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author Radomir Dopieralski <radomir.dopieralski@allegro.pl>
date Wed, 27 Jun 2012 16:35:54 +0200
parents 4e6cfa066d1a
children
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1 JavaScript objects
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2 ==================
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3
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4 Like Python classes
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5 -------------------
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6
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7 In JavaScript all objects are part of an inheritance tree. The
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8 ``create`` function adds a node to the inheritance tree.
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9
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10 .. code-block:: javascript
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11
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12 // A JavaScript object.
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13 js> root = {}
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14
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15 // Prototype inheritance.
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16 js> create = function (obj) {
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17 var f = function () {return this;};
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18 f.prototype = obj;
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19 return new f;
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20 }
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22 js> a = create(root)
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23 js> b = create(a)
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24
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25 js> a.name = 5
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26 js> a.name
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27 5
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28 js> b.name
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29 5
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31 In Python classes inherit in the same way.
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32
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33 .. doctest::
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34
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35 >>> root = type # Most classes are instance of type.
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36 >>> class a(root): pass
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37 >>> class b(a): pass # Class inheritance.
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38
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39 >>> a.name = 5 # Just like JavaScript.
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40 >>> a.name
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41 5
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42 >>> b.name
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43 5
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44
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45 class explanation
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46 ^^^^^^^^^^^^^^^^^
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47
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48 In Python we can subclass anything whose type is ``type`` (or a
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49 subclass of type). A subclass (and its instances) inherits properties
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50 from the super-class.
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51
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52 >>> type(root) == type(a) == type(b) == type
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53 True
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57 Custom item methods
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58 -------------------
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59
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60 In JavaScript attribute and item access are the same.
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61
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62 .. code-block:: javascript
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64 js> a = create(root)
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66 js> a.name = 5
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67 js> a['name']
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68 5
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70 js> a['key'] = 6
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71 js> a.key
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72 6
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74 js> a[1] = 6
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75 js> a['1']
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76 6
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77
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78 In Python we can define our own item methods. (The programmer owns
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79 the dot.)
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80
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81 .. doctest::
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82
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83 >>> class A(object):
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84 ...
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85 ... def __getitem__(self, key):
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86 ... return getattr(self, str(key))
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87 ... def __setitem__(self, key, value):
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88 ... return setattr(self, str(key), value)
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89
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90 >>> a = A()
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91 >>> a.name = 5
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93 >>> a['name']
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94 5
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95
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96 >>> a['key'] = 6
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97 >>> a.key
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98 6
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100 >>> a[1] = 6
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101 >>> a['1']
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102 6
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103
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104 Because ``type(a)`` is ``A``, which has the special item methods, we
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105 get the special item behaviour.
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106
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107 .. doctest::
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108
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109 >>> type(a) is A
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110 True
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113 On metaclass
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114 ------------
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115
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116 Using previous definition, we cannot subclass ``a`` to create ``b``.
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118 .. doctest::
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119
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120 >>> class b(a): pass
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121 Traceback (most recent call last):
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122 class b(a): pass
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123 TypeError: Error when calling the metaclass bases
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124 object.__new__() takes no parameters
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125
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126 This is because ``a`` is not a type. The solution involves Python
94
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127 metaclasses (an advanced topic).
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128
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129 .. doctest::
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130
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131 >>> isinstance(a, type)
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132 False
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133
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134
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135 metaclass construction
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136 ^^^^^^^^^^^^^^^^^^^^^^
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137 We will subclass type, not object, and add to it the special item
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138 methods.
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139
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140
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141 .. doctest::
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142
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143 >>> class ObjectType(type):
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144 ...
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145 ... def __getitem__(self, key):
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146 ... return getattr(self, str(key))
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147 ...
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148 ... def __setitem__(self, key, value):
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149 ... return setattr(self, str(key), value)
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150
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151 Here is a fancy way of calling **ObjectType**.
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152
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153 .. doctest::
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154
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155 >>> class root(object):
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156 ... __metaclass__ = ObjectType
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157
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158
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159 Here is a more direct (and equivalent) construction (create an
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160 instance of ``ObjectType``, whose instances are objects).
94
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161
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162 .. doctest::
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163
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164 >>> root = ObjectType('root', (object,), {})
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165 >>> isinstance(root(), object)
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166 True
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167
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168 metaclass demonstration
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169 ^^^^^^^^^^^^^^^^^^^^^^^
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170 .. doctest::
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171
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172 >>> class a(root): pass
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173 >>> class b(a): pass
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174
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175 >>> a.name = 5
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176 >>> a.name
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177 5
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178 >>> b.name
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179 5
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180 >>> a['name']
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181 5
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182 >>> b['name']
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183 5
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184
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185 >>> a[1] = 6
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186 >>> a['1']
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187 6
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188
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189
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190 metaclass explanation
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191 ^^^^^^^^^^^^^^^^^^^^^
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192
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193 Because ``type(root)`` is a subclass of type we can subclass root.
94
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194
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195 .. doctest::
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196
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197 >>> issubclass(type(root), type)
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198 True
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199
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200 Because the ``type(root)`` is ``ObjectType``, which has special item
94
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201 methods, we get the special item behaviour.
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202
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203 .. doctest::
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204
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205 >>> type(root) == type(a) == type(b) == ObjectType
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206 True
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207
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208
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209 Never instantiated
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210 ------------------
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211
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212 We can't call JavaScript objects (unless they are a function). But
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213 ``create`` creates ordinary JavaScript objects.
94
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214
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215 .. code-block:: javascript
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216
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217 js> a = create(root)
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218 js> a(1, 2, 3)
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219 TypeError: a is not a function
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220
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221
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222 We will monkey-patch the previous Python class, to provide custom
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223 behaviour when called.
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224
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225 .. doctest::
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226
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227 >>> def raise_not_a_function(obj, *argv, **kwargs):
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228 ... raise TypeError, obj.__name__ + ' is not a function'
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229
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230 >>> ObjectType.__call__ = raise_not_a_function
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231
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232 >>> a(1, 2, 3)
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233 Traceback (most recent call last):
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234 a(1, 2, 3)
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235 TypeError: a is not a function
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236
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237
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238 Conclusion
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239 ----------
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240
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241 JavaScript objects are like Python classes (because they inherit like
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242 Python classes).
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243
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244 For JavaScript attribute and item access are the same. This is
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245 achieved in Python by providing custom item methods.
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246
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247 In Python the custom item methods must be placed on the type of the
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248 object (or a superclass of its type).
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249
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250 Ordinary JavaScript objects are not functions and cannot be called. A
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251 Python class can be called (to create an instance of the object). But
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252 we can override this behaviour by supplying a custom method for call.
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253
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254 To summarize:
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255 ..
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256
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257 **JavaScript objects are like Python classes with custom item
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258 methods (on the metaclass) that are never instantiated.**
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259
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260 It's worth saying again:
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261
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262 ..
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263
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264 **JavaScript objects are like Python classes with custom item
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265 methods (on the metaclass) that are never instantiated.**
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266
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267
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268