Mercurial > cpython-lru_cache-weakref

annotate Doc/tutorial/introduction.rst @ 84007:dbfbdf2b5c19 3.3

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Fix #17967: For ftp urls CWD to target instead of hopping to each directory towards target. This fixes a bug where target is accessible, but parent directories are restricted.
author Senthil Kumaran <senthil@uthcode.com>
date Sat, 01 Jun 2013 08:27:06 -0700
parents bde91dddbcbc
children 6f16fa5223cc
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1 .. _tut-informal:
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2
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3 **********************************
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4 An Informal Introduction to Python
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5 **********************************
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6
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7 In the following examples, input and output are distinguished by the presence or
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8 absence of prompts (:term:`>>>` and :term:`...`): to repeat the example, you must type
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9 everything after the prompt, when the prompt appears; lines that do not begin
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10 with a prompt are output from the interpreter. Note that a secondary prompt on a
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11 line by itself in an example means you must type a blank line; this is used to
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12 end a multi-line command.
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13
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14 Many of the examples in this manual, even those entered at the interactive
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15 prompt, include comments. Comments in Python start with the hash character,
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16 ``#``, and extend to the end of the physical line. A comment may appear at the
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17 start of a line or following whitespace or code, but not within a string
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18 literal. A hash character within a string literal is just a hash character.
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19 Since comments are to clarify code and are not interpreted by Python, they may
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20 be omitted when typing in examples.
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21
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22 Some examples::
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23
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24 # this is the first comment
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25 spam = 1 # and this is the second comment
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26 # ... and now a third!
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27 text = "# This is not a comment because it's inside quotes."
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28
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29
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30 .. _tut-calculator:
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31
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32 Using Python as a Calculator
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33 ============================
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34
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35 Let's try some simple Python commands. Start the interpreter and wait for the
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36 primary prompt, ``>>>``. (It shouldn't take long.)
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37
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38
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39 .. _tut-numbers:
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40
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41 Numbers
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42 -------
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43
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44 The interpreter acts as a simple calculator: you can type an expression at it
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45 and it will write the value. Expression syntax is straightforward: the
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46 operators ``+``, ``-``, ``*`` and ``/`` work just like in most other languages
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47 (for example, Pascal or C); parentheses (``()``) can be used for grouping.
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48 For example::
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49
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50 >>> 2 + 2
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51 4
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52 >>> 50 - 5*6
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53 20
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54 >>> (50 - 5*6) / 4
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55 5.0
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56 >>> 8 / 5 # division always returns a floating point number
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57 1.6
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58
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59 The integer numbers (e.g. ``2``, ``4``, ``20``) have type :class:`int`,
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60 the ones with a fractional part (e.g. ``5.0``, ``1.6``) have type
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61 :class:`float`. We will see more about numberic types later in the tutorial.
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62
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63 Division (``/``) always returns a float. To do :term:`floor division` and
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64 get an integer result (discarding any fractional result) you can use the ``//``
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65 operator; to calculate the remainder you can use ``%``::
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66
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67 >>> 17 / 3 # classic division returns a float
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68 5.666666666666667
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69 >>>
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70 >>> 17 // 3 # floor division discards the fractional part
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71 5
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72 >>> 17 % 3 # the % operator returns the remainder of the division
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73 2
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74 >>> 5 * 3 + 2 # result * divisor + remainder
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75 17
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76
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77 With Python is possible to use the ``**`` operator to calculate powers [#]_::
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78
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79 >>> 5 ** 2 # 5 squared
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80 25
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81 >>> 2 ** 7 # 2 to the power of 7
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82 128
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83
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84 The equal sign (``=``) is used to assign a value to a variable. Afterwards, no
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85 result is displayed before the next interactive prompt::
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86
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87 >>> width = 20
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88 >>> height = 5 * 9
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89 >>> width * height
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90 900
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91
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92 If a variable is not "defined" (assigned a value), trying to use it will
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93 give you an error::
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94
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95 >>> n # try to access an undefined variable
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96 Traceback (most recent call last):
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97 File "<stdin>", line 1, in <module>
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98 NameError: name 'n' is not defined
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99
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100 There is full support for floating point; operators with mixed type operands
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101 convert the integer operand to floating point::
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102
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103 >>> 3 * 3.75 / 1.5
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104 7.5
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105 >>> 7.0 / 2
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106 3.5
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107
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108 In interactive mode, the last printed expression is assigned to the variable
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109 ``_``. This means that when you are using Python as a desk calculator, it is
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110 somewhat easier to continue calculations, for example::
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111
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112 >>> tax = 12.5 / 100
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113 >>> price = 100.50
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114 >>> price * tax
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115 12.5625
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116 >>> price + _
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117 113.0625
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118 >>> round(_, 2)
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119 113.06
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120
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121 This variable should be treated as read-only by the user. Don't explicitly
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122 assign a value to it --- you would create an independent local variable with the
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123 same name masking the built-in variable with its magic behavior.
