Baoding-FenShuaJiang
/
Green_GoodERP18_FSJ_Standard


			
							<!DOCTYPE html>

<html lang="en" data-content_root="../">
  <head>
    <meta charset="utf-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" /><meta name="viewport" content="width=device-width, initial-scale=1" />
<meta property="og:title" content="copy — Shallow and deep copy operations" />
<meta property="og:type" content="website" />
<meta property="og:url" content="https://docs.python.org/3/library/copy.html" />
<meta property="og:site_name" content="Python documentation" />
<meta property="og:description" content="Source code: Lib/copy.py 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 ..." />
<meta property="og:image" content="https://docs.python.org/3/_static/og-image.png" />
<meta property="og:image:alt" content="Python documentation" />
<meta name="description" content="Source code: Lib/copy.py 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 ..." />
<meta property="og:image:width" content="200" />
<meta property="og:image:height" content="200" />
<meta name="theme-color" content="#3776ab" />

    <title>copy — Shallow and deep copy operations &#8212; Python 3.12.3 documentation</title><meta name="viewport" content="width=device-width, initial-scale=1.0">
    
    <link rel="stylesheet" type="text/css" href="../_static/pygments.css?v=80d5e7a1" />
    <link rel="stylesheet" type="text/css" href="../_static/pydoctheme.css?v=bb723527" />
    <link id="pygments_dark_css" media="(prefers-color-scheme: dark)" rel="stylesheet" type="text/css" href="../_static/pygments_dark.css?v=b20cc3f5" />
    
    <script src="../_static/documentation_options.js?v=2c828074"></script>
    <script src="../_static/doctools.js?v=888ff710"></script>
    <script src="../_static/sphinx_highlight.js?v=dc90522c"></script>
    
    <script src="../_static/sidebar.js"></script>
    
    <link rel="search" type="application/opensearchdescription+xml"
          title="Search within Python 3.12.3 documentation"
          href="../_static/opensearch.xml"/>
    <link rel="author" title="About these documents" href="../about.html" />
    <link rel="index" title="Index" href="../genindex.html" />
    <link rel="search" title="Search" href="../search.html" />
    <link rel="copyright" title="Copyright" href="../copyright.html" />
    <link rel="next" title="pprint — Data pretty printer" href="pprint.html" />
    <link rel="prev" title="types — Dynamic type creation and names for built-in types" href="types.html" />
    <link rel="canonical" href="https://docs.python.org/3/library/copy.html" />
    
      
    <style>
      @media only screen {
        table.full-width-table {
            width: 100%;
        }
      }
    </style>
<link rel="stylesheet" href="../_static/pydoctheme_dark.css" media="(prefers-color-scheme: dark)" id="pydoctheme_dark_css">
    <link rel="shortcut icon" type="image/png" href="../_static/py.svg" />
            <script type="text/javascript" src="../_static/copybutton.js"></script>
            <script type="text/javascript" src="../_static/menu.js"></script>
            <script type="text/javascript" src="../_static/search-focus.js"></script>
            <script type="text/javascript" src="../_static/themetoggle.js"></script> 

  </head>
<body>
<div class="mobile-nav">
    <input type="checkbox" id="menuToggler" class="toggler__input" aria-controls="navigation"
           aria-pressed="false" aria-expanded="false" role="button" aria-label="Menu" />
    <nav class="nav-content" role="navigation">
        <label for="menuToggler" class="toggler__label">
            <span></span>
        </label>
        <span class="nav-items-wrapper">
            <a href="https://www.python.org/" class="nav-logo">
                <img src="../_static/py.svg" alt="Python logo"/>
            </a>
            <span class="version_switcher_placeholder"></span>
            <form role="search" class="search" action="../search.html" method="get">
                <svg xmlns="http://www.w3.org/2000/svg" width="20" height="20" viewBox="0 0 24 24" class="search-icon">
                    <path fill-rule="nonzero" fill="currentColor" d="M15.5 14h-.79l-.28-.27a6.5 6.5 0 001.48-5.34c-.47-2.78-2.79-5-5.59-5.34a6.505 6.505 0 00-7.27 7.27c.34 2.8 2.56 5.12 5.34 5.59a6.5 6.5 0 005.34-1.48l.27.28v.79l4.25 4.25c.41.41 1.08.41 1.49 0 .41-.41.41-1.08 0-1.49L15.5 14zm-6 0C7.01 14 5 11.99 5 9.5S7.01 5 9.5 5 14 7.01 14 9.5 11.99 14 9.5 14z"></path>
                </svg>
                <input placeholder="Quick search" aria-label="Quick search" type="search" name="q" />
                <input type="submit" value="Go"/>
            </form>
        </span>
    </nav>
    <div class="menu-wrapper">
        <nav class="menu" role="navigation" aria-label="main navigation">
            <div class="language_switcher_placeholder"></div>
            
