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13 <body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="chapter" id="ch5func"> |
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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> |
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18 <dt><span class="article"><a href="#id2436407">Functional Approach</a></span></dt> |
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19 <dd><dl> |
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20 <dt><span class="section"><a href="#id2487445">1. Function scope</a></span></dt> |
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21 <dt><span class="section"><a href="#id2487503">2. Default Arguments</a></span></dt> |
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22 <dt><span class="section"><a href="#id2487542">3. Keyword Arguments</a></span></dt> |
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23 <dt><span class="section"><a href="#id2487658">4. Parameter Packing and Unpacking</a></span></dt> |
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24 <dt><span class="section"><a href="#id2487758">5. Nested Functions and Scopes</a></span></dt> |
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25 <dt><span class="section"><a href="#id2487811">6. map, reduce and filter functions</a></span></dt> |
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26 <dd><dl><dt><span class="section"><a href="#id2488001">6.1. List Comprehensions</a></span></dt></dl></dd> |
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27 </dl></dd> |
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28 </dl> |
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29 </div> |
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30 <div class="article" title="Functional Approach"> |
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31 <div class="titlepage"> |
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32 <div><div><h2 class="title"> |
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33 <a name="id2436407"></a>Functional Approach</h2></div></div> |
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34 <hr> |
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35 </div> |
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36 <div class="toc"> |
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37 <p><b>Table of Contents</b></p> |
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38 <dl> |
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39 <dt><span class="section"><a href="#id2487445">1. Function scope</a></span></dt> |
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40 <dt><span class="section"><a href="#id2487503">2. Default Arguments</a></span></dt> |
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41 <dt><span class="section"><a href="#id2487542">3. Keyword Arguments</a></span></dt> |
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42 <dt><span class="section"><a href="#id2487658">4. Parameter Packing and Unpacking</a></span></dt> |
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43 <dt><span class="section"><a href="#id2487758">5. Nested Functions and Scopes</a></span></dt> |
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44 <dt><span class="section"><a href="#id2487811">6. map, reduce and filter functions</a></span></dt> |
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45 <dd><dl><dt><span class="section"><a href="#id2488001">6.1. List Comprehensions</a></span></dt></dl></dd> |
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46 </dl> |
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47 </div> |
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48 <p id="ch5func_1"></a><span class="emphasis"><em>Functions</em></span> allow us to enclose a set of statements and call the function again |
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49 and again instead of repeating the group of statements everytime. Functions also |
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50 allow us to isolate a piece of code from all the other code and provides the |
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51 convenience of not polluting the global variables.</p> |
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52 <p id="ch5func_2"></a><span class="emphasis"><em>Function</em></span> in python is defined with the keyword <span class="strong"><strong>def</strong></span> followed by the name |
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53 of the function, in turn followed by a pair of parenthesis which encloses the |
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54 list of parameters to the function. The definition line ends with a ':'. The |
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55 definition line is followed by the body of the function intended by one block. |
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56 The <span class="emphasis"><em>Function</em></span> must return a value:</p> |
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57 <pre class="programlisting"> def factorial(n): |
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58 fact = 1 |
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59 for i in range(2, n): |
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60 fact *= i |
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61 |
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62 return fact</pre> |
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63 <p id="ch5func_3"></a>The code snippet above defines a function with the name factorial, takes the |
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64 number for which the factorial must be computed, computes the factorial and |
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65 returns the value.</p> |
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66 <p id="ch5func_4"></a>A <span class="emphasis"><em>Function</em></span> once defined can be used or called anywhere else in the program. We |
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67 call a fucntion with its name followed by a pair of parenthesis which encloses |
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68 the arguments to the function.</p> |
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69 <p id="ch5func_5"></a>The value that function returns can be assigned to a variable. Let's call the |
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70 above function and store the factorial in a variable:</p> |
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71 <pre class="programlisting"> fact5 = factorial(5)</pre> |
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72 <p id="ch5func_6"></a>The value of fact5 will now be 120, which is the factorial of 5. Note that we |
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73 passed 5 as the argument to the function.</p> |
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74 <p id="ch5func_7"></a>It may be necessary to document what the function does, for each of the function |
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75 to help the person who reads our code to understand it better. In order to do |
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76 this Python allows the first line of the function body to be a string. This |
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77 string is called as <span class="emphasis"><em>Documentation String</em></span> or <span class="emphasis"><em>docstring</em></span>. <span class="emphasis"><em>docstrings</em></span> prove |
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78 to be very handy since there are number of tools which can pull out all the |
