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  &lt;div class=&quot;section&quot; id=&quot;module-copy&quot;&gt;
&lt;span id=&quot;copy-shallow-and-deep-copy-operations&quot;&gt;&lt;/span&gt;&lt;h1&gt;8.17. &lt;a class=&quot;reference internal&quot; href=&quot;#module-copy&quot; title=&quot;copy: Shallow and deep copy operations.&quot;&gt;&lt;tt class=&quot;xref py py-mod docutils literal&quot;&gt;&lt;span class=&quot;pre&quot;&gt;copy&lt;/span&gt;&lt;/tt&gt;&lt;/a&gt; &amp;#8212; Shallow and deep copy operations&lt;a class=&quot;headerlink&quot; href=&quot;#module-copy&quot; title=&quot;Permalink to this headline&quot;&gt;¶&lt;/a&gt;&lt;/h1&gt;
&lt;p&gt;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).&lt;/p&gt;
&lt;p&gt;Interface summary:&lt;/p&gt;
&lt;dl class=&quot;function&quot;&gt;
&lt;dt id=&quot;copy.copy&quot;&gt;
&lt;tt class=&quot;descclassname&quot;&gt;copy.&lt;/tt&gt;&lt;tt class=&quot;descname&quot;&gt;copy&lt;/tt&gt;&lt;big&gt;(&lt;/big&gt;&lt;em&gt;x&lt;/em&gt;&lt;big&gt;)&lt;/big&gt;&lt;a class=&quot;headerlink&quot; href=&quot;#copy.copy&quot; title=&quot;Permalink to this definition&quot;&gt;¶&lt;/a&gt;&lt;/dt&gt;
&lt;dd&gt;&lt;p&gt;Return a shallow copy of &lt;em&gt;x&lt;/em&gt;.&lt;/p&gt;
&lt;/dd&gt;&lt;/dl&gt;

&lt;dl class=&quot;function&quot;&gt;
&lt;dt id=&quot;copy.deepcopy&quot;&gt;
&lt;tt class=&quot;descclassname&quot;&gt;copy.&lt;/tt&gt;&lt;tt class=&quot;descname&quot;&gt;deepcopy&lt;/tt&gt;&lt;big&gt;(&lt;/big&gt;&lt;em&gt;x&lt;/em&gt;&lt;big&gt;)&lt;/big&gt;&lt;a class=&quot;headerlink&quot; href=&quot;#copy.deepcopy&quot; title=&quot;Permalink to this definition&quot;&gt;¶&lt;/a&gt;&lt;/dt&gt;
&lt;dd&gt;&lt;p&gt;Return a deep copy of &lt;em&gt;x&lt;/em&gt;.&lt;/p&gt;
&lt;/dd&gt;&lt;/dl&gt;

&lt;dl class=&quot;exception&quot;&gt;
&lt;dt id=&quot;copy.error&quot;&gt;
&lt;em class=&quot;property&quot;&gt;exception &lt;/em&gt;&lt;tt class=&quot;descclassname&quot;&gt;copy.&lt;/tt&gt;&lt;tt class=&quot;descname&quot;&gt;error&lt;/tt&gt;&lt;a class=&quot;headerlink&quot; href=&quot;#copy.error&quot; title=&quot;Permalink to this definition&quot;&gt;¶&lt;/a&gt;&lt;/dt&gt;
&lt;dd&gt;&lt;p&gt;Raised for module specific errors.&lt;/p&gt;
&lt;/dd&gt;&lt;/dl&gt;