<label class="theme-selector-label">
    Theme
    <select class="theme-selector" oninput="activateTheme(this.value)">
        <option value="auto" selected>Auto</option>
        <option value="light">Light</option>
        <option value="dark">Dark</option>
    </select>
</label>
  <div>
    <h4>Previous topic</h4>
    <p class="topless"><a href="types.html"
                          title="previous chapter"><code class="xref py py-mod docutils literal notranslate"><span class="pre">types</span></code> — Dynamic type creation and names for built-in types</a></p>
  </div>
  <div>
    <h4>Next topic</h4>
    <p class="topless"><a href="pprint.html"
                          title="next chapter"><code class="xref py py-mod docutils literal notranslate"><span class="pre">pprint</span></code> — Data pretty printer</a></p>
  </div>
  <div role="note" aria-label="source link">
    <h3>This Page</h3>
    <ul class="this-page-menu">
      <li><a href="../bugs.html">Report a Bug</a></li>
      <li>
        <a href="https://github.com/python/cpython/blob/main/Doc/library/copy.rst"
            rel="nofollow">Show Source
        </a>
      </li>
    </ul>
  </div>
        </nav>
    </div>
</div>

  
    <div class="related" role="navigation" aria-label="related navigation">
      <h3>Navigation</h3>
      <ul>
        <li class="right" style="margin-right: 10px">
          <a href="../genindex.html" title="General Index"
             accesskey="I">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="pprint — Data pretty printer"
             accesskey="N">next</a> |</li>
        <li class="right" >
          <a href="types.html" title="types — Dynamic type creation and names for built-in types"
             accesskey="P">previous</a> |</li>

          <li><img src="../_static/py.svg" alt="Python logo" style="vertical-align: middle; margin-top: -1px"/></li>
          <li><a href="https://www.python.org/">Python</a> &#187;</li>
          <li class="switchers">
            <div class="language_switcher_placeholder"></div>
            <div class="version_switcher_placeholder"></div>
          </li>
          <li>
              
          </li>
    <li id="cpython-language-and-version">
      <a href="../index.html">3.12.3 Documentation</a> &#187;
    </li>

          <li class="nav-item nav-item-1"><a href="index.html" >The Python Standard Library</a> &#187;</li>
          <li class="nav-item nav-item-2"><a href="datatypes.html" accesskey="U">Data Types</a> &#187;</li>
        <li class="nav-item nav-item-this"><a href=""><code class="xref py py-mod docutils literal notranslate"><span class="pre">copy</span></code> — Shallow and deep copy operations</a></li>
                <li class="right">
                    

    <div class="inline-search" role="search">
        <form class="inline-search" action="../search.html" method="get">
          <input placeholder="Quick search" aria-label="Quick search" type="search" name="q" id="search-box" />
          <input type="submit" value="Go" />
        </form>
    </div>
                     |
                </li>
            <li class="right">
<label class="theme-selector-label">
    Theme
    <select class="theme-selector" oninput="activateTheme(this.value)">
        <option value="auto" selected>Auto</option>
        <option value="light">Light</option>
        <option value="dark">Dark</option>
    </select>
</label> |</li>
            
      </ul>
    </div>    

    <div class="document">
      <div class="documentwrapper">
        <div class="bodywrapper">
          <div class="body" role="main">
            
  <section id="module-copy">
<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="Link to this heading">¶</a></h1>
<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>
<hr class="docutils" />
<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="py function">
<dt class="sig sig-object py" id="copy.copy">
<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="Link to this definition">¶</a></dt>
<dd><p>Return a shallow copy of <em>x</em>.</p>
</dd></dl>

<dl class="py function">
<dt class="sig sig-object py" id="copy.deepcopy">
<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="Link to this definition">¶</a></dt>
<dd><p>Return a deep copy of <em>x</em>.</p>
</dd></dl>

<dl class="py exception">
<dt class="sig sig-object py" id="copy.Error">
<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="Link to this definition">¶</a></dt>
<dd><p>Raised for module specific errors.</p>
</dd></dl>

