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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 Objects
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2 =======
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4 JavaScript and Python have rather different object models and here we
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5 give a broad view of the differences. The simplest way to create a
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6 JavaScript object ``jso`` is to write:
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8 .. code-block:: javascript
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10 jso = {}
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13 The closest builtin equivalent to jso in Python is the dictionary such
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14 as **pyo**, which is created using:
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16 .. code-block:: python
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18 pyo = {}
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21 The two objects jso and pyo have important similarities and
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22 differences (mostly differences).
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454054afcc0a $ cp -piv ../python-classes-tutorial/source/javascript-objects.rst source/ # and some other changes
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454054afcc0a $ cp -piv ../python-classes-tutorial/source/javascript-objects.rst source/ # and some other changes
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25 Late news
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26 ---------
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28 Not incorporated in this document is a recent discovery, that
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30 **JavaScript objects are like Python classes with custom item
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31 methods (on the metaclass) that are never instantiated.**
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33 I've put this material in a page on :doc:`javascript-objects`.
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36 Similarities
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37 ------------
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39 Both objects can be used to store data. Here's a lightly edited
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40 command line session (using the SpiderMonkey JavaScript interpreter):
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42 .. code-block:: javascript
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44 js> jso = {}
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45 js> jso['A'] = 'apple'
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46 js> jso['A']
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47 apple
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50 Here's the corresponding Python session (with *sys.ps1='py> '*):
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52 .. code-block:: python
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54 py> pyo = {}
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55 py> pyo['A'] = 'apple'
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56 py> pyo['A']
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57 'apple'
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58
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59 In the examples above ``'A'`` is the key and ``'apple'`` is the value.
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60 In both cases we are able to store values against keys. In both cases
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61 the value can be any object. In most other respects, ``jso`` and ``pyo`` are
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62 different.
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64 Differences
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65 -----------
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67 Keys
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68 ^^^^
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70 In JavaScript all keys are converted to strings.
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72 .. code-block:: javascript
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74 js> jso = {}
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75 js> jso['1'] = 'one'
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76 js> jso[1]
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77 one
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78
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79 Here's an even more surprising example:
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81 .. code-block:: javascript
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83 js> jso = {}
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84 js> a = {} ; b = {};
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85 js> jso[a] = 'apple'
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86 apple
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87 js> jso[b]
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88 apple
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91 What's going on here? Well, objects ``a`` and ``b`` are converted to
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92 strings, and they have the same string representation, namely *'[object
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93 Object]'*. (We'll see later how we can change this.) So let's try
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94 this out (continuing the previous example):
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96 .. code-block:: javascript
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98 js> jso['[object Object]']
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99 apple
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101 Here, for comparison, is what happens with Python:
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103 .. code-block:: python
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104
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105 py> pyo = {}
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106 py> pyo['1'] = 'one'
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107 py> pyo[1]
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108 Traceback (most recent call last):
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109 File "<stdin>", line 1, in <module>
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110 KeyError: 1
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111 py> a = {}; b = {}
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112 py> pyo[a] = 'apple'
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113 Traceback (most recent call last):
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114 File "<stdin>", line 1, in <module>
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115 TypeError: unhashable type: 'dict'
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119 Here JavaScript, faced with input of the wrong type, has converted it
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120 to the required type, which is a string.
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121
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122 .. note::
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123
2
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124 In JavaScript anything can be a key, and all keys are converted to
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125 strings before lookup. In Python, keys are unchanged, and have to
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126 be hashable.
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127
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128 .. note::
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129
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130 Python built-in objects throw an error when given incorrect input.
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131 JavaScript converts the input to something that might work.
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132
5
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133 .. _object-missing-keys:
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134
1
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135 Missing keys
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136 ^^^^^^^^^^^^
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137 The notation ``a[b]`` is called **indexing**. In Python, indexing with
1
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138 a key that is not present in the dictionary raises an exception.
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139
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140 .. code-block:: python
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141
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142 py> pyo = {}
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143 py> pyo['dne']
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144 Traceback (most recent call last):
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145 File "<stdin>", line 1, in <module>
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146 KeyError: 'dne'
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148
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149 In JavaScript, we get *undefined*, which is a special JavaScript
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150 object (which is diferent from the *null* JavaScript object).
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151
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152 .. code-block:: javascript
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153
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154 js> jso = {}
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155 [object Object]
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156 js> jso['dne']
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157 js> jso['dne'] === undefined
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158 true
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159
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160 (SpiderMonkey does not echo *undefined* at the command prompt, just as
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161 Python does not echo *None*. Therefore we use equality to show that
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162 we truly get undefined.)
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163
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164 .. note::
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165
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166 In JavaScript you won't get a KeyError (or IndexError). Instead,
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167 you get **undefined**.
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168
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169 In practice, JavaScript left to itself rarely throws an error. It
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170 most commonly happens with code such as
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171
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172 .. code-block: javascript
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173
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174 obj_a.method(args)
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175 obj_b.name
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176
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177 where respectively ``obj_a.method`` and ``obj_name`` are undefined.
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178
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179
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180 Attributes
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181 ^^^^^^^^^^
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182
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183 In JavaScript an object has a single set of properties, which can be
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184 accessed both by indexing and as attributes. Here is an example:
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185
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186 .. code-block:: javascript
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187
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188 js> jso = {}
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189 js> jso['a'] = 'apple'
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190 js> jso.a
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191 apple
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192 js> jso.b = 'banana'
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193 banana
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194 js> jso['b']
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195 banana
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196
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197 In Python a dictionary has a set of key-value pairs and in addition it
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198 has dictionary attributes, which are methods.
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199
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200
1
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201 .. note::
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202
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203 In JavaScript the three statements below are completely equivalent
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204 (except for assignment to ``tmp`` as a side effect).
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205
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206 .. code-block:: javascript
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207
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208 x = a.b
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209 x = a['b']
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210 tmp = 'b'; x = a[tmp]
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211
211
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212 In addition, when ``a.b`` is a function then
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213
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214 .. code-block:: javascript
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215
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216 x = a.b(arg1, arg2, arg3)
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217 tmp = 'b'; x = a[tmp](arg1, arg2, arg3)
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218
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219 are equivalent.
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220
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221 But note also that
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222
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223 .. code-block:: javascript
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224
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225 x = a.b(arg1, arg2, arg3)
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226 tmp = a.b; x = tmp(arg1, arg2, arg3
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227
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228 are sometimes completely different.