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Block 2.6-only doc change.
author Georg Brandl <georg@python.org>
date Sat, 26 Apr 2008 18:28:08 +0000
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1 .. _tut-structures:
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2
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3 ***************
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4 Data Structures
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5 ***************
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6
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7 This chapter describes some things you've learned about already in more detail,
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8 and adds some new things as well.
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9
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10 .. _tut-tuples:
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11
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12 Tuples and Sequences
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13 ====================
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14
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15 We saw that lists and strings have many common properties, such as indexing and
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16 slicing operations. They are two examples of *sequence* data types (see
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17 :ref:`typesseq`). Since Python is an evolving language, other sequence data
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18 types may be added. There is also another standard sequence data type: the
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19 *tuple*.
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20
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21 A tuple consists of a number of values separated by commas, for instance::
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22
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23 >>> t = 12345, 54321, 'hello!'
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24 >>> t[0]
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25 12345
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26 >>> t
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27 (12345, 54321, 'hello!')
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28 >>> # Tuples may be nested:
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29 ... u = t, (1, 2, 3, 4, 5)
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30 >>> u
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31 ((12345, 54321, 'hello!'), (1, 2, 3, 4, 5))
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32
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33 As you see, on output tuples are always enclosed in parentheses, so that nested
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34 tuples are interpreted correctly; they may be input with or without surrounding
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35 parentheses, although often parentheses are necessary anyway (if the tuple is
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36 part of a larger expression).
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37
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38 Tuples have many uses. For example: (x, y) coordinate pairs, employee records
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39 from a database, etc. Tuples, like strings, are immutable: it is not possible
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40 to assign to the individual items of a tuple (you can simulate much of the same
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41 effect with slicing and concatenation, though). It is also possible to create
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42 tuples which contain mutable objects, such as lists.
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43
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44 A special problem is the construction of tuples containing 0 or 1 items: the
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45 syntax has some extra quirks to accommodate these. Empty tuples are constructed
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46 by an empty pair of parentheses; a tuple with one item is constructed by
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47 following a value with a comma (it is not sufficient to enclose a single value
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48 in parentheses). Ugly, but effective. For example::
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49
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50 >>> empty = ()
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51 >>> singleton = 'hello', # <-- note trailing comma
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52 >>> len(empty)
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53 0
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54 >>> len(singleton)
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55 1
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56 >>> singleton
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57 ('hello',)
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58
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59 The statement ``t = 12345, 54321, 'hello!'`` is an example of *tuple packing*:
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60 the values ``12345``, ``54321`` and ``'hello!'`` are packed together in a tuple.
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61 The reverse operation is also possible::
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62
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63 >>> x, y, z = t
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64
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65 This is called, appropriately enough, *sequence unpacking*. Sequence unpacking
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66 requires the list of variables on the left to have the same number of elements
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67 as the length of the sequence. Note that multiple assignment is really just a
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68 combination of tuple packing and sequence unpacking!
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69
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70 There is a small bit of asymmetry here: packing multiple values always creates
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71 a tuple, and unpacking works for any sequence.
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72
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73 .. % XXX Add a bit on the difference between tuples and lists.
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75
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76 .. _tut-morelists:
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77
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78 More on Lists
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79 =============
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80
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81 The list data type has some more methods. Here are all of the methods of list
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82 objects:
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83
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84
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85 .. method:: list.append(x)
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86 :noindex:
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87
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88 Add an item to the end of the list; equivalent to ``a[len(a):] = [x]``.
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89
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91 .. method:: list.extend(L)
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92 :noindex:
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93
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94 Extend the list by appending all the items in the given list; equivalent to
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95 ``a[len(a):] = L``.
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96
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97
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98 .. method:: list.insert(i, x)
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99 :noindex:
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100
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101 Insert an item at a given position. The first argument is the index of the
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102 element before which to insert, so ``a.insert(0, x)`` inserts at the front of
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103 the list, and ``a.insert(len(a), x)`` is equivalent to ``a.append(x)``.
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104
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105
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106 .. method:: list.remove(x)
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107 :noindex:
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108
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109 Remove the first item from the list whose value is *x*. It is an error if there
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110 is no such item.
