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  <div class="section" id="module-copy">
<span id="copy-shallow-and-deep-copy-operations"></span><h1>8.17. <a class="reference internal" href="#module-copy" title="copy: Shallow and deep copy operations."><tt class="xref py py-mod docutils literal"><span class="pre">copy</span></tt></a> &#8212; Shallow and deep copy operations<a class="headerlink" href="#module-copy" title="Permalink to this headline">¶</a></h1>
<p>Assignment statements in Python do not copy objects, they create bindings
between a target and an object. For collections that are mutable or contain
mutable items, a copy is sometimes needed so one can change one copy without
changing the other. This module provides generic shallow and deep copy
operations (explained below).</p>
<p>Interface summary:</p>
<dl class="function">
<dt id="copy.copy">
<tt class="descclassname">copy.</tt><tt class="descname">copy</tt><big>(</big><em>x</em><big>)</big><a class="headerlink" href="#copy.copy" title="Permalink to this definition">¶</a></dt>
<dd><p>Return a shallow copy of <em>x</em>.</p>
</dd></dl>

<dl class="function">
<dt id="copy.deepcopy">
<tt class="descclassname">copy.</tt><tt class="descname">deepcopy</tt><big>(</big><em>x</em><big>)</big><a class="headerlink" href="#copy.deepcopy" title="Permalink to this definition">¶</a></dt>
<dd><p>Return a deep copy of <em>x</em>.</p>
</dd></dl>

<dl class="exception">
<dt id="copy.error">
<em class="property">exception </em><tt class="descclassname">copy.</tt><tt class="descname">error</tt><a class="headerlink" href="#copy.error" title="Permalink to this definition">¶</a></dt>
<dd><p>Raised for module specific errors.</p>
</dd></dl>

<p>The difference between shallow and deep copying is only relevant for compound
objects (objects that contain other objects, like lists or class instances):</p>
<ul class="simple">
<li>A <em>shallow copy</em> constructs a new compound object and then (to the extent
possible) inserts <em>references</em> into it to the objects found in the original.</li>
<li>A <em>deep copy</em> constructs a new compound object and then, recursively, inserts
<em>copies</em> into it of the objects found in the original.</li>
</ul>
<p>Two problems often exist with deep copy operations that don&#8217;t exist with shallow
copy operations:</p>
<ul class="simple">
<li>Recursive objects (compound objects that, directly or indirectly, contain a
reference to themselves) may cause a recursive loop.</li>
<li>Because deep copy copies <em>everything</em> it may copy too much, e.g.,
administrative data structures that should be shared even between copies.</li>
</ul>
<p>The <a class="reference internal" href="#copy.deepcopy" title="copy.deepcopy"><tt class="xref py py-func docutils literal"><span class="pre">deepcopy()</span></tt></a> function avoids these problems by:</p>
<ul class="simple">
<li>keeping a &#8220;memo&#8221; dictionary of objects already copied during the current
copying pass; and</li>
<li>letting user-defined classes override the copying operation or the set of
components copied.</li>
</ul>
<p>This module does not copy types like module, method, stack trace, stack frame,
file, socket, window, array, or any similar types.  It does &#8220;copy&#8221; functions and
classes (shallow and deeply), by returning the original object unchanged; this
is compatible with the way these are treated by the <a class="reference internal" href="pickle.html#module-pickle" title="pickle: Convert Python objects to streams of bytes and back."><tt class="xref py py-mod docutils literal"><span class="pre">pickle</span></tt></a> module.</p>
<p>Shallow copies of dictionaries can be made using <a class="reference internal" href="stdtypes.html#dict.copy" title="dict.copy"><tt class="xref py py-meth docutils literal"><span class="pre">dict.copy()</span></tt></a>, and
of lists by assigning a slice of the entire list, for example,
<tt class="docutils literal"><span class="pre">copied_list</span> <span class="pre">=</span> <span class="pre">original_list[:]</span></tt>.</p>
<p class="versionchanged">
<span class="versionmodified">Changed in version 2.5: </span>Added copying functions.</p>
<p id="index-0">Classes can use the same interfaces to control copying that they use to control
pickling.  See the description of module <a class="reference internal" href="pickle.html#module-pickle" title="pickle: Convert Python objects to streams of bytes and back."><tt class="xref py py-mod docutils literal"><span class="pre">pickle</span></tt></a> for information on these
methods.  The <a class="reference internal" href="#module-copy" title="copy: Shallow and deep copy operations."><tt class="xref py py-mod docutils literal"><span class="pre">copy</span></tt></a> module does not use the <a class="reference internal" href="copy_reg.html#module-copy_reg" title="copy_reg: Register pickle support functions."><tt class="xref py py-mod docutils literal"><span class="pre">copy_reg</span></tt></a> registration
module.</p>
<p id="index-1">In order for a class to define its own copy implementation, it can define
special methods <tt class="xref py py-meth docutils literal"><span class="pre">__copy__()</span></tt> and <tt class="xref py py-meth docutils literal"><span class="pre">__deepcopy__()</span></tt>.  The former is called
to implement the shallow copy operation; no additional arguments are passed.
The latter is called to implement the deep copy operation; it is passed one
argument, the memo dictionary.  If the <tt class="xref py py-meth docutils literal"><span class="pre">__deepcopy__()</span></tt> implementation needs
to make a deep copy of a component, it should call the <a class="reference internal" href="#copy.deepcopy" title="copy.deepcopy"><tt class="xref py py-func docutils literal"><span class="pre">deepcopy()</span></tt></a> function
with the component as first argument and the memo dictionary as second argument.</p>
<div class="admonition-see-also admonition seealso">
<p class="first admonition-title">See also</p>
<dl class="last docutils">
<dt>Module <a class="reference internal" href="pickle.html#module-pickle" title="pickle: Convert Python objects to streams of bytes and back."><tt class="xref py py-mod docutils literal"><span class="pre">pickle</span></tt></a></dt>
<dd>Discussion of the special methods used to support object state retrieval and
restoration.</dd>
</dl>
</div>
</div>


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