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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"

"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

<head>

<title>8.10. copy — Shallow and deep copy operations — Python 3.7.0 說明文件</title>

title="在 Python 3.7.0 說明文件中搜尋"

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<span id="copy-shallow-and-deep-copy-operations"></span><h1>8.10. <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="本標題的永久連結">¶</a></h1>

<p><strong>Source code:</strong> <a class="reference external" href="https://github.com/python/cpython/tree/3.7/Lib/copy.py">Lib/copy.py</a></p>

<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>

<dd><p>Return a shallow copy of <em>x</em>.</p>

</dd></dl>

<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="本定義的永久連結">¶</a></dt>

<dd><p>Return a deep copy of <em>x</em>.</p>

</dd></dl>

<em class="property">exception </em><code class="descclassname">copy.</code><code class="descname">error</code><a class="headerlink" href="#copy.error" title="本定義的永久連結">¶</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>

<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>

<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 notranslate"><span class="pre">deepcopy()</span></code></a> function avoids these problems by:</p>

<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 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 memo 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.</p>

<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>Discussion of the special methods used to support object state retrieval and

restoration.</dd>

</dl>

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title="下一章">8.11. <code class="docutils literal notranslate"><span class="pre">pprint</span></code> — Data pretty printer</a></p>

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