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111
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112
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113 .. method:: list.pop([i])
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114 :noindex:
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115
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116 Remove the item at the given position in the list, and return it. If no index
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117 is specified, ``a.pop()`` removes and returns the last item in the list. (The
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118 square brackets around the *i* in the method signature denote that the parameter
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119 is optional, not that you should type square brackets at that position. You
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120 will see this notation frequently in the Python Library Reference.)
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121
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122
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123 .. method:: list.index(x)
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124 :noindex:
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125
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126 Return the index in the list of the first item whose value is *x*. It is an
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127 error if there is no such item.
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128
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129
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130 .. method:: list.count(x)
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131 :noindex:
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132
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133 Return the number of times *x* appears in the list.
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134
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135
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136 .. method:: list.sort()
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137 :noindex:
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138
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139 Sort the items of the list, in place.
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140
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141
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142 .. method:: list.reverse()
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143 :noindex:
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144
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145 Reverse the elements of the list, in place.
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146
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147 An example that uses most of the list methods::
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148
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149 >>> a = [66.25, 333, 333, 1, 1234.5]
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150 >>> print(a.count(333), a.count(66.25), a.count('x'))
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151 2 1 0
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152 >>> a.insert(2, -1)
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153 >>> a.append(333)
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154 >>> a
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155 [66.25, 333, -1, 333, 1, 1234.5, 333]
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156 >>> a.index(333)
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157 1
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158 >>> a.remove(333)
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159 >>> a
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160 [66.25, -1, 333, 1, 1234.5, 333]
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161 >>> a.reverse()
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162 >>> a
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163 [333, 1234.5, 1, 333, -1, 66.25]
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164 >>> a.sort()
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165 >>> a
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166 [-1, 1, 66.25, 333, 333, 1234.5]
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167
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168
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169 .. _tut-lists-as-stacks:
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170
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171 Using Lists as Stacks
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172 ---------------------
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173
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174 .. sectionauthor:: Ka-Ping Yee <ping@lfw.org>
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175
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176
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177 The list methods make it very easy to use a list as a stack, where the last
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178 element added is the first element retrieved ("last-in, first-out"). To add an
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179 item to the top of the stack, use :meth:`append`. To retrieve an item from the
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180 top of the stack, use :meth:`pop` without an explicit index. For example::
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181
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182 >>> stack = [3, 4, 5]
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183 >>> stack.append(6)
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184 >>> stack.append(7)
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185 >>> stack
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186 [3, 4, 5, 6, 7]
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187 >>> stack.pop()
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188 7
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189 >>> stack
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190 [3, 4, 5, 6]
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191 >>> stack.pop()
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192 6
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193 >>> stack.pop()
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194 5
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195 >>> stack
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196 [3, 4]
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197
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198
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199 .. _tut-lists-as-queues:
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200
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201 Using Lists as Queues
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202 ---------------------
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203
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204 .. sectionauthor:: Ka-Ping Yee <ping@lfw.org>
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205
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206
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207 You can also use a list conveniently as a queue, where the first element added
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208 is the first element retrieved ("first-in, first-out"). To add an item to the
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209 back of the queue, use :meth:`append`. To retrieve an item from the front of
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210 the queue, use :meth:`pop` with ``0`` as the index. For example::
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211
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212 >>> queue = ["Eric", "John", "Michael"]
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213 >>> queue.append("Terry") # Terry arrives
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214 >>> queue.append("Graham") # Graham arrives
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215 >>> queue.pop(0)
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216 'Eric'
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217 >>> queue.pop(0)
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218 'John'
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219 >>> queue
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220 ['Michael', 'Terry', 'Graham']
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221
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222
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223 List Comprehensions
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224 -------------------
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225
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226 List comprehensions provide a concise way to create lists from sequences.
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227 Common applications are to make lists where each element is the result of
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228 some operations applied to each member of the sequence, or to create a
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229 subsequence of those elements that satisfy a certain condition.
