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<h3>The <tt cLASS="monofont">class</tt> statement</h3>
<p>A <I>class</I> defines a set of attributes that are associated with a collection of objects known as <I>instances</I>. These attributes typically include variables that are known as <i>class variables</i> and functions that are known as <i>methods</i>.</P>
<P>Classes are defined using the <TT class="monofont">class</tt> statement. The body of a class contains a series of statements that are executed when the class is first defined (see <a href="74#2.html">Listing 7.1</a>).</p>
<h5>
<a name="2"></A>Listing 7.1 Classes</h5>
<pRe clAss="monofont">
class Account:
"A simple class"
account_type = "Basic"
def _ _init_ _(self,name,balance):
"Initialize a new Account instance"
self.name = name
self.balance = balance
def deposit(self,amt):
"Add to the balance"
self.balance = self.balance + amt
def withdraw(self,amt):
"Subtract from the balance"
self.balance = self.balance - amt
def inquiry(self):
"Return the current balance"
return self.balance </prE>
<p>The objects created during the execution of the class body are placed into a class object that serves as a namespace. For example, the members of the <tt CLASs="monofont">Account</tt> class are accessible as follows:</p>
<PRE>
Account.account_type
Account._ _init_ _
Account.deposit
Account. withdrawAccount.
inquiry </Pre>
<p>It抯 important to note that a class statement doesn抰 create any instances of a class (for example, no accounts are actually created in the preceding example). Rather, a class only defines the set of attributes that are shared by all of the instances that will be created.</p>
<P>The functions defined within a class (methods) always operate on a class instance that抯 passed as the first argument. By convention, this argument is called <TT Class="monofont">self</TT>, although any legal identifier name can be used. Class variables such as <TT class="monofont">account_type</tt> are shared among all instances of a class (that is, they抮e not individually assigned to each instance). Although a class defines a namespace, this namespace is not a scope for code appearing inside the class body. Thus, references to other attributes of a class must use a fully qualified name, as shown in the following example:</p>
<pre>
class Foo:
def bar(self):
print "bar!"
def spam(self):
bar(self) # Incorrect! 'bar' generates a NameError
Foo.bar(self) # This works </pre>
<p>Finally, you cannot define class methods that don抰 operate on instances:</p>
<pre>
class Foo:
def add(x,y):
return x+y
a = Foo.add(3,4) # TypeError. Need class instance as first argument </pRe>
</fOnt>
<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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