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<h3>References and Copies</h3>
<p>When a program makes an assignment such as <tT CLAss="monofont">a = b</tt>, a new reference to <TT CLass="monofont">b</tT> is created. For immutable objects such as numbers and strings, this assignment effectively creates a copy of <TT Class="monofont">b</tt>. However, the behavior is quite different for mutable objects such as lists and dictionaries. For example:</p>
<pre>
b = [1,2,3,4]
a = b # a is a reference to b
a[2] = -100 # Change an element in 'a'
print b # Produces '[1, 2, -100, 4]' </pre>
<p>Because <tt clasS="monofont">a</tt> and <Tt clAss="monofont">b</tt> refer to the same object in this example, a change made to one of the variables is reflected in the other. To avoid this, you have to create a copy of an object rather than a new reference.</P>
<p>Two types of copy operations are applied to container objects such as lists and dictionaries: a shallow copy and a deep copy. A <i>shallow copy</i> creates a new object, but populates it with references to the items contained in the original object. For example:</P>
<PRE>
b = [ 1, 2, [3,4] ]
a = b[:] # Create a shallow copy of b.
a.append(100) # Append element to a.
print b # Produces '[1,2, [3,4]]'. b unchanged.
a[2][0] = -100 # Modify an element of a.
print b # Produces '[1,2, [-100,4]]'. </pre>
<p>In this case, <TT CLass="monofont">a</tT> and <TT Class="monofont">b</TT> are separate list objects, but the elements they contain are shared. Thus, a modification to one of the elements of <TT class="monofont">a</tt> also modifies an element of <tt class="monofont">b</tt> as shown.</p>
<p>A <i>deep copy</i> creates a new object and recursively copies all the objects it contains. There is no built-in function to create deep copies of objects. However, the <Tt cLass="monofont">copy.deepcopy()</Tt> function in the standard library can be used, as shown in the following example:</p>
<prE>
import copy
b = [1, 2, [3, 4] ]
a = copy.deepcopy(b) </pre>
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<P><TABLE width="100%" border=0><TR valign="top"><TD><font size=1 color="#C0C0C0"><br></font></TD><TD align=right><font size=1 color="#C0C0C0">Last updated on 3/28/2002<br>Python Essential Reference, Second Edition, © 2002 New Riders Publishing</font></TD></TR></TABLE></P>
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