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230
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231
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232 Each list comprehension consists of an expression followed by a :keyword:`for`
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233 clause, then zero or more :keyword:`for` or :keyword:`if` clauses. The result
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234 will be a list resulting from evaluating the expression in the context of the
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235 :keyword:`for` and :keyword:`if` clauses which follow it. If the expression
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236 would evaluate to a tuple, it must be parenthesized.
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237
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238 Here we take a list of numbers and return a list of three times each number::
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239
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240 >>> vec = [2, 4, 6]
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241 >>> [3*x for x in vec]
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242 [6, 12, 18]
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243
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244 Now we get a little fancier::
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245
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246 >>> [[x, x**2] for x in vec]
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247 [[2, 4], [4, 16], [6, 36]]
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248
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249 Here we apply a method call to each item in a sequence::
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250
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251 >>> freshfruit = [' banana', ' loganberry ', 'passion fruit ']
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252 >>> [weapon.strip() for weapon in freshfruit]
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253 ['banana', 'loganberry', 'passion fruit']
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254
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255 Using the :keyword:`if` clause we can filter the stream::
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256
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257 >>> [3*x for x in vec if x > 3]
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258 [12, 18]
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259 >>> [3*x for x in vec if x < 2]
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260 []
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261
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262 Tuples can often be created without their parentheses, but not here::
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263
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264 >>> [x, x**2 for x in vec] # error - parens required for tuples
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265 File "<stdin>", line 1, in ?
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266 [x, x**2 for x in vec]
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267 ^
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268 SyntaxError: invalid syntax
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269 >>> [(x, x**2) for x in vec]
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270 [(2, 4), (4, 16), (6, 36)]
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271
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272 Here are some nested for loops and other fancy behavior::
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273
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274 >>> vec1 = [2, 4, 6]
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275 >>> vec2 = [4, 3, -9]
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276 >>> [x*y for x in vec1 for y in vec2]
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277 [8, 6, -18, 16, 12, -36, 24, 18, -54]
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278 >>> [x+y for x in vec1 for y in vec2]
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279 [6, 5, -7, 8, 7, -5, 10, 9, -3]
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280 >>> [vec1[i]*vec2[i] for i in range(len(vec1))]
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281 [8, 12, -54]
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282
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283 List comprehensions can be applied to complex expressions and nested functions::
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284
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285 >>> [str(round(355/113, i)) for i in range(1, 6)]
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286 ['3.1', '3.14', '3.142', '3.1416', '3.14159']
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287
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288
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289 Nested List Comprehensions
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290 --------------------------
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291
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292 If you've got the stomach for it, list comprehensions can be nested. They are a
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293 powerful tool but -- like all powerful tools -- they need to be used carefully,
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294 if at all.
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295
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296 Consider the following example of a 3x3 matrix held as a list containing three
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297 lists, one list per row::
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298
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299 >>> mat = [
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300 ... [1, 2, 3],
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301 ... [4, 5, 6],
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302 ... [7, 8, 9],
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303 ... ]
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304
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305 Now, if you wanted to swap rows and columns, you could use a list
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306 comprehension::
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307
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308 >>> print [[row[i] for row in mat] for i in [0, 1, 2]]
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309 [[1, 4, 7], [2, 5, 8], [3, 6, 9]]
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310
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311 Special care has to be taken for the *nested* list comprehension:
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312
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313 To avoid apprehension when nesting list comprehensions, read from right to
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314 left.
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315
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316 A more verbose version of this snippet shows the flow explicitly::
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317
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318 for i in [0, 1, 2]:
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319 for row in mat:
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320 print row[i],
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321 print
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322
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323 In real world, you should prefer builtin functions to complex flow statements.
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324 The :func:`zip` function would do a great job for this use case::
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325
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326 >>> zip(*mat)
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327 [(1, 4, 7), (2, 5, 8), (3, 6, 9)]
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328
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329 See :ref:`tut-unpacking-arguments` for details on the asterisk in this line.
