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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> <html xmlns="http://www.w3.org/1999/xhtml"> <head> <meta http-equiv="Content-Type" content="text/html; charset=utf-8" /> <title>8.17. copy — Shallow and deep copy operations — Python 2.7.5 documentation</title> <link rel="stylesheet" href="../_static/default.css" type="text/css" /> <link rel="stylesheet" href="../_static/pygments.css" type="text/css" /> <script type="text/javascript"> var DOCUMENTATION_OPTIONS = { URL_ROOT: '../', VERSION: '2.7.5', COLLAPSE_INDEX: false, FILE_SUFFIX: '.html', HAS_SOURCE: true }; </script> <script type="text/javascript" src="../_static/jquery.js"></script> <script type="text/javascript" src="../_static/underscore.js"></script> <script type="text/javascript" src="../_static/doctools.js"></script> <script type="text/javascript" src="../_static/sidebar.js"></script> <link rel="search" type="application/opensearchdescription+xml" title="Search within Python 2.7.5 documentation" href="../_static/opensearch.xml"/> <link rel="author" title="About these documents" href="../about.html" /> <link rel="copyright" title="Copyright" href="../copyright.html" /> <link rel="top" title="Python 2.7.5 documentation" href="../index.html" /> <link rel="up" title="8. 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Data Types</a> »</li> </ul> </div> <div class="document"> <div class="documentwrapper"> <div class="bodywrapper"> <div class="body"> <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> — 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’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 “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."><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> </div> </div> </div> <div class="sphinxsidebar"> <div class="sphinxsidebarwrapper"> <h4>Previous topic</h4> <p class="topless"><a href="new.html" title="previous chapter">8.16. <tt class="docutils literal"><span class="pre">new</span></tt> — Creation of runtime internal objects</a></p> <h4>Next topic</h4> <p class="topless"><a href="pprint.html" title="next chapter">8.18. <tt class="docutils literal"><span class="pre">pprint</span></tt> — Data pretty printer</a></p> <h3>This Page</h3> <ul class="this-page-menu"> <li><a href="../bugs.html">Report a Bug</a></li> <li><a href="../_sources/library/copy.txt" rel="nofollow">Show Source</a></li> </ul> <div id="searchbox" style="display: none"> <h3>Quick search</h3> <form class="search" action="../search.html" method="get"> <input type="text" name="q" /> <input type="submit" value="Go" /> <input type="hidden" name="check_keywords" value="yes" /> <input type="hidden" name="area" value="default" /> </form> <p class="searchtip" style="font-size: 90%"> Enter search terms or a module, class or function name. </p> </div> <script type="text/javascript">$('#searchbox').show(0);</script> </div> </div> <div class="clearer"></div> </div> <div class="related"> <h3>Navigation</h3> <ul> <li class="right" style="margin-right: 10px"> <a href="../genindex.html" title="General Index" >index</a></li> <li class="right" > <a href="../py-modindex.html" title="Python Module Index" >modules</a> |</li> <li class="right" > <a href="pprint.html" title="8.18. pprint — Data pretty printer" >next</a> |</li> <li class="right" > <a href="new.html" title="8.16. new — Creation of runtime internal objects" >previous</a> |</li> <li><img src="../_static/py.png" alt="" style="vertical-align: middle; margin-top: -1px"/></li> <li><a href="http://www.python.org/">Python</a> »</li> <li> <a href="../index.html">Python 2.7.5 documentation</a> » </li> <li><a href="index.html" >The Python Standard Library</a> »</li> <li><a href="datatypes.html" >8. 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