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79 docstrings from Python functions and generate the documentation automatically |
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80 from it. <span class="emphasis"><em>docstrings</em></span> for functions can be written as follows:</p> |
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81 <pre class="programlisting"> def factorial(n): |
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82 'Returns the factorial for the number n.' |
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83 fact = 1 |
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84 for i in range(2, n): |
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85 fact *= i |
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86 |
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87 return fact</pre> |
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88 <p id="ch5func_8"></a>An important point to note at this point is that, a function can return any |
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89 Python value or a Python object, which also includes a <span class="emphasis"><em>Tuple</em></span>. A <span class="emphasis"><em>Tuple</em></span> is |
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90 just a collection of values and those values themselves can be of any other |
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91 valid Python datatypes, including <span class="emphasis"><em>Lists</em></span>, <span class="emphasis"><em>Tuples</em></span>, <span class="emphasis"><em>Dictionaries</em></span> among other |
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92 things. So effectively, if a function can return a tuple, it can return any |
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93 number of values through a tuple</p> |
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94 <p id="ch5func_9"></a>Let us write a small function to swap two values:</p> |
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95 <pre class="programlisting"> def swap(a, b): |
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96 return b, a |
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97 |
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98 c, d = swap(a, b)</pre> |
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99 <div class="section" title="1. Function scope"> |
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100 <div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
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101 <a name="id2487445"></a>1. Function scope</h2></div></div></div> |
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102 <p id="ch5func_a"></a>The variables used inside the function are confined to the function's scope |
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103 and doesn't pollute the variables of the same name outside the scope of the |
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104 function. Also the arguments passed to the function are passed by-value if |
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105 it is of basic Python data type:</p> |
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106 <pre class="programlisting"> def cant_change(n): |
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107 n = 10 |
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108 |
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109 n = 5 |
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110 cant_change(n)</pre> |
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111 <p id="ch5func_b"></a>Upon running this code, what do you think would have happened to value of n |
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112 which was assigned 5 before the function call? If you have already tried out |
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113 that snippet on the interpreter you already know that the value of n is not |
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114 changed. This is true of any immutable types of Python like <span class="emphasis"><em>Numbers</em></span>, <span class="emphasis"><em>Strings</em></span> |
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115 and <span class="emphasis"><em>Tuples</em></span>. But when you pass mutable objects like <span class="emphasis"><em>Lists</em></span> and <span class="emphasis"><em>Dictionaries</em></span> |
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116 the values are manipulated even outside the function:</p> |
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117 <pre class="programlisting"> >>> def can_change(n): |
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118 ... n[1] = James |
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119 ... |
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120 |
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121 >>> name = ['Mr.', 'Steve', 'Gosling'] |
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122 >>> can_change(name) |
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123 >>> name |
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124 ['Mr.', 'James', 'Gosling']</pre> |
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125 <p id="ch5func_c"></a>If nothing is returned by the function explicitly, Python takes care to return |
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126 None when the funnction is called.</p> |
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127 </div> |
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128 <div class="section" title="2. Default Arguments"> |
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129 <div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
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130 <a name="id2487503"></a>2. Default Arguments</h2></div></div></div> |
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131 <p id="ch5func_d"></a>There may be situations where we need to allow the functions to take the |
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132 arguments optionally. Python allows us to define function this way by providing |
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133 a facility called <span class="emphasis"><em>Default Arguments</em></span>. For example, we need to write a function |
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134 that returns a list of fibonacci numbers. Since our function cannot generate an |
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135 infinite list of fibonacci numbers, we need to specify the number of elements |
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136 that the fibonacci sequence must contain. Suppose, additionally, we want to the |
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137 function to return 10 numbers in the sequence if no option is specified we can |
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138 define the function as follows:</p> |
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139 <pre class="programlisting"> def fib(n=10): |
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140 fib_list = [0, 1] |
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141 for i in range(n - 2): |
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142 next = fib_list[-2] + fib_list[-1] |
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143 fib_list.append(next) |
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144 return fib_list</pre> |
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145 <p id="ch5func_e"></a>When we call this function, we can optionally specify the value for the |
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146 parameter n, during the call as an argument. Calling with no argument and |