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330
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331 .. _tut-del:
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332
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333 The :keyword:`del` statement
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334 ============================
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335
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336 There is a way to remove an item from a list given its index instead of its
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337 value: the :keyword:`del` statement. This differs from the :meth:`pop` method
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338 which returns a value. The :keyword:`del` statement can also be used to remove
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339 slices from a list or clear the entire list (which we did earlier by assignment
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340 of an empty list to the slice). For example::
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341
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342 >>> a = [-1, 1, 66.25, 333, 333, 1234.5]
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343 >>> del a[0]
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344 >>> a
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345 [1, 66.25, 333, 333, 1234.5]
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346 >>> del a[2:4]
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347 >>> a
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348 [1, 66.25, 1234.5]
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349 >>> del a[:]
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350 >>> a
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351 []
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352
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353 :keyword:`del` can also be used to delete entire variables::
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354
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355 >>> del a
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356
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357 Referencing the name ``a`` hereafter is an error (at least until another value
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358 is assigned to it). We'll find other uses for :keyword:`del` later.
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359
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360
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361
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362 Tuples and Sequences
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363 ====================
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364
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365 We saw that lists and strings have many common properties, such as indexing and
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366 slicing operations. They are two examples of *sequence* data types (see
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367 :ref:`typesseq`). Since Python is an evolving language, other sequence data
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368 types may be added. There is also another standard sequence data type: the
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369 *tuple*.
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370
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371 A tuple consists of a number of values separated by commas, for instance::
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372
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373 >>> t = 12345, 54321, 'hello!'
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374 >>> t[0]
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375 12345
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376 >>> t
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377 (12345, 54321, 'hello!')
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378 >>> # Tuples may be nested:
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379 ... u = t, (1, 2, 3, 4, 5)
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380 >>> u
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381 ((12345, 54321, 'hello!'), (1, 2, 3, 4, 5))
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382
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383 As you see, on output tuples are always enclosed in parentheses, so that nested
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384 tuples are interpreted correctly; they may be input with or without surrounding
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385 parentheses, although often parentheses are necessary anyway (if the tuple is
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386 part of a larger expression).
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387
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388 Tuples have many uses. For example: (x, y) coordinate pairs, employee records
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389 from a database, etc. Tuples, like strings, are immutable: it is not possible
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390 to assign to the individual items of a tuple (you can simulate much of the same
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391 effect with slicing and concatenation, though). It is also possible to create
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392 tuples which contain mutable objects, such as lists.
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393
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394 A special problem is the construction of tuples containing 0 or 1 items: the
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395 syntax has some extra quirks to accommodate these. Empty tuples are constructed
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396 by an empty pair of parentheses; a tuple with one item is constructed by
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397 following a value with a comma (it is not sufficient to enclose a single value
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398 in parentheses). Ugly, but effective. For example::
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399
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400 >>> empty = ()
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401 >>> singleton = 'hello', # <-- note trailing comma
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402 >>> len(empty)
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403 0
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404 >>> len(singleton)
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405 1
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406 >>> singleton
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407 ('hello',)
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408
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409 The statement ``t = 12345, 54321, 'hello!'`` is an example of *tuple packing*:
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410 the values ``12345``, ``54321`` and ``'hello!'`` are packed together in a tuple.
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411 The reverse operation is also possible::
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412
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413 >>> x, y, z = t
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414
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415 This is called, appropriately enough, *sequence unpacking*. Sequence unpacking
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416 requires the list of variables on the left to have the same number of elements
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417 as the length of the sequence. Note that multiple assignment is really just a
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418 combination of tuple packing and sequence unpacking!
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419
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420 There is a small bit of asymmetry here: packing multiple values always creates
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421 a tuple, and unpacking works for any sequence.
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422
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423 .. XXX Add a bit on the difference between tuples and lists.
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424
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425
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426 .. _tut-sets:
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427
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428 Sets
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429 ====
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430
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431 Python also includes a data type for *sets*. A set is an unordered collection
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432 with no duplicate elements. Basic uses include membership testing and
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433 eliminating duplicate entries. Set objects also support mathematical operations
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434 like union, intersection, difference, and symmetric difference.
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435
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436 Curly braces or the :func:`set` function can be use to create sets. Note:
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437 To create an empty set you have to use set(), not {}; the latter creates
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438 an empty dictionary, a data structure that we discuss in the next section.
