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8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	4	<title>Chapter 6. OOP</title>
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8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	13	<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="chapter" id="ch6oop">
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	14	<div class="titlepage"></div>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	15	<div class="toc">
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	16	<p><b>Table of Contents</b></p>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	17	<dl><dt><span class="article"><a href="#id2582702">Classes and Objects</a></span></dt></dl>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	18	</div>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	19	<div class="article" title="Classes and Objects">
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	20	<div class="titlepage">
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	21	<div><div><h2 class="title">
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	22	<a name="id2582702"></a>Classes and Objects</h2></div></div>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	23	<hr>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	24	</div>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	25	<p id="ch6oop_1"></a>In the previous sections we learnt about functions which provide certain level
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	26	of abstraction to our code by holding the code which performs one or more
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	27	specific functionalities. We were able to use this function as many times as we
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	28	wanted. In addition to functions, Python also higher level of abstractions
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	29	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
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	30	collection of a set of data items and a set of methods. The data items can be
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	31	any valid Python variable or any Python object. Functions enclosed within a class
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	32	are called as <span class="emphasis"><em>methods</em></span>. If you are thinking if methods are functions why is there
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	33	a distinction between the two? The answer to this will be given as we walk through
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	34	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
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	35	<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>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	36	<p id="ch6oop_2"></a>A class is defined using the keyword <span class="strong"><strong>class</strong></span> followed by the class name, in
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	37	turn followed by a semicolon. The statements that a <span class="emphasis"><em>Class</em></span> encloses are written
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	38	in a new block, i.e on the next indentation level:</p>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	39	<pre class="programlisting"> class Employee:
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	40	def setName(self, name):
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	41	self.name = name
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	42
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	43	def getName(self):
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	44	return self.name</pre>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	45	<p id="ch6oop_3"></a>In the above example, we defined a class with the name Employee. We also defined
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	46	two methods, setName and getName for this class. It is important to note the
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	47	differences between the normal Python functions and class methods defined above.
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	48	Each method of the class must take the same instance of the class(object) from
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	49	which it was called as the first argument. It is conventionally given the name,
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	50	<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
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	51	the first argument to the method will always be the same object of the class
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	52	from which the method was called. The data memebers that belong to the class are
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	53	called as <span class="emphasis"><em>class attributes</em></span>. <span class="emphasis"><em>Class attributes</em></span> are preceded by the object of
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	54	the class and a dot. In the above example, <span class="emphasis"><em>name</em></span> is a class attribute since it
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	55	is preceded by the <span class="emphasis"><em>self</em></span> object. <span class="emphasis"><em>Class attributes</em></span> can be accessed from
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	56	anywhere within the class.</p>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	57	<p id="ch6oop_4"></a>We can create objects of a class outside the class definition by using the same
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	58	syntax we use to call a function with no parameters. We can assign this object
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	59	to a variable:</p>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	60	<pre class="programlisting"> emp = Employee()</pre>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	61	<p id="ch6oop_5"></a>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>.
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	62	All the attributes and methods of the class can be accessed by the object of the
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	63	class using the standard notation <span class="emphasis"><em>object.attribute</em></span> or <span class="emphasis"><em>object.method()</em></span>.
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	64	Although the first parameter of a class method is the self object, it must not
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	65	be passed as an argument when calling the method. The <span class="emphasis"><em>self</em></span> object is implicitly
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	66	passed to the method by the Python interpreter. All other arguments passing rules
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	67	like default arguments, keyword arguments, argument packing and unpacking follow
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	68	the same rules as those for ordinary Python functions:</p>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	69	<pre class="programlisting"> >>> emp.setName('John')
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	70	>>> name = emp.getName()
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	71	>>> print name
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	72	John
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	73	>>> print emp.name
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	74	John</pre>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	75	<p id="ch6oop_6"></a>If we at all try to access a class attribute before assigning a value to it, i.e
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	76	before creating it, Python raises the same error as it would raise for the
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	77	accessing undefined variable:</p>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	78	<pre class="programlisting"> >>> emp = Employee()
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	79	>>> emp.name
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	80	Traceback (most recent call last):
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	81	File "class.py", line 10, in <module>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	82	print e.name
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	83	AttributeError: Employee instance has no attribute 'name'</pre>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	84	</div>
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8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	86	</html>

author	amit@thunder
	Mon, 25 Jan 2010 18:56:45 +0530
changeset 0	8083d21c0020
child 1	672eaaab9204
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