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124
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125 In addition to :class:`int` and :class:`float`, Python supports other types of
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126 numbers, such as :class:`~decimal.Decimal` and :class:`~fractions.Fraction`.
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127 Python also has built-in support for :ref:`complex numbers <typesnumeric>`,
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128 and uses the ``j`` or ``J`` suffix to indicate the imaginary part
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129 (e.g. ``3+5j``).
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130
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131
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132 .. _tut-strings:
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133
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134 Strings
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135 -------
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136
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137 Besides numbers, Python can also manipulate strings, which can be expressed
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138 in several ways. They can be enclosed in single quotes (``'...'``) or
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139 double quotes (``"..."``) with the same result [#]_. ``\`` can be used
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140 to escape quotes::
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141
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142 >>> 'spam eggs' # single quotes
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143 'spam eggs'
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144 >>> 'doesn\'t' # use \' to escape the single quote...
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145 "doesn't"
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146 >>> "doesn't" # ...or use double quotes instead
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147 "doesn't"
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148 >>> '"Yes," he said.'
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149 '"Yes," he said.'
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150 >>> "\"Yes,\" he said."
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151 '"Yes," he said.'
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152 >>> '"Isn\'t," she said.'
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153 '"Isn\'t," she said.'
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154
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155 In the interactive interpreter, the output string is enclosed in quotes and
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156 special characters are escaped with backslashes. While this might sometimes
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157 look different from the input (the enclosing quotes could change), the two
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158 strings are equivalent. The string is enclosed in double quotes if
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159 the string contains a single quote and no double quotes, otherwise it is
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160 enclosed in single quotes. The :func:`print` function produces a more
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161 readable output, by omitting the enclosing quotes and by printing escaped
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162 and special characters::
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163
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164 >>> '"Isn\'t," she said.'
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165 '"Isn\'t," she said.'
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166 >>> print('"Isn\'t," she said.')
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167 "Isn't," she said.
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168 >>> s = 'First line.\nSecond line.' # \n means newline
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169 >>> s # without print(), \n is included in the output
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170 'First line.\nSecond line.'
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171 >>> print(s) # with print(), \n produces a new line
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172 First line.
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173 Second line.
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174
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175 If you don't want characters prefaced by ``\`` to be interpreted as
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176 special characters, you can use *raw strings* by adding an ``r`` before
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177 the first quote::
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178
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179 >>> print('C:\some\name') # here \n means newline!