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439
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440 Here is a brief demonstration::
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441
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442 >>> basket = {'apple', 'orange', 'apple', 'pear', 'orange', 'banana'}
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443 >>> print(basket)
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444 {'orange', 'bananna', 'pear', 'apple'}
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445 >>> fruit = ['apple', 'orange', 'apple', 'pear', 'orange', 'banana']
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446 >>> fruit = set(basket) # create a set without duplicates
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447 >>> fruit
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448 {'orange', 'pear', 'apple', 'banana'}
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449 >>> fruit = {'orange', 'apple'} # {} syntax is equivalent to [] for lists
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450 >>> fruit
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451 {'orange', 'apple'}
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452 >>> 'orange' in fruit # fast membership testing
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453 True
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454 >>> 'crabgrass' in fruit
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455 False
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456
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457 >>> # Demonstrate set operations on unique letters from two words
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458 ...
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459 >>> a = set('abracadabra')
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460 >>> b = set('alacazam')
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461 >>> a # unique letters in a
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462 {'a', 'r', 'b', 'c', 'd'}
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463 >>> a - b # letters in a but not in b
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464 {'r', 'd', 'b'}
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465 >>> a | b # letters in either a or b
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466 {'a', 'c', 'r', 'd', 'b', 'm', 'z', 'l'}
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467 >>> a & b # letters in both a and b
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468 {'a', 'c'}
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469 >>> a ^ b # letters in a or b but not both
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470 {'r', 'd', 'b', 'm', 'z', 'l'}
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471
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472 Like for lists, there is a set comprehension syntax::
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473
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474 >>> a = {x for x in 'abracadabra' if x not in 'abc'}
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475 >>> a
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476 {'r', 'd'}
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477
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478
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479
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480 .. _tut-dictionaries:
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481
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482 Dictionaries
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483 ============
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484
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485 Another useful data type built into Python is the *dictionary* (see
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486 :ref:`typesmapping`). Dictionaries are sometimes found in other languages as
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487 "associative memories" or "associative arrays". Unlike sequences, which are
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488 indexed by a range of numbers, dictionaries are indexed by *keys*, which can be
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489 any immutable type; strings and numbers can always be keys. Tuples can be used
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490 as keys if they contain only strings, numbers, or tuples; if a tuple contains
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491 any mutable object either directly or indirectly, it cannot be used as a key.
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492 You can't use lists as keys, since lists can be modified in place using index
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493 assignments, slice assignments, or methods like :meth:`append` and
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494 :meth:`extend`.
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495
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496 It is best to think of a dictionary as an unordered set of *key: value* pairs,
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497 with the requirement that the keys are unique (within one dictionary). A pair of
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498 braces creates an empty dictionary: ``{}``. Placing a comma-separated list of
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499 key:value pairs within the braces adds initial key:value pairs to the
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500 dictionary; this is also the way dictionaries are written on output.
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501
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502 The main operations on a dictionary are storing a value with some key and
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503 extracting the value given the key. It is also possible to delete a key:value
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504 pair with ``del``. If you store using a key that is already in use, the old
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505 value associated with that key is forgotten. It is an error to extract a value
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506 using a non-existent key.
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507
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508 The :meth:`keys` method of a dictionary object returns a list of all the keys
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509 used in the dictionary, in arbitrary order (if you want it sorted, just apply
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510 the :meth:`sort` method to the list of keys). To check whether a single key is
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511 in the dictionary, use the :keyword:`in` keyword.
