19" id="id9">[9]</a>
In 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">pickle</code></a> module, you need to derive a subclass from
<code class="xref py py-class docutils literal notranslate">Unpickler</code>, overriding the <code class="xref py py-meth docutils literal notranslate">load_global()</code> method.
<code class="xref py py-meth docutils literal notranslate">load_global()</code> should read two lines from the pickle data stream where the
first line will the name of the module containing the class and the second line
will be the name of the instance’s class. It then looks up the class, possibly
importing the module and digging out the attribute, then it appends what it
finds to the unpickler’s stack. Later on, this class will be assigned to the
<code class="xref py py-attr docutils literal notranslate">__class__</code> attribute of an empty class, as a way of magically creating an
instance without calling its class’s <a class="reference internal" href="../reference/datamodel.html#object.__init__" title="object.__init__"><code class="xref py py-meth docutils literal notranslate">__init__()</code></a>. Your job (should you
choose to accept it), would be to have <code class="xref py py-meth docutils literal notranslate">load_global()</code> push onto the
unpickler’s stack, a known safe version of any class you deem safe to unpickle.
It is up to you to produce such a class. Or you could raise an error if you
want to disallow all unpickling of instances. If this sounds like a hack,
you’re right. Refer to the source code to make this work.
Things are a little cleaner with <a class="reference internal" href="#module-cPickle" title="cPickle: Faster version of pickle, but not subclassable."><code class="xref py py-mod docutils literal notranslate">cPickle</code></a>, but not by much. To control
what gets unpickled, you can set the unpickler’s <code class="xref py py-attr docutils literal notranslate">find_global</code>
attribute to a function or <code class="docutils literal notranslate">None</code>. If it is <code class="docutils literal notranslate">None</code> then any attempts to
unpickle instances will raise an <code class="xref py py-exc docutils literal notranslate">UnpicklingError</code>. If it is a function,
then it should accept a module name and a class name, and return the
corresponding class object. It is responsible for looking up the class and
performing any necessary imports, and it may raise an error to prevent
instances of the class from being unpickled.
The moral of the story is that you should be really careful about the source of
the strings your application unpickles.
</div>
<div class="section" id="example">
<h2>11.1.7. Example<a class="headerlink" href="#example" title="Permalink to this headline">¶</a></h2>
For the simplest code, use the <code class="xref py py-func docutils literal notranslate">dump()</code> and <code class="xref py py-func docutils literal notranslate">load()</code> functions. Note
that a self-referencing list is pickled and restored correctly.
<div class="highlight-default notranslate"><div class="highlight"><pre>import pickle

data1 = {&#39;a&#39;: [1, 2.0, 3, 4+6j],
 &#39;b&#39;: (&#39;string&#39;, u&#39;Unicode string&#39;),
 &#39;c&#39;: None}

selfref_list = [1, 2, 3]
selfref_list.append(selfref_list)

output = open(&#39;data.pkl&#39;, &#39;wb&#39;)

# Pickle dictionary using protocol 0.
pickle.dump(data1, output)

# Pickle the list using the highest protocol available.
pickle.dump(selfref_list, output, -1)

output.close()
</pre></div>
</div>
The following example reads the resulting pickled data. When reading a
pickle-containing file, you should open the file in binary mode because you
can’t be sure if the ASCII or binary format was used.
<div class="highlight-default notranslate"><div class="highlight"><pre>import pprint, pickle

pkl_file = open(&#39;data.pkl&#39;, &#39;rb&#39;)

data1 = pickle.load(pkl_file)
pprint.pprint(data1)

data2 = pickle.load(pkl_file)
pprint.pprint(data2)

pkl_file.close()
</pre></div>
</div>
Here’s a larger example that shows how to modify pickling behavior for a class.
The <code class="xref py py-class docutils literal notranslate">TextReader</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">readline()</code> method is called. If a
<code class="xref py py-class docutils literal notranslate">TextReader</code> instance is pickled, all attributes except 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">__setstate__()</code></a> and
<a class="reference internal" href="#object.__getstate__" title="object.__getstate__"><code class="xref py py-meth docutils literal notranslate">__getstate__()</code></a> methods are used to implement this behavior.
<div class="highlight-default notranslate"><div class="highlight"><pre>#!/usr/local/bin/python