&lt;p&gt;The difference between shallow and deep copying is only relevant for compound
objects (objects that contain other objects, like lists or class instances):&lt;/p&gt;
&lt;ul class=&quot;simple&quot;&gt;
&lt;li&gt;A &lt;em&gt;shallow copy&lt;/em&gt; constructs a new compound object and then (to the extent
possible) inserts &lt;em&gt;references&lt;/em&gt; into it to the objects found in the original.&lt;/li&gt;
&lt;li&gt;A &lt;em&gt;deep copy&lt;/em&gt; constructs a new compound object and then, recursively, inserts
&lt;em&gt;copies&lt;/em&gt; into it of the objects found in the original.&lt;/li&gt;
&lt;/ul&gt;
&lt;p&gt;Two problems often exist with deep copy operations that don&amp;#8217;t exist with shallow
copy operations:&lt;/p&gt;
&lt;ul class=&quot;simple&quot;&gt;
&lt;li&gt;Recursive objects (compound objects that, directly or indirectly, contain a
reference to themselves) may cause a recursive loop.&lt;/li&gt;
&lt;li&gt;Because deep copy copies &lt;em&gt;everything&lt;/em&gt; it may copy too much, e.g.,
administrative data structures that should be shared even between copies.&lt;/li&gt;
&lt;/ul&gt;
&lt;p&gt;The &lt;a class=&quot;reference internal&quot; href=&quot;#copy.deepcopy&quot; title=&quot;copy.deepcopy&quot;&gt;&lt;tt class=&quot;xref py py-func docutils literal&quot;&gt;&lt;span class=&quot;pre&quot;&gt;deepcopy()&lt;/span&gt;&lt;/tt&gt;&lt;/a&gt; function avoids these problems by:&lt;/p&gt;
&lt;ul class=&quot;simple&quot;&gt;
&lt;li&gt;keeping a &amp;#8220;memo&amp;#8221; dictionary of objects already copied during the current
copying pass; and&lt;/li&gt;
&lt;li&gt;letting user-defined classes override the copying operation or the set of
components copied.&lt;/li&gt;
&lt;/ul&gt;
&lt;p&gt;This module does not copy types like module, method, stack trace, stack frame,
file, socket, window, array, or any similar types.  It does &amp;#8220;copy&amp;#8221; functions and
classes (shallow and deeply), by returning the original object unchanged; this
is compatible with the way these are treated by the &lt;a class=&quot;reference internal&quot; href=&quot;pickle.html#module-pickle&quot; title=&quot;pickle: Convert Python objects to streams of bytes and back.&quot;&gt;&lt;tt class=&quot;xref py py-mod docutils literal&quot;&gt;&lt;span class=&quot;pre&quot;&gt;pickle&lt;/span&gt;&lt;/tt&gt;&lt;/a&gt; module.&lt;/p&gt;
&lt;p&gt;Shallow copies of dictionaries can be made using &lt;a class=&quot;reference internal&quot; href=&quot;stdtypes.html#dict.copy&quot; title=&quot;dict.copy&quot;&gt;&lt;tt class=&quot;xref py py-meth docutils literal&quot;&gt;&lt;span class=&quot;pre&quot;&gt;dict.copy()&lt;/span&gt;&lt;/tt&gt;&lt;/a&gt;, and
of lists by assigning a slice of the entire list, for example,
&lt;tt class=&quot;docutils literal&quot;&gt;&lt;span class=&quot;pre&quot;&gt;copied_list&lt;/span&gt; &lt;span class=&quot;pre&quot;&gt;=&lt;/span&gt; &lt;span class=&quot;pre&quot;&gt;original_list[:]&lt;/span&gt;&lt;/tt&gt;.&lt;/p&gt;
&lt;p class=&quot;versionchanged&quot;&gt;
&lt;span class=&quot;versionmodified&quot;&gt;Changed in version 2.5: &lt;/span&gt;Added copying functions.&lt;/p&gt;
&lt;p id=&quot;index-0&quot;&gt;Classes can use the same interfaces to control copying that they use to control
pickling.  See the description of module &lt;a class=&quot;reference internal&quot; href=&quot;pickle.html#module-pickle&quot; title=&quot;pickle: Convert Python objects to streams of bytes and back.&quot;&gt;&lt;tt class=&quot;xref py py-mod docutils literal&quot;&gt;&lt;span class=&quot;pre&quot;&gt;pickle&lt;/span&gt;&lt;/tt&gt;&lt;/a&gt; for information on these
methods.  The &lt;a class=&quot;reference internal&quot; href=&quot;#module-copy&quot; title=&quot;copy: Shallow and deep copy operations.&quot;&gt;&lt;tt class=&quot;xref py py-mod docutils literal&quot;&gt;&lt;span class=&quot;pre&quot;&gt;copy&lt;/span&gt;&lt;/tt&gt;&lt;/a&gt; module does not use the &lt;a class=&quot;reference internal&quot; href=&quot;copy_reg.html#module-copy_reg&quot; title=&quot;copy_reg: Register pickle support functions.&quot;&gt;&lt;tt class=&quot;xref py py-mod docutils literal&quot;&gt;&lt;span class=&quot;pre&quot;&gt;copy_reg&lt;/span&gt;&lt;/tt&gt;&lt;/a&gt; registration
module.&lt;/p&gt;
&lt;p id=&quot;index-1&quot;&gt;In order for a class to define its own copy implementation, it can define
special methods &lt;tt class=&quot;xref py py-meth docutils literal&quot;&gt;&lt;span class=&quot;pre&quot;&gt;__copy__()&lt;/span&gt;&lt;/tt&gt; and &lt;tt class=&quot;xref py py-meth docutils literal&quot;&gt;&lt;span class=&quot;pre&quot;&gt;__deepcopy__()&lt;/span&gt;&lt;/tt&gt;.  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 &lt;tt class=&quot;xref py py-meth docutils literal&quot;&gt;&lt;span class=&quot;pre&quot;&gt;__deepcopy__()&lt;/span&gt;&lt;/tt&gt; implementation needs
to make a deep copy of a component, it should call the &lt;a class=&quot;reference internal&quot; href=&quot;#copy.deepcopy&quot; title=&quot;copy.deepcopy&quot;&gt;&lt;tt class=&quot;xref py py-func docutils literal&quot;&gt;&lt;span class=&quot;pre&quot;&gt;deepcopy()&lt;/span&gt;&lt;/tt&gt;&lt;/a&gt; function
with the component as first argument and the memo dictionary as second argument.&lt;/p&gt;
&lt;div class=&quot;admonition-see-also admonition seealso&quot;&gt;
&lt;p class=&quot;first admonition-title&quot;&gt;See also&lt;/p&gt;
&lt;dl class=&quot;last docutils&quot;&gt;
&lt;dt&gt;Module &lt;a class=&quot;reference internal&quot; href=&quot;pickle.html#module-pickle&quot; title=&quot;pickle: Convert Python objects to streams of bytes and back.&quot;&gt;&lt;tt class=&quot;xref py py-mod docutils literal&quot;&gt;&lt;span class=&quot;pre&quot;&gt;pickle&lt;/span&gt;&lt;/tt&gt;&lt;/a&gt;&lt;/dt&gt;
&lt;dd&gt;Discussion of the special methods used to support object state retrieval and
restoration.&lt;/dd&gt;
&lt;/dl&gt;
&lt;/div&gt;
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