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512
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513 Here is a small example using a dictionary::
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514
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515 >>> tel = {'jack': 4098, 'sape': 4139}
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516 >>> tel['guido'] = 4127
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517 >>> tel
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518 {'sape': 4139, 'guido': 4127, 'jack': 4098}
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519 >>> tel['jack']
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520 4098
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521 >>> del tel['sape']
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522 >>> tel['irv'] = 4127
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523 >>> tel
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524 {'guido': 4127, 'irv': 4127, 'jack': 4098}
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525 >>> list(tel.keys())
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526 ['guido', 'irv', 'jack']
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527 >>> 'guido' in tel
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528 True
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529 >>> 'jack' not in tel
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530 False
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531
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532 The :func:`dict` constructor builds dictionaries directly from lists of
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533 key-value pairs stored as tuples. When the pairs form a pattern, list
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534 comprehensions can compactly specify the key-value list. ::
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535
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536 >>> dict([('sape', 4139), ('guido', 4127), ('jack', 4098)])
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537 {'sape': 4139, 'jack': 4098, 'guido': 4127}
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538
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539 In addition, dict comprehensions can be used to create dictionaries from
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540 arbitrary key and value expressions::
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541
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542 >>> {x: x**2 for x in (2, 4, 6)}
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543 {2: 4, 4: 16, 6: 36}
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544
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545 When the keys are simple strings, it is sometimes easier to specify pairs using
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546 keyword arguments::
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547
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548 >>> dict(sape=4139, guido=4127, jack=4098)
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549 {'sape': 4139, 'jack': 4098, 'guido': 4127}
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550
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551
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552 .. XXX Find out the right way to do these DUBOIS
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553 .. _tut-loopidioms:
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554
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555 Looping Techniques
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556 ==================
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557
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558 When looping through dictionaries, the key and corresponding value can be
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559 retrieved at the same time using the :meth:`items` method. ::
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560
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561 >>> knights = {'gallahad': 'the pure', 'robin': 'the brave'}
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562 >>> for k, v in knights.items():
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563 ... print(k, v)
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564 ...
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diff changeset
565 gallahad the pure
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
566 robin the brave
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
567
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
568 When looping through a sequence, the position index and corresponding value can
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
569 be retrieved at the same time using the :func:`enumerate` function. ::
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
570
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
571 >>> for i, v in enumerate(['tic', 'tac', 'toe']):
43115
95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois.
Guido van Rossum <guido@python.org>
parents: 42604
diff changeset
572 ... print(i, v)
42604
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
573 ...
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
574 0 tic
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
575 1 tac
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
576 2 toe
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
577
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
578 To loop over two or more sequences at the same time, the entries can be paired
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
579 with the :func:`zip` function. ::
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
580
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
581 >>> questions = ['name', 'quest', 'favorite color']
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
582 >>> answers = ['lancelot', 'the holy grail', 'blue']
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
583 >>> for q, a in zip(questions, answers):
43115
95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois.
Guido van Rossum <guido@python.org>
parents: 42604
diff changeset
584 ... print('What is your %s? It is %s.' % (q, a))
42604
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
585 ...
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
586 What is your name? It is lancelot.
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
587 What is your quest? It is the holy grail.
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
588 What is your favorite color? It is blue.
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
589
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
590 To loop over a sequence in reverse, first specify the sequence in a forward
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
591 direction and then call the :func:`reversed` function. ::
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
592
43201
4902671dd678 Tutorial formatting patch by Robin Stocker.
Georg Brandl <georg@python.org>
parents: 43117
diff changeset
593 >>> for i in reversed(range(1, 10, 2)):
43115
95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois.
Guido van Rossum <guido@python.org>
parents: 42604
diff changeset
594 ... print(i)
42604
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
595 ...
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
596 9
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
597 7
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
598 5
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
599 3
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
600 1
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
601
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
602 To loop over a sequence in sorted order, use the :func:`sorted` function which
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
603 returns a new sorted list while leaving the source unaltered. ::
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
604
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
605 >>> basket = ['apple', 'orange', 'apple', 'pear', 'orange', 'banana']
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
606 >>> for f in sorted(set(basket)):
43115
95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois.
Guido van Rossum <guido@python.org>
parents: 42604
diff changeset
607 ... print(f)
42604
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
608 ...
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
609 apple
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
610 banana
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
611 orange
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
612 pear
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
613
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
614
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
615 .. _tut-conditions:
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
616
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
617 More on Conditions
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
618 ==================
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
619
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
620 The conditions used in ``while`` and ``if`` statements can contain any
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
621 operators, not just comparisons.
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
622
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
623 The comparison operators ``in`` and ``not in`` check whether a value occurs
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
624 (does not occur) in a sequence. The operators ``is`` and ``is not`` compare
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
625 whether two objects are really the same object; this only matters for mutable
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
626 objects like lists. All comparison operators have the same priority, which is
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
627 lower than that of all numerical operators.
