| author | amit@thunder |
| Thu, 25 Feb 2010 00:45:20 +0530 | |
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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> |
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<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="chapter" id="ch6oop"> |
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<div class="titlepage"></div> |
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<div class="toc"> |
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<p><b>Table of Contents</b></p> |
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<dl><dt><span class="article"><a href="#id2745996">Classes and Objects</a></span></dt></dl> |
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</div> |
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<div class="article" title="Classes and Objects"> |
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<div class="titlepage"> |
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<div><div><h2 class="title"> |
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<a name="id2745996"></a>Classes and Objects</h2></div></div> |
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<hr /> |
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</div> |
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<p id="ch6oop_1">In the previous sections we learnt about functions which provide certain level |
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of abstraction to our code by holding the code which performs one or more |
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specific functionalities. We were able to use this function as many times as we |
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wanted. In addition to functions, Python also higher level of abstractions |
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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 |
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collection of a set of data items and a set of methods. The data items can be |
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any valid Python variable or any Python object. Functions enclosed within a class |
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are called as <span class="emphasis"><em>methods</em></span>. If you are thinking if methods are functions why is there |
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a distinction between the two? The answer to this will be given as we walk through |
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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 |
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<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> |
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<p id="ch6oop_2">A class is defined using the keyword <span class="strong"><strong>class</strong></span> followed by the class name, in |
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turn followed by a semicolon. The statements that a <span class="emphasis"><em>Class</em></span> encloses are written |
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in a new block, i.e on the next indentation level:</p> |
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<pre class="programlisting"> |
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class Employee: |
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def setName(self, name): |
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self.name = name |
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def getName(self): |
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return self.name</pre> |
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<p id="ch6oop_3">In the above example, we defined a class with the name Employee. We also defined |
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two methods, setName and getName for this class. It is important to note the |
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differences between the normal Python functions and class methods defined above. |
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Each method of the class must take the same instance of the class(object) from |
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which it was called as the first argument. It is conventionally given the name, |
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<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 |
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the first argument to the method will always be the same object of the class |
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from which the method was called. The data memebers that belong to the class are |
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called as <span class="emphasis"><em>class attributes</em></span>. <span class="emphasis"><em>Class attributes</em></span> are preceded by the object of |
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the class and a dot. In the above example, <span class="emphasis"><em>name</em></span> is a class attribute since it |
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is preceded by the <span class="emphasis"><em>self</em></span> object. <span class="emphasis"><em>Class attributes</em></span> can be accessed from |
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anywhere within the class.</p> |
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<p id="ch6oop_4">We can create objects of a class outside the class definition by using the same |
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syntax we use to call a function with no parameters. We can assign this object |
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to a variable:</p> |
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<pre class="programlisting"> |
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emp = Employee()</pre> |
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<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>. |
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All the attributes and methods of the class can be accessed by the object of the |
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class using the standard notation <span class="emphasis"><em>object.attribute</em></span> or <span class="emphasis"><em>object.method()</em></span>. |
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Although the first parameter of a class method is the self object, it must not |
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be passed as an argument when calling the method. The <span class="emphasis"><em>self</em></span> object is implicitly |
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passed to the method by the Python interpreter. All other arguments passing rules |
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like default arguments, keyword arguments, argument packing and unpacking follow |
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the same rules as those for ordinary Python functions:</p> |
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<pre class="programlisting"> |
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>>> emp.setName('John')
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>>> name = emp.getName() |
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>>> print name |
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John |
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>>> print emp.name |
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John</pre> |
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<p id="ch6oop_6">If we at all try to access a class attribute before assigning a value to it, i.e |
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before creating it, Python raises the same error as it would raise for the |
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accessing undefined variable:</p> |
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<pre class="programlisting"> |
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>>> emp = Employee() |
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>>> emp.name |
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Traceback (most recent call last): |
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File "class.py", line 10, in <module> |
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print e.name |
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AttributeError: Employee instance has no attribute 'name'</pre> |
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</div> |
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</div></body> |
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</html> |
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