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180 C:\some
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181 ame
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182 >>> print(r'C:\some\name') # note the r before the quote
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183 C:\some\name
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184
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185 String literals can span multiple lines. One way is using triple-quotes:
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186 ``"""..."""`` or ``'''...'''``. End of lines are automatically
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187 included in the string, but it's possible to prevent this by adding a ``\`` at
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188 the end of the line. The following example::
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189
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190 print("""\
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191 Usage: thingy [OPTIONS]
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192 -h Display this usage message
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193 -H hostname Hostname to connect to
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194 """)
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195
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196 produces the following output (note that the initial newline is not included):
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197
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198 .. code-block:: text
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199
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200 Usage: thingy [OPTIONS]
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201 -h Display this usage message
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202 -H hostname Hostname to connect to
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203
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204 Strings can be concatenated (glued together) with the ``+`` operator, and
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205 repeated with ``*``::
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206
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207 >>> # 3 times 'un', followed by 'ium'
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208 >>> 3 * 'un' + 'ium'
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209 'unununium'
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210
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211 Two or more *string literals* (i.e. the ones enclosed between quotes) next
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212 to each other are automatically concatenated. ::
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213
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214 >>> 'Py' 'thon'
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215 'Python'
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216
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217 This only works with two literals though, not with variables or expressions::
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218
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219 >>> prefix = 'Py'
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220 >>> prefix 'thon' # can't concatenate a variable and a string literal
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221 ...
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222 SyntaxError: invalid syntax
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223 >>> ('un' * 3) 'ium'
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224 ...
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225 SyntaxError: invalid syntax
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226
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227 If you want to concatenate variables or a variable and a literal, use ``+``::
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228
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229 >>> prefix + 'thon'
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230 'Python'
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231
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232 This feature is particularly useful when you want to break long strings::
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233
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234 >>> text = ('Put several strings within parentheses '
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235 'to have them joined together.')
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236 >>> text
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237 'Put several strings within parentheses to have them joined together.'
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238
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239 Strings can be *indexed* (subscripted), with the first character having index 0.
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240 There is no separate character type; a character is simply a string of size
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241 one::
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242
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243 >>> word = 'Python'
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244 >>> word[0] # character in position 0
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245 'P'
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246 >>> word[5] # character in position 5
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247 'n'
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248
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249 Indices may also be negative numbers, to start counting from the right::
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250
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251 >>> word[-1] # last character
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252 'n'
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253 >>> word[-2] # second-last character
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254 'o'
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255 >>> word[-6]
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256 'P'
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257
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258 Note that since -0 is the same as 0, negative indices start from -1.
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259
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260 In addition to indexing, *slicing* is also supported. While indexing is used
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261 to obtain individual characters, *slicing* allows you to obtain substring::
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262
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263 >>> word[0:2] # characters from position 0 (included) to 2 (excluded)
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264 'Py'
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265 >>> word[2:5] # characters from position 2 (included) to 4 (excluded)
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266 'tho'
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267
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268 Note how the start is always included, and the end always excluded. This
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269 makes sure that ``s[:i] + s[i:]`` is always equal to ``s``::
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270
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271 >>> word[:2] + word[2:]
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272 'Python'
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273 >>> word[:4] + word[4:]
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274 'Python'
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275
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276 Slice indices have useful defaults; an omitted first index defaults to zero, an
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277 omitted second index defaults to the size of the string being sliced. ::
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278
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279 >>> word[:2] # character from the beginning to position 2 (excluded)
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280 'Py'
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281 >>> word[4:] # characters from position 4 (included) to the end
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282 'on'
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283 >>> word[-2:] # characters from the second-last (included) to the end
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284 'on'
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285
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286 One way to remember how slices work is to think of the indices as pointing
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287 *between* characters, with the left edge of the first character numbered 0.
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288 Then the right edge of the last character of a string of *n* characters has
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289 index *n*, for example::
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290
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291 +---+---+---+---+---+---+
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292 | P | y | t | h | o | n |
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293 +---+---+---+---+---+---+
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294 0 1 2 3 4 5 6
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295 -6 -5 -4 -3 -2 -1
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296
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297 The first row of numbers gives the position of the indices 0...6 in the string;
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298 the second row gives the corresponding negative indices. The slice from *i* to
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299 *j* consists of all characters between the edges labeled *i* and *j*,
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300 respectively.
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301
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302 For non-negative indices, the length of a slice is the difference of the
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303 indices, if both are within bounds. For example, the length of ``word[1:3]`` is
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304 2.