<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">
<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>
<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>
<p>Two problems often exist with deep copy operations that don’t exist with shallow
copy operations:</p>
<ul class="simple">
<li><p>Recursive objects (compound objects that, directly or indirectly, contain a
reference to themselves) may cause a recursive loop.</p></li>
<li><p>Because deep copy copies everything it may copy too much, such as data
which is intended to be shared between copies.</p></li>
</ul>
<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>
<li><p>letting user-defined classes override the copying operation or the set of
components copied.</p></li>
</ul>
<p>This module does not copy types like module, method, stack trace, stack frame,
file, socket, window, 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 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,
<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">
<p class="admonition-title">See also</p>
<dl class="simple">
<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
restoration.</p>
</dd>
</dl>
</div>
</section>


            <div class="clearer"></div>
          </div>
        </div>
      </div>
      <div class="sphinxsidebar" role="navigation" aria-label="main navigation">
        <div class="sphinxsidebarwrapper">
  <div>
    <h4>Previous topic</h4>
    <p class="topless"><a href="types.html"
                          title="previous chapter"><code class="xref py py-mod docutils literal notranslate"><span class="pre">types</span></code> — Dynamic type creation and names for built-in types</a></p>
  </div>
  <div>
    <h4>Next topic</h4>
    <p class="topless"><a href="pprint.html"
                          title="next chapter"><code class="xref py py-mod docutils literal notranslate"><span class="pre">pprint</span></code> — Data pretty printer</a></p>
  </div>
  <div role="note" aria-label="source link">
    <h3>This Page</h3>
    <ul class="this-page-menu">
      <li><a href="../bugs.html">Report a Bug</a></li>
      <li>
        <a href="https://github.com/python/cpython/blob/main/Doc/library/copy.rst"
            rel="nofollow">Show Source
        </a>
      </li>
    </ul>
  </div>
        </div>
<div id="sidebarbutton" title="Collapse sidebar">
<span>«</span>
</div>

      </div>
      <div class="clearer"></div>
    </div>  
    <div class="related" role="navigation" aria-label="related navigation">
      <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="pprint — Data pretty printer"
             >next</a> |</li>
        <li class="right" >
          <a href="types.html" title="types — Dynamic type creation and names for built-in types"
             >previous</a> |</li>

          <li><img src="../_static/py.svg" alt="Python logo" style="vertical-align: middle; margin-top: -1px"/></li>
          <li><a href="https://www.python.org/">Python</a> &#187;</li>
          <li class="switchers">
            <div class="language_switcher_placeholder"></div>
            <div class="version_switcher_placeholder"></div>
          </li>
          <li>
              
          </li>
    <li id="cpython-language-and-version">
      <a href="../index.html">3.12.3 Documentation</a> &#187;
    </li>

          <li class="nav-item nav-item-1"><a href="index.html" >The Python Standard Library</a> &#187;</li>
          <li class="nav-item nav-item-2"><a href="datatypes.html" >Data Types</a> &#187;</li>
        <li class="nav-item nav-item-this"><a href=""><code class="xref py py-mod docutils literal notranslate"><span class="pre">copy</span></code> — Shallow and deep copy operations</a></li>
                <li class="right">
                    

    <div class="inline-search" role="search">
        <form class="inline-search" action="../search.html" method="get">
          <input placeholder="Quick search" aria-label="Quick search" type="search" name="q" id="search-box" />
          <input type="submit" value="Go" />
        </form>
    </div>
                     |
                </li>
            <li class="right">
<label class="theme-selector-label">
    Theme
    <select class="theme-selector" oninput="activateTheme(this.value)">
        <option value="auto" selected>Auto</option>
        <option value="light">Light</option>
        <option value="dark">Dark</option>
    </select>
</label> |</li>
            
      </ul>
    </div>  
    <div class="footer">
    &copy; 
      <a href="../copyright.html">
    
    Copyright
    
      </a>
     2001-2024, Python Software Foundation.
    <br />
    This page is licensed under the Python Software Foundation License Version 2.
    <br />
    Examples, recipes, and other code in the documentation are additionally licensed under the Zero Clause BSD License.
    <br />
    
      See <a href="/license.html">History and License</a> for more information.<br />
    
    
    <br />

    The Python Software Foundation is a non-profit corporation.
<a href="https://www.python.org/psf/donations/">Please donate.</a>
<br />
    <br />
      Last updated on Apr 09, 2024 (13:47 UTC).
    
      <a href="/bugs.html">Found a bug</a>?
    
    <br />

    Created using <a href="https://www.sphinx-doc.org/">Sphinx</a> 7.2.6.
    </div>

  </body>
</html>