class TextReader:
 &quot;&quot;&quot;Print and number lines in a text file.&quot;&quot;&quot;
 def __init__(self, file):
 self.file = file
 self.fh = open(file)
 self.lineno = 0

def readline(self):
 self.lineno = self.lineno + 1
 line = self.fh.readline()
 if not line:
 return None
 if line.endswith(&quot;\n&quot;):
 line = line[:-1]
 return &quot;%d: %s&quot; % (self.lineno, line)

def __getstate__(self):
 odict = self.__dict__.copy() # copy the dict since we change it
 del odict[&#39;fh&#39;] # remove filehandle entry
 return odict

def __setstate__(self, dict):
 fh = open(dict[&#39;file&#39;]) # reopen file
 count = dict[&#39;lineno&#39;] # read from file...
 while count: # until line count is restored
 fh.readline()
 count = count - 1
 self.__dict__.update(dict) # update attributes
 self.fh = fh # save the file object
</pre></div>
</div>
A sample usage might be something like this:
<div class="highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; import TextReader
&gt;&gt;&gt; obj = TextReader.TextReader(&quot;TextReader.py&quot;)
&gt;&gt;&gt; obj.readline()
&#39;1: #!/usr/local/bin/python&#39;
&gt;&gt;&gt; obj.readline()
&#39;2: &#39;
&gt;&gt;&gt; obj.readline()
&#39;3: class TextReader:&#39;
&gt;&gt;&gt; import pickle
&gt;&gt;&gt; pickle.dump(obj, open(&#39;save.p&#39;, &#39;wb&#39;))
</pre></div>
</div>
If you want to see that <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">pickle</code></a> works across Python processes, start
another Python session, before continuing. What follows can happen from either
the same process or a new process.
<div class="highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; import pickle
&gt;&gt;&gt; reader = pickle.load(open(&#39;save.p&#39;, &#39;rb&#39;))
&gt;&gt;&gt; reader.readline()
&#39;4: &quot;&quot;&quot;Print and number lines in a text file.&quot;&quot;&quot;&#39;
</pre></div>
</div>
<div class="admonition seealso">
See also
<dl class="last docutils">
<dt>Module <a class="reference internal" href="copy_reg.html#module-copy_reg" title="copy_reg: Register pickle support functions."><code class="xref py py-mod docutils literal notranslate">copy_reg</code></a></dt>
<dd>Pickle interface constructor registration for extension types.</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">shelve</code></a></dt>
<dd>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">pickle</code></a>.</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">copy</code></a></dt>
<dd>Shallow and deep object copying.</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">marshal</code></a></dt>
<dd>High-performance serialization of built-in types.</dd>
</dl>
</div>
</div>
</div>
<div class="section" id="module-cPickle">
<h1>11.2. <a class="reference internal" href="#module-cPickle" title="cPickle: Faster version of pickle, but not subclassable."><code class="xref py py-mod docutils literal notranslate">cPickle</code></a> — A faster <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">pickle</code></a><a class="headerlink" href="#module-cPickle" title="Permalink to this headline">¶</a></h1>
The <a class="reference internal" href="#module-cPickle" title="cPickle: Faster version of pickle, but not subclassable."><code class="xref py py-mod docutils literal notranslate">cPickle</code></a> module supports serialization and de-serialization of Python
objects, providing an interface and functionality nearly identical to 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">pickle</code></a> module. There are several differences, the most important being
performance and subclassability.
First, <a class="reference internal" href="#module-cPickle" title="cPickle: Faster version of pickle, but not subclassable."><code class="xref py py-mod docutils literal notranslate">cPickle</code></a> can be up to 1000 times faster than <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">pickle</code></a> because
the former is implemented in C. Second, in the <a class="reference internal" href="#module-cPickle" title="cPickle: Faster version of pickle, but not subclassable."><code class="xref py py-mod docutils literal notranslate">cPickle</code></a> module the
callables <code class="xref py py-func docutils literal notranslate">Pickler()</code> and <code class="xref py py-func docutils literal notranslate">Unpickler()</code> are functions, not classes.
This means that you cannot use them to derive custom pickling and unpickling
subclasses. Most applications have no need for this functionality and should
benefit from the greatly improved performance of the <a class="reference internal" href="#module-cPickle" title="cPickle: Faster version of pickle, but not subclassable."><code class="xref py py-mod docutils literal notranslate">cPickle</code></a> module.
The pickle data stream produced 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">pickle</code></a> and <a class="reference internal" href="#module-cPickle" title="cPickle: Faster version of pickle, but not subclassable."><code class="xref py py-mod docutils literal notranslate">cPickle</code></a> are
identical, so it is possible to use <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">pickle</code></a> and <a class="reference internal" href="#module-cPickle" title="cPickle: Faster version of pickle, but not subclassable."><code class="xref py py-mod docutils literal notranslate">cPickle</code></a>
interchangeably with existing pickles. <a class="footnote-reference" href="#id20" id="id10">[10]</a>
There are additional minor differences in API between <a class="reference internal" href="#module-cPickle" title="cPickle: Faster version of pickle, but not subclassable."><code class="xref py py-mod docutils literal notranslate">cPickle</code></a> and
<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">pickle</code></a>, however for most applications, they are interchangeable. More
documentation is provided in 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">pickle</code></a> module documentation, which
includes a list of the documented differences.
Footnotes
<table class="docutils footnote" frame="void" id="id11" rules="none">
