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<section id="module-copy">
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<span id="copy-shallow-and-deep-copy-operations"></span><h1><a class="reference internal" href="#module-copy" title="copy: Shallow and deep copy operations."><code class="xref py py-mod docutils literal notranslate"><span class="pre">copy</span></code></a> — Shallow and deep copy operations<a class="headerlink" href="#module-copy" title="Permalink to this headline">¶</a></h1>
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<p><strong>Source code:</strong> <a class="reference external" href="https://github.com/python/cpython/tree/3.12/Lib/copy.py">Lib/copy.py</a></p>
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<hr class="docutils" />
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<p>Assignment statements in Python do not copy objects, they create bindings
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between a target and an object. For collections that are mutable or contain
|
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mutable items, a copy is sometimes needed so one can change one copy without
|
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changing the other. This module provides generic shallow and deep copy
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operations (explained below).</p>
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<p>Interface summary:</p>
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||
<dl class="py function">
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||
<dt class="sig sig-object py" id="copy.copy">
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<span class="sig-prename descclassname"><span class="pre">copy.</span></span><span class="sig-name descname"><span class="pre">copy</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">x</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#copy.copy" title="Permalink to this definition">¶</a></dt>
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<dd><p>Return a shallow copy of <em>x</em>.</p>
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||
</dd></dl>
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<dl class="py function">
|
||
<dt class="sig sig-object py" id="copy.deepcopy">
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<span class="sig-prename descclassname"><span class="pre">copy.</span></span><span class="sig-name descname"><span class="pre">deepcopy</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">x</span></span></em><span class="optional">[</span>, <em class="sig-param"><span class="n"><span class="pre">memo</span></span></em><span class="optional">]</span><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>
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||
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<dl class="py exception">
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||
<dt class="sig sig-object py" id="copy.Error">
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||
<em class="property"><span class="pre">exception</span><span class="w"> </span></em><span class="sig-prename descclassname"><span class="pre">copy.</span></span><span class="sig-name descname"><span class="pre">Error</span></span><a class="headerlink" href="#copy.Error" title="Permalink to this definition">¶</a></dt>
|
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<dd><p>Raised for module specific errors.</p>
|
||
</dd></dl>
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||
|
||
<p id="shallow-vs-deep-copy">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">
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||
<li><p>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.</p></li>
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||
<li><p>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.</p></li>
|
||
</ul>
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||
<p>Two problems often exist with deep copy operations that don’t exist with shallow
|
||
copy operations:</p>
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||
<ul class="simple">
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||
<li><p>Recursive objects (compound objects that, directly or indirectly, contain a
|
||
reference to themselves) may cause a recursive loop.</p></li>
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<li><p>Because deep copy copies everything it may copy too much, such as data
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||
which is intended to be shared between copies.</p></li>
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||
</ul>
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||
<p>The <a class="reference internal" href="#copy.deepcopy" title="copy.deepcopy"><code class="xref py py-func docutils literal notranslate"><span class="pre">deepcopy()</span></code></a> function avoids these problems by:</p>
|
||
<ul class="simple">
|
||
<li><p>keeping a <code class="docutils literal notranslate"><span class="pre">memo</span></code> dictionary of objects already copied during the current
|
||
copying pass; and</p></li>
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<li><p>letting user-defined classes override the copying operation or the set of
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components copied.</p></li>
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||
</ul>
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<p>This module does not copy types like module, method, stack trace, stack frame,
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||
file, socket, window, or any similar types. It does “copy” functions and
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||
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 notranslate"><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 notranslate"><span class="pre">dict.copy()</span></code></a>, and
|
||
of lists by assigning a slice of the entire list, for example,
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<code class="docutils literal notranslate"><span class="pre">copied_list</span> <span class="pre">=</span> <span class="pre">original_list[:]</span></code>.</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."><code class="xref py py-mod docutils literal notranslate"><span class="pre">pickle</span></code></a> for information on these
|
||
methods. In fact, the <a class="reference internal" href="#module-copy" title="copy: Shallow and deep copy operations."><code class="xref py py-mod docutils literal notranslate"><span class="pre">copy</span></code></a> module uses the registered
|
||
pickle functions from the <a class="reference internal" href="copyreg.html#module-copyreg" title="copyreg: Register pickle support functions."><code class="xref py py-mod docutils literal notranslate"><span class="pre">copyreg</span></code></a> module.</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 notranslate"><span class="pre">__copy__()</span></code> and <code class="xref py py-meth docutils literal notranslate"><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 <code class="docutils literal notranslate"><span class="pre">memo</span></code> dictionary. If the <code class="xref py py-meth docutils literal notranslate"><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 notranslate"><span class="pre">deepcopy()</span></code></a> function
|
||
with the component as first argument and the memo dictionary as second argument.
|
||
The memo dictionary should be treated as an opaque object.</p>
|
||
<div class="admonition seealso">
|
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<p class="admonition-title">See also</p>
|
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<dl class="simple">
|
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<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 notranslate"><span class="pre">pickle</span></code></a></dt><dd><p>Discussion of the special methods used to support object state retrieval and
|
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restoration.</p>
|
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</dd>
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Last updated on Oct 02, 2023.
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