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147 argument with n=5 returns the following fibonacci sequences:</p> |
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148 <pre class="programlisting"> fib() |
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149 [0, 1, 1, 2, 3, 5, 8, 13, 21, 34] |
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150 fib(5) |
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151 [0, 1, 1, 2, 3]</pre> |
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152 </div> |
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153 <div class="section" title="3. Keyword Arguments"> |
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154 <div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
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155 <a name="id2487542"></a>3. Keyword Arguments</h2></div></div></div> |
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156 <p id="ch5func_f"></a>When a function takes a large number of arguments, it may be difficult to |
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157 remember the order of the parameters in the function definition or it may |
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158 be necessary to pass values to only certain parameters since others take |
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159 the default value. In either of these cases, Python provides the facility |
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160 of passing arguments by specifying the name of the parameter as defined in |
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161 the function definition. This is known as <span class="emphasis"><em>Keyword Arguments</em></span>.</p> |
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162 <p id="ch5func_10"></a>In a function call, <span class="emphasis"><em>Keyword arguments</em></span> can be used for each argument, in the |
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163 following fashion:</p> |
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164 <pre class="programlisting"> argument_name=argument_value |
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165 Also denoted as: keyword=argument |
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166 |
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167 def wish(name='World', greetings='Hello'): |
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168 print "%s, %s!" % (greetings, name)</pre> |
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169 <p id="ch5func_11"></a>This function can be called in one of the following ways. It is important to |
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170 note that no restriction is imposed in the order in which <span class="emphasis"><em>Keyword arguments</em></span> |
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171 can be specified. Also note, that we have combined <span class="emphasis"><em>Keyword arguments</em></span> with |
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172 <span class="emphasis"><em>Default arguments</em></span> in this example, however it is not necessary:</p> |
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173 <pre class="programlisting"> wish(name='Guido', greetings='Hey') |
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174 wish(greetings='Hey', name='Guido')</pre> |
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175 <p id="ch5func_12"></a>Calling functions by specifying arguments in the order of parameters specified |
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176 in the function definition is called as <span class="emphasis"><em>Positional arguments</em></span>, as opposed to |
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177 <span class="emphasis"><em>Keyword arguments</em></span>. It is possible to use both <span class="emphasis"><em>Positional arguments</em></span> and |
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178 <span class="emphasis"><em>Keyword arguments</em></span> in a single function call. But Python doesn't allow us to |
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179 bungle up both of them. The arguments to the function, in the call, must always |
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180 start with <span class="emphasis"><em>Positional arguments</em></span> which is in turn followed by <span class="emphasis"><em>Keyword |
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181 arguments</em></span>:</p> |
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182 <pre class="programlisting"> def my_func(x, y, z, u, v, w): |
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183 # initialize variables. |
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184 ... |
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185 # do some stuff |
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186 ... |
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187 # return the value</pre> |
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188 <p id="ch5func_13"></a>It is valid to call the above functions in the following ways:</p> |
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189 <pre class="programlisting"> my_func(10, 20, 30, u=1.0, v=2.0, w=3.0) |
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190 my_func(10, 20, 30, 1.0, 2.0, w=3.0) |
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191 my_func(10, 20, z=30, u=1.0, v=2.0, w=3.0) |
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192 my_func(x=10, y=20, z=30, u=1.0, v=2.0, w=3.0)</pre> |
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193 <p id="ch5func_14"></a>Following lists some of the invalid calls:</p> |
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194 <pre class="programlisting"> my_func(10, 20, z=30, 1.0, 2.0, 3.0) |
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195 my_func(x=10, 20, z=30, 1.0, 2.0, 3.0) |
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196 my_func(x=10, y=20, z=30, u=1.0, v=2.0, 3.0)</pre> |
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197 </div> |
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198 <div class="section" title="4. Parameter Packing and Unpacking"> |
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199 <div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
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200 <a name="id2487658"></a>4. Parameter Packing and Unpacking</h2></div></div></div> |
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201 <p id="ch5func_15"></a>The positional arguments passed to a function can be collected in a tuple |
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202 parameter and keyword arguments can be collected in a dictionary. Since keyword |
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203 arguments must always be the last set of arguments passed to a function, the |
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204 keyword dictionary parameter must be the last parameter. The function definition |
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205 must include a list explicit parameters, followed by tuple paramter collecting |
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206 parameter, whose name is preceded by a <span class="strong"><strong>*</strong></span>, for collecting positional |
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207 parameters, in turn followed by the dictionary collecting parameter, whose name |
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208 is preceded by a <span class="strong"><strong>**</strong></span></p> |
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209 <pre class="programlisting"> def print_report(title, *args, **name): |
