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author Radomir Dopieralski <radomir.dopieralski@allegro.pl>
date Wed, 27 Jun 2012 16:35:54 +0200
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1 Classes
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2 =======
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4 In Python, *class* is a language primitive. JavaScript has no
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5 built-in concept of class. Therefore, if we are to have classes in
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6 JavaScript then we (or some library) must implement them. Here we
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7 give an outline implementation of classes in JavaScript, so that the
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8 basic language features can be understood.
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9
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10 :ref:`Recall <inheritance-tree>` that each object has a parent, which
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11 in turn has a parent, all the way up to the root object (which has no
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12 parent). In Python (and other languages), classes are a way for
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13 instances to share data and methods. As a first approximation, let us
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14 say *two objects belong to the same class if they have the same
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15 parent*.
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17 As well as sharing class data, each instance of a class has its own
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18 data, which is added to the instance as part of its creation. In
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19 Python, this is usually done using the ``__init__`` method (although
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20 sometimes ``__new__`` is also used).
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22 Point in Python
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23 ---------------
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25 In Python we might write:
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27 .. code-block:: python
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29 class Point(object):
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31 def __init__(self, x, y):
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32 self.x = x
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33 self.y = y
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35 def move(self, dx, dy):
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36 '''Move point by changes dx and dy.'''
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37 self.x += dx
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38 self.y += dy
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40 mypoint = Point(2, 3)
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41 mypoint.move(1, 1)
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44 Point in JavaScript
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45 -------------------
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47 To implement a similar class in JavaScript we need a function Point
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48 that returns a point, and an object that is a parent for all the
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49 points created by the ``Point`` function. In Python we had a single
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50 object, namely the class ``Point``, that performed both roles. In
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51 JavaScript we need a constructor function and a parent object.
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54 Parent object
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55 ^^^^^^^^^^^^^
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57 We'll start with the parent object:
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59 .. code-block:: javascript
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61 var base = {}; // Grandparent for all instances.
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62 var point = create(base); // Parent to all points.
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64 point.__init__ = function(x, y){
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65 this.x = x;
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66 this.y = y;
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67 };
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69 point.move = function(dx, dy){
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70 this.x += dx;
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71 this.y += dy;
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72 };
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73
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74 See [link] for why we use ``this`` rather than ``self``.
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76
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77 Constructor
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78 ^^^^^^^^^^^
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80 We also need a constructor function
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81
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82 .. code-block:: javascript
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83
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84 var Point = function(x, y){
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85
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86 var instance = create(point);
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87 instance.__init__(x, y);
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88 return instance;
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89 };
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91
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92 In production we would use ``arguments`` and ``apply`` rather than
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93 directly calling ``__init__``. Do you know why?
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94
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95 Advanced features
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96 -----------------
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97
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98 This can be omitted at a first reading.
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99
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100 Improved ``toString``
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101 ^^^^^^^^^^^^^^^^^
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102
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103 In Python we have, for example:
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104
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105 .. code-block:: python
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106
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107 py> from point import Point
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108 py> Point(2, 3)
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109 <point.Point object at 0x20ead90>
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110
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111 We can get something similar in JavaScript by writing:
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112
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113 .. code-block:: javascript
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114
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115 point.__classname__ = 'Point';
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116 base.toString = function(){
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117 return '<' + this.__classname__ + ' object>'
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118 };
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119
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120 Constructor factory
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121 ^^^^^^^^^^^^^^^^^^^
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122
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123 Here is a factory function for producing constructor functions,
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124 such as ``Point`` above.
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125
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126 .. code-block:: javascript
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127
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128 var constructor_factory(parent){
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129 return function(){
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130 var instance = create(parent);
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131 instance.__init__.apply(instance, arguments);
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132 return instance;
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133 };
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134 };
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135
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136 var Point = constructor_factory(point);
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137