<colgroup><col class="label" /><col /></colgroup>
<tbody valign="top">
<tr><td class="label"><a class="fn-backref" href="#id1">[1]</a></td><td>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">marshal</code></a> module</td></tr>
</tbody>
</table>
<table class="docutils footnote" frame="void" id="id12" rules="none">
<colgroup><col class="label" /><col /></colgroup>
<tbody valign="top">
<tr><td class="label"><a class="fn-backref" href="#id2">[2]</a></td><td>In 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">pickle</code></a> module these callables are classes, which you could
subclass to customize the behavior. However, in the <a class="reference internal" href="#module-cPickle" title="cPickle: Faster version of pickle, but not subclassable."><code class="xref py py-mod docutils literal notranslate">cPickle</code></a> module these
callables are factory functions and so cannot be subclassed. One common reason
to subclass is to control what objects can actually be unpickled. See section
<a class="reference internal" href="#pickle-sub">Subclassing Unpicklers</a> for more details.</td></tr>
</tbody>
</table>
<table class="docutils footnote" frame="void" id="id13" rules="none">
<colgroup><col class="label" /><col /></colgroup>
<tbody valign="top">
<tr><td class="label"><a class="fn-backref" href="#id3">[3]</a></td><td>Warning: this is intended for pickling multiple objects without intervening
modifications to the objects or their parts. If you modify an object and then
pickle it again using the same <code class="xref py py-class docutils literal notranslate">Pickler</code> instance, the object is not
pickled again — a reference to it is pickled and the <code class="xref py py-class docutils literal notranslate">Unpickler</code> will
return the old value, not the modified one. There are two problems here: (1)
detecting changes, and (2) marshalling a minimal set of changes. Garbage
Collection may also become a problem here.</td></tr>
</tbody>
</table>
<table class="docutils footnote" frame="void" id="id14" rules="none">
<colgroup><col class="label" /><col /></colgroup>
<tbody valign="top">
<tr><td class="label"><a class="fn-backref" href="#id4">[4]</a></td><td>The exception raised will likely be an <a class="reference internal" href="exceptions.html#exceptions.ImportError" title="exceptions.ImportError"><code class="xref py py-exc docutils literal notranslate">ImportError</code></a> or an
<a class="reference internal" href="exceptions.html#exceptions.AttributeError" title="exceptions.AttributeError"><code class="xref py py-exc docutils literal notranslate">AttributeError</code></a> but it could be something else.</td></tr>
</tbody>
</table>
<table class="docutils footnote" frame="void" id="id15" rules="none">
<colgroup><col class="label" /><col /></colgroup>
<tbody valign="top">
<tr><td class="label"><a class="fn-backref" href="#id5">[5]</a></td><td>These methods can also be used to implement copying class instances.</td></tr>
</tbody>
</table>
<table class="docutils footnote" frame="void" id="id16" rules="none">
<colgroup><col class="label" /><col /></colgroup>
<tbody valign="top">
<tr><td class="label"><a class="fn-backref" href="#id6">[6]</a></td><td>This protocol is also used by the shallow and deep copying operations defined in
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">copy</code></a> module.</td></tr>
</tbody>
</table>
<table class="docutils footnote" frame="void" id="id17" rules="none">
<colgroup><col class="label" /><col /></colgroup>
<tbody valign="top">
<tr><td class="label"><a class="fn-backref" href="#id7">[7]</a></td><td>The actual mechanism for associating these user defined functions is slightly
different for <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">pickle</code></a> and <a class="reference internal" href="#module-cPickle" title="cPickle: Faster version of pickle, but not subclassable."><code class="xref py py-mod docutils literal notranslate">cPickle</code></a>. The description given here
works the same for both implementations. Users of 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">pickle</code></a> module
could also use subclassing to effect the same results, overriding the
<code class="xref py py-meth docutils literal notranslate">persistent_id()</code> and <code class="xref py py-meth docutils literal notranslate">persistent_load()</code> methods in the derived
classes.</td></tr>
</tbody>
</table>
<table class="docutils footnote" frame="void" id="id18" rules="none">
<colgroup><col class="label" /><col /></colgroup>
<tbody valign="top">
<tr><td class="label"><a class="fn-backref" href="#id8">[8]</a></td><td>We’ll leave you with the image of Guido and Jim sitting around sniffing pickles
in their living rooms.</td></tr>
</tbody>
</table>
<table class="docutils footnote" frame="void" id="id19" rules="none">
<colgroup><col class="label" /><col /></colgroup>
<tbody valign="top">
<tr><td class="label"><a class="fn-backref" href="#id9">[9]</a></td><td>A word of caution: the mechanisms described here use internal attributes and
methods, which are subject to change in future versions of Python. We intend to
someday provide a common interface for controlling this behavior, which will
work in either <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">pickle</code></a> or <a class="reference internal" href="#module-cPickle" title="cPickle: Faster version of pickle, but not subclassable."><code class="xref py py-mod docutils literal notranslate">cPickle</code></a>.</td></tr>
</tbody>
</table>
<table class="docutils footnote" frame="void" id="id20" rules="none">
<colgroup><col class="label" /><col /></colgroup>
<tbody valign="top">
<tr><td class="label"><a class="fn-backref" href="#id10">[10]</a></td><td>Since the pickle data format is actually a tiny stack-oriented programming
language, and some freedom is taken in the encodings of certain objects, it is
possible that the two modules produce different data streams for the same input
objects. However it is guaranteed that they will always be able to read each
other’s data streams.</td></tr>
</tbody>
</table>
</div>