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210 """Structure of *args* |
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211 (age, email-id) |
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212 Structure of *name* |
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213 { |
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214 'first': First Name |
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215 'middle': Middle Name |
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216 'last': Last Name |
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217 } |
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218 """ |
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219 |
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220 print "Title: %s" % (title) |
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221 print "Full name: %(first)s %(middle)s %(last)s" % name |
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222 print "Age: %d\nEmail-ID: %s" % args</pre> |
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223 <p id="ch5func_16"></a>The above function can be called as. Note, the order of keyword parameters can |
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224 be interchanged:</p> |
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225 <pre class="programlisting"> >>> print_report('Employee Report', 29, 'johny@example.com', first='Johny', |
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226 last='Charles', middle='Douglas') |
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227 Title: Employee Report |
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228 Full name: Johny Douglas Charles |
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229 Age: 29 |
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230 Email-ID: johny@example.com</pre> |
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231 <p id="ch5func_17"></a>The reverse of this can also be achieved by using a very identical syntax while |
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232 calling the function. A tuple or a dictionary can be passed as arguments in |
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233 place of a list of <span class="emphasis"><em>Positional arguments</em></span> or <span class="emphasis"><em>Keyword arguments</em></span> respectively |
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234 using <span class="strong"><strong>*</strong></span> or <span class="strong"><strong>**</strong></span></p> |
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235 <pre class="programlisting"> def print_report(title, age, email, first, middle, last): |
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236 print "Title: %s" % (title) |
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237 print "Full name: %s %s %s" % (first, middle, last) |
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238 print "Age: %d\nEmail-ID: %s" % (age, email) |
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239 |
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240 >>> args = (29, 'johny@example.com') |
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241 >>> name = { |
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242 'first': 'Johny', |
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243 'middle': 'Charles', |
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244 'last': 'Douglas' |
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245 } |
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246 >>> print_report('Employee Report', *args, **name) |
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247 Title: Employee Report |
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248 Full name: Johny Charles Douglas |
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249 Age: 29 |
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250 Email-ID: johny@example.com</pre> |
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251 </div> |
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252 <div class="section" title="5. Nested Functions and Scopes"> |
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253 <div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
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254 <a name="id2487758"></a>5. Nested Functions and Scopes</h2></div></div></div> |
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255 <p id="ch5func_18"></a>Python allows nesting one function inside another. This style of programming |
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256 turns out to be extremely flexible and powerful features when we use <span class="emphasis"><em>Python |
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257 decorators</em></span>. We will not talk about decorators is beyond the scope of this |
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258 course. If you are interested in knowing more about <span class="emphasis"><em>decorator programming</em></span> in |
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259 Python you are suggested to read:</p> |
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260 <span style="color: black"><line_block><span style="color: black"><line><div class="reference"> |
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261 <div class="titlepage"><hr></div>http://avinashv.net/2008/04/python-decorators-syntactic-sugar/</div></line></span><span style="color: black"><line><div class="reference"> |
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262 <div class="titlepage"><hr></div>http://personalpages.tds.net/~kent37/kk/00001.html</div></line></span></line_block></span><p id="ch5func_19"></a>However, the following is an example for nested functions in Python:</p> |
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263 <pre class="programlisting"> def outer(): |
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264 print "Outer..." |
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265 def inner(): |
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266 print "Inner..." |
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267 print "Outer..." |
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268 inner() |
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269 |
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270 >>> outer()</pre> |
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271 </div> |
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272 <div class="section" title="6. map, reduce and filter functions"> |
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273 <div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
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274 <a name="id2487811"></a>6. map, reduce and filter functions</h2></div></div></div> |
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275 <p id="ch5func_1a"></a>Python provides several built-in functions for convenience. The <span class="strong"><strong>map()</strong></span>, |
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276 <span class="strong"><strong>reduce()</strong></span> and <span class="strong"><strong>filter()</strong></span> functions prove to be very useful with sequences like |
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277 <span class="emphasis"><em>Lists</em></span>.</p> |
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278 <p id="ch5func_1b"></a>The <span class="strong"><strong>map</strong></span> (<span class="emphasis"><em>function</em></span>, <span class="emphasis"><em>sequence</em></span>) function takes two arguments: <span class="emphasis"><em>function</em></span> |
