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<h3>Terminology</h3>
<p>Every piece of data stored in a program is an object. Each object has an identity, a type, and a value.</p>
<P>For example, when you write <TT Class="monofont">a = 42</TT>, an integer object is created with the value of <TT clasS="monofont">42</TT>. You can view the <I>identity</i> of an object as a pointer to its location in memory. In this example, <tt class="monofont">a</tt> is a name that refers to this specific location.</p>
<p>The <i>type</i> of an object (which is itself a special kind of object) describes the internal representation of the object as well as the methods and operations that it supports. When an object of a particular type is created, that object is sometimes called an <i>instance</i> of that type (although an instance of a type should not be confused with an instance of a user-defined class). After an object is created, its identity and type cannot be changed. If an object抯 value can be modified, the object is said to be <i>mutable</i>. If the value cannot be modified, the object is said to be <i>immutable</i>. An object that contains references to other objects is said to be a <I>container</i> or <i>collection</I>.</p>
<p>In addition to holding a value, many objects define a number of data attributes and methods. An <i>attribute</I> is a property or value associated with an object. A <i>method</i> is a function that performs some sort of operation on an object when the method is invoked. Attributes and methods are accessed using the dot (<tt ClasS="monofont">.</TT>) operator, as shown in the following example:</P>
<pre>
a = 3 + 4j # Create a complex number
r = a.real # Get the real part (an attribute)
b = [1, 2, 3] # Create a list
b.append(7) # Add a new element using the append method </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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