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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"><!--z--></A>
<P>
This module provides generic (shallow and deep) copying operations.
<a id='l2h-664'><!--x--></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 <i>shallow copy</i> constructs a new compound object and then (to the
extent possible) inserts <i>references</i> into it to the objects found
in the original.
<P>
</LI>
<LI>A <i>deep copy</i> constructs a new compound object and then,
recursively, inserts <i>copies</i> 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 <i>everything</i> 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-667'><!--x--></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-666'><!--x--></a>
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<div class="seealso">
<p class="heading"><b>See Also:</b></p>
<dl compact class="seemodule">
<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 2.3.4, documentation updated on May 20, 2004.</span>
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