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
628
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
629 Comparisons can be chained. For example, ``a < b == c`` tests whether ``a`` is
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
630 less than ``b`` and moreover ``b`` equals ``c``.
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
631
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
632 Comparisons may be combined using the Boolean operators ``and`` and ``or``, and
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
633 the outcome of a comparison (or of any other Boolean expression) may be negated
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
634 with ``not``. These have lower priorities than comparison operators; between
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
635 them, ``not`` has the highest priority and ``or`` the lowest, so that ``A and
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
636 not B or C`` is equivalent to ``(A and (not B)) or C``. As always, parentheses
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
637 can be used to express the desired composition.
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
638
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
639 The Boolean operators ``and`` and ``or`` are so-called *short-circuit*
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
640 operators: their arguments are evaluated from left to right, and evaluation
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
641 stops as soon as the outcome is determined. For example, if ``A`` and ``C`` are
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
642 true but ``B`` is false, ``A and B and C`` does not evaluate the expression
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
643 ``C``. When used as a general value and not as a Boolean, the return value of a
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
644 short-circuit operator is the last evaluated argument.
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
645
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
646 It is possible to assign the result of a comparison or other Boolean expression
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
647 to a variable. For example, ::
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
648
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
649 >>> string1, string2, string3 = '', 'Trondheim', 'Hammer Dance'
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
650 >>> non_null = string1 or string2 or string3
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
651 >>> non_null
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
652 'Trondheim'
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
653
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
654 Note that in Python, unlike C, assignment cannot occur inside expressions. C
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
655 programmers may grumble about this, but it avoids a common class of problems
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
656 encountered in C programs: typing ``=`` in an expression when ``==`` was
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
657 intended.
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
658
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
659
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
660 .. _tut-comparing:
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
661
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
662 Comparing Sequences and Other Types
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
663 ===================================
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
664
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
665 Sequence objects may be compared to other objects with the same sequence type.
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
666 The comparison uses *lexicographical* ordering: first the first two items are
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
667 compared, and if they differ this determines the outcome of the comparison; if
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
668 they are equal, the next two items are compared, and so on, until either
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
669 sequence is exhausted. If two items to be compared are themselves sequences of
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
670 the same type, the lexicographical comparison is carried out recursively. If
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
671 all items of two sequences compare equal, the sequences are considered equal.
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
672 If one sequence is an initial sub-sequence of the other, the shorter sequence is
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
673 the smaller (lesser) one. Lexicographical ordering for strings uses the ASCII
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
674 ordering for individual characters. Some examples of comparisons between
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
675 sequences of the same type::
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
676
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
677 (1, 2, 3) < (1, 2, 4)
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
678 [1, 2, 3] < [1, 2, 4]
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
679 'ABC' < 'C' < 'Pascal' < 'Python'
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
680 (1, 2, 3, 4) < (1, 2, 4)
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
681 (1, 2) < (1, 2, -1)
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
682 (1, 2, 3) == (1.0, 2.0, 3.0)
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
683 (1, 2, ('aa', 'ab')) < (1, 2, ('abc', 'a'), 4)
9e1529bf0442 Move the 3k reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
diff changeset
684
43497
6614d42d7f0a #1228: new comparison behavior.
Georg Brandl <georg@python.org>
parents: 43303
diff changeset
685 Note that comparing objects of different types with ``<`` or ``>`` is legal
6614d42d7f0a #1228: new comparison behavior.
Georg Brandl <georg@python.org>
parents: 43303
diff changeset
686 provided that the objects have appropriate comparison methods. For example,
6614d42d7f0a #1228: new comparison behavior.
Georg Brandl <georg@python.org>
parents: 43303
diff changeset
687 mixed numeric types are compared according to their numeric value, so 0 equals
6614d42d7f0a #1228: new comparison behavior.
Georg Brandl <georg@python.org>
parents: 43303
diff changeset
688 0.0, etc. Otherwise, rather than providing an arbitrary ordering, the
6614d42d7f0a #1228: new comparison behavior.
Georg Brandl <georg@python.org>
parents: 43303
diff changeset
689 interpreter will raise a :exc:`TypeError` exception.