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305
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306 Attempting to use a index that is too large will result in an error::
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307
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308 >>> word[42] # the word only has 7 characters
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309 Traceback (most recent call last):
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310 File "<stdin>", line 1, in <module>
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311 IndexError: string index out of range
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312
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313 However, out of range slice indexes are handled gracefully when used for
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314 slicing::
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315
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316 >>> word[4:42]
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317 'on'
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318 >>> word[42:]
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319 ''
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320
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321 Python strings cannot be changed --- they are :term:`immutable`.
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322 Therefore, assigning to an indexed position in the string results in an error::
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323
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324 >>> word[0] = 'J'
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325 ...
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326 TypeError: 'str' object does not support item assignment
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327 >>> word[2:] = 'py'
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328 ...
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329 TypeError: 'str' object does not support item assignment
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330
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331 If you need a different string, you should create a new one::
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332
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333 >>> 'J' + word[1:]
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334 'Jython'
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335 >>> word[:2] + 'py'
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336 'Pypy'
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337
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338 The built-in function :func:`len` returns the length of a string::
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339
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340 >>> s = 'supercalifragilisticexpialidocious'
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341 >>> len(s)
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342 34
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343
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344
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345 .. seealso::
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346
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347 :ref:`textseq`
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348 Strings are examples of *sequence types*, and support the common
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349 operations supported by such types.
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350
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351 :ref:`string-methods`
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352 Strings support a large number of methods for
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353 basic transformations and searching.
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354
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355 :ref:`string-formatting`
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356 Information about string formatting with :meth:`str.format` is described
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357 here.
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358
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359 :ref:`old-string-formatting`
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360 The old formatting operations invoked when strings and Unicode strings are
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361 the left operand of the ``%`` operator are described in more detail here.
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362
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363
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364 .. _tut-lists:
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365
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366 Lists
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367 -----
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368
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369 Python knows a number of *compound* data types, used to group together other
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370 values. The most versatile is the *list*, which can be written as a list of
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371 comma-separated values (items) between square brackets. Lists might contain
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372 items of different types, but usually the items all have the same type. ::
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373
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374 >>> squares = [1, 2, 4, 9, 16, 25]
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375 >>> squares
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376 [1, 2, 4, 9, 16, 25]
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377
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378 Like strings (and all other built-in :term:`sequence` type), lists can be
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379 indexed and sliced::
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380
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381 >>> squares[0] # indexing returns the item
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382 1
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383 >>> squares[-1]
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384 25
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385 >>> squares[-3:] # slicing returns a new list
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386 [9, 16, 25]
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387
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388 All slice operations return a new list containing the requested elements. This
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389 means that the following slice returns a new (shallow) copy of the list::
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390
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391 >>> squares[:]
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392 [1, 2, 4, 9, 16, 25]
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393
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394 Lists also supports operations like concatenation::
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395
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396 >>> squares + [36, 49, 64, 81, 100]
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397 [1, 2, 4, 9, 16, 25, 36, 49, 64, 81, 100]
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398
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399 Unlike strings, which are :term:`immutable`, lists are a :term:`mutable`
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400 type, i.e. it is possible to change their content::
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401
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402 >>> cubes = [1, 8, 27, 65, 125] # something's wrong here
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403 >>> 4 ** 3 # the cube of 4 is 64, not 65!
