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