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279 and a <span class="emphasis"><em>sequence</em></span> argument. The <span class="emphasis"><em>function</em></span> argument must be the name of the |
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280 function which in turn takes a single argument, the individual element of the |
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281 <span class="emphasis"><em>sequence</em></span>. The <span class="strong"><strong>map</strong></span> function calls <span class="emphasis"><em>function(item)</em></span>, for each item in the |
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282 sequence and returns a list of values, where each value is the value returned |
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283 by each call to <span class="emphasis"><em>function(item)</em></span>. <span class="strong"><strong>map()</strong></span> function allows to pass more than |
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284 one sequence. In this case, the first argument, <span class="emphasis"><em>function</em></span> must take as many |
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285 arguments as the number of sequences passed. This function is called with each |
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286 corresponding element in the each of the sequences, or <span class="strong"><strong>None</strong></span> if one of the |
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287 sequence is exhausted:</p> |
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288 <pre class="programlisting"> def square(x): |
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289 return x*x |
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290 |
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291 >>> map(square, [1, 2, 3, 4]) |
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292 [1, 4, 9, 16] |
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293 |
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294 def mul(x, y): |
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295 return x*y |
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296 |
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297 >>> map(mul, [1, 2, 3, 4], [6, 7, 8, 9])</pre> |
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298 <p id="ch5func_1c"></a>The <span class="strong"><strong>filter</strong></span> (<span class="emphasis"><em>function</em></span>, <span class="emphasis"><em>sequence</em></span>) function takes two arguments, similar to |
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299 the <span class="strong"><strong>map()</strong></span> function. The <span class="strong"><strong>filter</strong></span> function calls <span class="emphasis"><em>function(item)</em></span>, for each |
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300 item in the sequence and returns all the elements in the sequence for which |
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301 <span class="emphasis"><em>function(item)</em></span> returned True:</p> |
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302 <pre class="programlisting"> def even(x): |
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303 if x % 2: |
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304 return True |
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305 else: |
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306 return False |
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307 |
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308 >>> filter(even, range(1, 10)) |
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309 [1, 3, 5, 7, 9]</pre> |
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310 <p id="ch5func_1d"></a>The <span class="strong"><strong>reduce</strong></span> (<span class="emphasis"><em>function</em></span>, <span class="emphasis"><em>sequence</em></span>) function takes two arguments, similar to |
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311 <span class="strong"><strong>map</strong></span> function, however multiple sequences are not allowed. The <span class="strong"><strong>reduce</strong></span> |
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312 function calls <span class="emphasis"><em>function</em></span> with first two consecutive elements in the sequence, |
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313 obtains the result, calls <span class="emphasis"><em>function</em></span> with the result and the subsequent element |
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314 in the sequence and so on until the end of the list and returns the final result:</p> |
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315 <pre class="programlisting"> def mul(x, y): |
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316 return x*y |
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317 |
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318 >>> reduce(mul, [1, 2, 3, 4]) |
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319 24</pre> |
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320 <div class="section" title="6.1. List Comprehensions"> |
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321 <div class="titlepage"><div><div><h3 class="title"> |
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322 <a name="id2488001"></a>6.1. List Comprehensions</h3></div></div></div> |
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323 <p id="ch5func_1e"></a>List Comprehension is a convenvience utility provided by Python. It is a |
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324 syntatic sugar to create <span class="emphasis"><em>Lists</em></span>. Using <span class="emphasis"><em>List Comprehensions</em></span> one can create |
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325 <span class="emphasis"><em>Lists</em></span> from other type of sequential data structures or other <span class="emphasis"><em>Lists</em></span> itself. |
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326 The syntax of <span class="emphasis"><em>List Comprehensions</em></span> consists of a square brackets to indicate |
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327 the result is a <span class="emphasis"><em>List</em></span> within which we include at least one <span class="strong"><strong>for</strong></span> clause and |
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328 multiple <span class="strong"><strong>if</strong></span> clauses. It will be more clear with an example:</p> |
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329 <pre class="programlisting"> >>> num = [1, 2, 3] |
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330 >>> sq = [x*x for x in num] |
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331 >>> sq |
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332 [1, 4, 9] |
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333 >>> all_num = [1, 2, 3, 4, 5, 6, 7, 8, 9] |
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334 >>> even = [x for x in all_num if x%2 == 0]</pre> |
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335 <p id="ch5func_1f"></a>The syntax used here is very clear from the way it is written. It can be |
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336 translated into english as, "for each element x in the list all_num, |
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337 if remainder of x divided by 2 is 0, add x to the list."</p> |
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338 </div> |
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339 </div> |
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340 </div> |
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341 </div></body> |
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342 </html> |