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<title>3.18 copy -- Shallow and deep copy operations</title>
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<H1><A NAME="SECTION0051800000000000000000">
3.18 <tt class="module">copy</tt> --
         Shallow and deep copy operations</A>
</H1>

<P>
<A NAME="module-copy"></A>

<P>
This module provides generic (shallow and deep) copying operations.
<a id='l2h-675' xml:id='l2h-675'></a>
<P>
Interface summary:

<P>
<div class="verbatim"><pre>
import copy

x = copy.copy(y)        # make a shallow copy of y
x = copy.deepcopy(y)    # make a deep copy of y
</pre></div>
For module specific errors, <tt class="exception">copy.error</tt> is raised.

<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>
<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.

<P>
</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.

<P>
</LI>
</UL>

<P>
Two problems often exist with deep copy operations that don't exist
with shallow copy operations:

<P>

<UL>
<LI>Recursive objects (compound objects that, directly or indirectly,
contain a reference to themselves) may cause a recursive loop.

<P>
</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.

<P>
</LI>
</UL>

<P>
The <tt class="function">deepcopy()</tt> function avoids these problems by:

<P>

<UL>
<LI>keeping a ``memo'' dictionary of objects already copied during the current
copying pass; and

<P>
</LI>
<LI>letting user-defined classes override the copying operation or the
set of components copied.

<P>
</LI>
</UL>

<P>
This version does not copy types like module, class, function, method,
stack trace, stack frame, file, socket, window, array, or any similar
types.

<P>
Classes can use the same interfaces to control copying that they use
to control pickling.  See the description of module
<tt class="module"><a href="module-pickle.html">pickle</a></tt><a id='l2h-678' xml:id='l2h-678'></a> for information on these
methods.  The <tt class="module">copy</tt> module does not use the
<tt class="module"><a href="module-copyreg.html">copy_reg</a></tt> registration module.

<P>
In order for a class to define its own copy implementation, it can
define special methods <tt class="method">__copy__()</tt> and
<tt class="method">__deepcopy__()</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="method">__deepcopy__()</tt>
implementation needs to make a deep copy of a component, it should
call the <tt class="function">deepcopy()</tt> function with the component as first
argument and the memo dictionary as second argument.
<a id='l2h-677' xml:id='l2h-677'></a>
<P>
<div class="seealso">
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    <dt>Module <b><tt class="module"><a href="module-pickle.html">pickle</a></tt>:</b>
    <dd>Discussion of the special methods used to
support object state retrieval and restoration.
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<span class="release-info">Release 2.4.2, documentation updated on 28 September 2005.</span>
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