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-<head><meta http-equiv="Content-Type" content="text/html; charset=utf-8" /><title>Chapter. oop</title><link rel="stylesheet" href="/review/support/styles.css" type="text/css" /><meta name="generator" content="DocBook XSL Stylesheets V1.74.3" /><link rel="shortcut icon" type="image/png" href="/review/support/figs/favicon.png" /><script type="text/javascript" src="/review/support/jquery-min.js"></script><script type="text/javascript" src="/review/support/form.js"></script><script type="text/javascript" src="/review/support/hsbook.js"></script></head>
-<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="chapter" id="ch6oop">
-<div class="titlepage"></div>
-<div class="toc">
-<p><b>Table of Contents</b></p>
-<dl><dt><span class="article"><a href="#id2745996">Classes and Objects</a></span></dt></dl>
-</div>
-<div class="article" title="Classes and Objects">
-<div class="titlepage">
-<div><div><h2 class="title">
-<a name="id2745996"></a>Classes and Objects</h2></div></div>
-<hr />
-</div>
-<p id="ch6oop_1">In the previous sections we learnt about functions which provide certain level
-of abstraction to our code by holding the code which performs one or more
-specific functionalities. We were able to use this function as many times as we
-wanted. In addition to functions, Python also higher level of abstractions
-through <span class="emphasis"><em>Classes</em></span> and <span class="emphasis"><em>Objects</em></span>. <span class="emphasis"><em>Objects</em></span> can be loosely defined as a
-collection of a set of data items and a set of methods. The data items can be
-any valid Python variable or any Python object. Functions enclosed within a class
-are called as <span class="emphasis"><em>methods</em></span>. If you are thinking if methods are functions why is there
-a distinction between the two? The answer to this will be given as we walk through
-the concepts of <span class="emphasis"><em>Classes</em></span> and <span class="emphasis"><em>Objects</em></span>. <span class="emphasis"><em>Classes</em></span> contain the definition for the
-<span class="emphasis"><em>Objects</em></span>. <span class="emphasis"><em>Objects</em></span> are instances of <span class="emphasis"><em>Classes</em></span>.</p>
-<p id="ch6oop_2">A class is defined using the keyword <span class="strong"><strong>class</strong></span> followed by the class name, in
-turn followed by a semicolon. The statements that a <span class="emphasis"><em>Class</em></span> encloses are written
-in a new block, i.e on the next indentation level:</p>
-<pre class="programlisting">
-class Employee:
- def setName(self, name):
- self.name = name
-
- def getName(self):
- return self.name</pre>
-<p id="ch6oop_3">In the above example, we defined a class with the name Employee. We also defined
-two methods, setName and getName for this class. It is important to note the
-differences between the normal Python functions and class methods defined above.
-Each method of the class must take the same instance of the class(object) from
-which it was called as the first argument. It is conventionally given the name,
-<span class="emphasis"><em>self</em></span>. Note that <span class="emphasis"><em>self</em></span> is only a convention. You can use any other name, but
-the first argument to the method will always be the same object of the class
-from which the method was called. The data memebers that belong to the class are
-called as <span class="emphasis"><em>class attributes</em></span>. <span class="emphasis"><em>Class attributes</em></span> are preceded by the object of
-the class and a dot. In the above example, <span class="emphasis"><em>name</em></span> is a class attribute since it
-is preceded by the <span class="emphasis"><em>self</em></span> object. <span class="emphasis"><em>Class attributes</em></span> can be accessed from
-anywhere within the class.</p>
-<p id="ch6oop_4">We can create objects of a class outside the class definition by using the same
-syntax we use to call a function with no parameters. We can assign this object
-to a variable:</p>
-<pre class="programlisting">
-emp = Employee()</pre>
-<p id="ch6oop_5">In the above example, we create an object named <span class="emphasis"><em>emp</em></span> of the class <span class="emphasis"><em>Employee</em></span>.
-All the attributes and methods of the class can be accessed by the object of the
-class using the standard notation <span class="emphasis"><em>object.attribute</em></span> or <span class="emphasis"><em>object.method()</em></span>.
-Although the first parameter of a class method is the self object, it must not
-be passed as an argument when calling the method. The <span class="emphasis"><em>self</em></span> object is implicitly
-passed to the method by the Python interpreter. All other arguments passing rules
-like default arguments, keyword arguments, argument packing and unpacking follow
-the same rules as those for ordinary Python functions:</p>
-<pre class="programlisting">
->>> emp.setName('John')
->>> name = emp.getName()
->>> print name
-John
->>> print emp.name
-John</pre>
-<p id="ch6oop_6">If we at all try to access a class attribute before assigning a value to it, i.e
-before creating it, Python raises the same error as it would raise for the
-accessing undefined variable:</p>
-<pre class="programlisting">
->>> emp = Employee()
->>> emp.name
-Traceback (most recent call last):
- File "class.py", line 10, in <module>
- print e.name
-AttributeError: Employee instance has no attribute 'name'</pre>
-</div>
-</div></body>
-</html>
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