</div>
 </div>
 </div>
 <div class="sphinxsidebar" role="navigation" aria-label="main navigation">
 <div class="sphinxsidebarwrapper">
 <h3><a href="../contents.html">Table Of Contents</a></h3>
 <ul>
<li><a class="reference internal" href="#">11.1. <code class="docutils literal notranslate">pickle</code> — Python object serialization</a><ul>
<li><a class="reference internal" href="#relationship-to-other-python-modules">11.1.1. Relationship to other Python modules</a></li>
<li><a class="reference internal" href="#data-stream-format">11.1.2. Data stream format</a></li>
<li><a class="reference internal" href="#usage">11.1.3. Usage</a></li>
<li><a class="reference internal" href="#what-can-be-pickled-and-unpickled">11.1.4. What can be pickled and unpickled?</a></li>
<li><a class="reference internal" href="#the-pickle-protocol">11.1.5. The pickle protocol</a><ul>
<li><a class="reference internal" href="#pickling-and-unpickling-normal-class-instances">11.1.5.1. Pickling and unpickling normal class instances</a></li>
<li><a class="reference internal" href="#pickling-and-unpickling-extension-types">11.1.5.2. Pickling and unpickling extension types</a></li>
<li><a class="reference internal" href="#pickling-and-unpickling-external-objects">11.1.5.3. Pickling and unpickling external objects</a></li>
</ul>
</li>
<li><a class="reference internal" href="#subclassing-unpicklers">11.1.6. Subclassing Unpicklers</a></li>
<li><a class="reference internal" href="#example">11.1.7. Example</a></li>
</ul>
</li>
<li><a class="reference internal" href="#module-cPickle">11.2. <code class="docutils literal notranslate">cPickle</code> — A faster <code class="docutils literal notranslate">pickle</code></a></li>
</ul>

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 title="next chapter">11.3. <code class="docutils literal notranslate">copy_reg</code> — Register <code class="docutils literal notranslate">pickle</code> support functions</a>
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