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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 What do you need to know in Python
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2 ==================================
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3
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4 Global and local scopes
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5 -----------------------
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7 Whenever you reference a name in Python, it looks for it in several scopes. First it tries to find it in the *local* scope, which is usually the body of the function that is being currently executed. If the name is not there, Python tries the *global* scope, which is the body of the module. When that fails, it tries the *buildins*.
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9 .. code-block:: python
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10
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11 import os
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12 # The name *os* gets defined in the *global* scope.
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13
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14 g = 3
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15 # Define variable *g* in the *global* scope with value 3.
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16
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17 def my_function(x):
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18 # The name *x* gets defined in *local* scope of *my_function*
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19
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20 # *x* is found in the *local* scope and its value is
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21 # assigned to new variable *y* created in *local* scope.
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22 y = x
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23
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24 # The name *os* is not found in the *local* scope, so the *global*
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25 # scope is tried. A new variable *os* is created in the *local* scope
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26 # with value of the *os* from the *global* scope.
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27 os = os
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28
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29 # The name *g* from *global* scope is imported into the *local* scope.
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30 global g
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31
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32 # The *global* variable *g* is assigned the value of *local*
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33 # variable *x*
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34 g = x
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35
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36 my_function(3)
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38 In JavaScript you have a whole hierarchy of scopes -- each new nested function
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39 creates a new scope. The scopes are searched beginning from the innermost up to
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40 the global object.
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43 Default scope
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44 -------------
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45
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46 In Python, when you create a new name, it goes by default to the *local* scope
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47 (when you are at the module scope, it's equal to the *global* scope).
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48 You don't need to do anything special to make that happen.
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49
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50 In JavaScript, variables go to the global object by default. You have to use
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51 the ``var`` keyword to have them defined locally.
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54 Functions passed as parameters
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55 ------------------------------
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56
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57 Everything is an object in Python. In particular, a function is an object. This
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58 means that you can assign a function to a variable or pass it as a function
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59 argument. This may be useful when you want to have some generic algorithms.
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60 This is used in the ``sort`` method so that you can, for example, sort strings in
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61 case-insensitive way:
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62
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63 .. code-block:: python
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64
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65 def lowercase(s):
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66 return s.lower()
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67
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68 my_list = ['a', 'B', 'c']
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69 my_list.sort(key=lowercase)
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70 print my_list
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71
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72 You can pass functions as parameters and assign them to variables in JavaScript
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73 too, although there is one important detail that is different about methods.
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75 Method binding
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76 --------------
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77
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78 Methods in Python are bound to object instances. This means that you can pass
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79 them around as shown above, and they will still remember what object they
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80 belong to.
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82 .. code-block:: python
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83
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84 def call_with_a(method):
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85 method('a')
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86
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87 my_list = []
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88 call_with_a(my_list.append)
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89 print my_list
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90
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91 This is not true for JavaScript. In JavaScript the functions and methods don't
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92 know where they came from, and they get the reference to their object at the
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93 moment when they are called.
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94
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95 Closures
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96 --------
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97
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98 You can define a function inside other function in Python. That new function
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99 then has access to all the variables in the local scope of the initial
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100 function. Moreover, if you return that new function from the original function,
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101 all the variables in the original function's scope are still alive -- because
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102 the new function still has access to them. This saved scope is called a
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103 closure.
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104
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105 An example use of a closure is a "private" variable that can be only accessed
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106 by ``get`` and ``set`` functions.
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107
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108 .. code-block:: python
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109
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110 def create_get_set():
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111 """
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112 Get a pair of functions for getting and setting a hidden value.
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113 Using a closure.
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114 """
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115
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116 value = None
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117
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118 def get():
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119 return value
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120
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121 def set(new_value):
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122 value = new_value
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123
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124 return get, set
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125
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126 The ``value`` variable stays in the memory even after ``create_get_set`` finishes,
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127 because the ``get`` and ``set`` functions still hold a reference to it. Because
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128 they are the only objects holding reference to it, the variable is effectively
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129 hidden from other code. You could have a similar effect by using an object:
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130
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131 .. code-block:: python
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132
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133 class GetSet(object):
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134 def __init__(self):
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135 self.value = None
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136
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137 def get(self):
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138 return self.value
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139
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140 def set(self, new_value):
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141 self.value = new_value
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142
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143 def create_get_set():
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144 """
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145 Get a pair of functions for getting and setting a hidden value.
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146 Using a closure.
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147 """
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148
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149 getset = GetSet()
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150 return getset.get, getset.set
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151
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152
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153 Here instead of a closure we have an explicit ``getset`` object. Again, this
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154 object stays in memory because the bound methods ``get`` and ``set`` hold a
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155 reference to it. This couldn't be done in JavaScript.
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156
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157
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158 Star arguments
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159 --------------
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160
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161 You can define a function that takes a variable number of arguments:
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162
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163 .. code-block:: python
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164
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165 def average(*args):
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166 total = 0
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167 for arg in args:
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168 total += arg
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169 return float(total)/len(args)
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170
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171 And you can even have a function that takes keyword arguments:
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172
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173 .. code-block:: python
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174
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175 def get_arguments(*args, **kwargs):
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176 return args, kwargs
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177
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178 get_argumenst(1, 2, 3, four=5)
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179
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180 You can also call a function and pass it a dictionary or list as the argument:
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181
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182 .. code-block:: python
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183
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184 my_list = [1, 2, 3]
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185 my_dict = {'four': 5}
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186
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187 get_arguments(*my_list, **my_dict)
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188
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189 In JavaScript there are other mechanisms for doing that: the *arguments* keyword
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190 has the list of all parameters passed to a function. These are all positional arguments, JavaScript doesn't have keyword arguments. The *apply* method of a function lets you pass an array of parameters in a function call.
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191