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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."><code class="xref py py-mod docutils literal"><span class="pre">copy</span></code></a> — 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">
<code class="descclassname">copy.</code><code class="descname">copy</code><span class="sig-paren">(</span><em>x</em><span class="sig-paren">)</span><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">
<code class="descclassname">copy.</code><code class="descname">deepcopy</code><span class="sig-paren">(</span><em>x</em><span class="sig-paren">)</span><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><code class="descclassname">copy.</code><code class="descname">error</code><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’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 everything it may copy too much, such as data
which is intended to be shared between copies.</li>
</ul>
<p>The <a class="reference internal" href="#copy.deepcopy" title="copy.deepcopy"><code class="xref py py-func docutils literal"><span class="pre">deepcopy()</span></code></a> function avoids these problems by:</p>
<ul class="simple">
<li>keeping a “memo” 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 “copy” 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."><code class="xref py py-mod docutils literal"><span class="pre">pickle</span></code></a> module.</p>
<p>Shallow copies of dictionaries can be made using <a class="reference internal" href="stdtypes.html#dict.copy" title="dict.copy"><code class="xref py py-meth docutils literal"><span class="pre">dict.copy()</span></code></a>, and
of lists by assigning a slice of the entire list, for example,
<code class="docutils literal"><span class="pre">copied_list</span> <span class="pre">=</span> <span class="pre">original_list[:]</span></code>.</p>
<div class="versionchanged">
<p><span class="versionmodified">Changed in version 2.5: </span>Added copying functions.</p>
</div>
<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."><code class="xref py py-mod docutils literal"><span class="pre">pickle</span></code></a> for information on these
methods. The <a class="reference internal" href="#module-copy" title="copy: Shallow and deep copy operations."><code class="xref py py-mod docutils literal"><span class="pre">copy</span></code></a> module does not use 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"><span class="pre">copy_reg</span></code></a> registration
module.</p>
<p id="index-1">In order for a class to define its own copy implementation, it can define
special methods <code class="xref py py-meth docutils literal"><span class="pre">__copy__()</span></code> and <code class="xref py py-meth docutils literal"><span class="pre">__deepcopy__()</span></code>. 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 <code class="xref py py-meth docutils literal"><span class="pre">__deepcopy__()</span></code> implementation needs
to make a deep copy of a component, it should call the <a class="reference internal" href="#copy.deepcopy" title="copy.deepcopy"><code class="xref py py-func docutils literal"><span class="pre">deepcopy()</span></code></a> function
with the component as first argument and the memo dictionary as second argument.</p>
<div class="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."><code class="xref py py-mod docutils literal"><span class="pre">pickle</span></code></a></dt>
<dd>Discussion of the special methods used to support object state retrieval and
restoration.</dd>
</dl>
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