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404 64
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405 >>> cubes[3] = 64 # replace the wrong value
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406 >>> cubes
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407 [1, 8, 27, 64, 125]
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408
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409 You can also add new items at the end of the list, by using
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410 the :meth:`~list.append` *method* (we will see more about methods later)::
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411
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412 >>> cubes.append(216) # add the cube of 6
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413 >>> cubes.append(7 ** 3) # and the cube of 7
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414 >>> cubes
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415 [1, 8, 27, 64, 125, 216, 343]
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416
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417 Assignment to slices is also possible, and this can even change the size of the
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418 list or clear it entirely::
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419
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420 >>> letters = ['a', 'b', 'c', 'd', 'e', 'f', 'g']
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421 >>> letters
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422 ['a', 'b', 'c', 'd', 'e', 'f', 'g']
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423 >>> # replace some values
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424 >>> letters[2:5] = ['C', 'D', 'E']
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425 >>> letters
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426 ['a', 'b', 'C', 'D', 'E', 'f', 'g']
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427 >>> # now remove them
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428 >>> letters[2:5] = []
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429 >>> letters
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430 ['a', 'b', 'f', 'g']
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431 >>> # clear the list by replacing all the elements with an empty list
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432 >>> letters[:] = []
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433 >>> letters
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434 []
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435
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436 The built-in function :func:`len` also applies to lists::
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437
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438 >>> letters = ['a', 'b', 'c', 'd']
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439 >>> len(letters)
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440 4
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441
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442 It is possible to nest lists (create lists containing other lists), for
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443 example::
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444
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445 >>> a = ['a', 'b', 'c']
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446 >>> n = [1, 2, 3]
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447 >>> x = [a, n]
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448 >>> x
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449 [['a', 'b', 'c'], [1, 2, 3]]
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450 >>> x[0]
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451 ['a', 'b', 'c']
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452 >>> x[0][1]
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453 'b'
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454
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455 .. _tut-firststeps:
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456
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457 First Steps Towards Programming
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458 ===============================
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459
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460 Of course, we can use Python for more complicated tasks than adding two and two
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461 together. For instance, we can write an initial sub-sequence of the *Fibonacci*
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462 series as follows::
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463
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464 >>> # Fibonacci series:
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465 ... # the sum of two elements defines the next
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466 ... a, b = 0, 1
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467 >>> while b < 10:
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468 ... print(b)
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469 ... a, b = b, a+b
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470 ...
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471 1
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472 1
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473 2
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474 3
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475 5
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476 8
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477
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478 This example introduces several new features.
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479
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480 * The first line contains a *multiple assignment*: the variables ``a`` and ``b``
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481 simultaneously get the new values 0 and 1. On the last line this is used again,
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482 demonstrating that the expressions on the right-hand side are all evaluated
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483 first before any of the assignments take place. The right-hand side expressions
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484 are evaluated from the left to the right.
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485
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486 * The :keyword:`while` loop executes as long as the condition (here: ``b < 10``)
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487 remains true. In Python, like in C, any non-zero integer value is true; zero is
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488 false. The condition may also be a string or list value, in fact any sequence;
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489 anything with a non-zero length is true, empty sequences are false. The test
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490 used in the example is a simple comparison. The standard comparison operators
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491 are written the same as in C: ``<`` (less than), ``>`` (greater than), ``==``
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492 (equal to), ``<=`` (less than or equal to), ``>=`` (greater than or equal to)
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493 and ``!=`` (not equal to).
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494
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495 * The *body* of the loop is *indented*: indentation is Python's way of grouping
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496 statements. At the interactive prompt, you have to type a tab or space(s) for
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497 each indented line. In practice you will prepare more complicated input
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498 for Python with a text editor; all decent text editors have an auto-indent
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499 facility. When a compound statement is entered interactively, it must be
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500 followed by a blank line to indicate completion (since the parser cannot
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501 guess when you have typed the last line). Note that each line within a basic
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502 block must be indented by the same amount.
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503
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504 * The :func:`print` function writes the value of the argument(s) it is given.
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505 It differs from just writing the expression you want to write (as we did
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506 earlier in the calculator examples) in the way it handles multiple arguments,
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507 floating point quantities, and strings. Strings are printed without quotes,
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508 and a space is inserted between items, so you can format things nicely, like
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509 this::
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510
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511 >>> i = 256*256
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512 >>> print('The value of i is', i)
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513 The value of i is 65536
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514
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515 The keyword argument *end* can be used to avoid the newline after the output,
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516 or end the output with a different string::
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517
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518 >>> a, b = 0, 1
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519 >>> while b < 1000:
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520 ... print(b, end=',')
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521 ... a, b = b, a+b
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522 ...
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523 1,1,2,3,5,8,13,21,34,55,89,144,233,377,610,987,
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524
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525
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526 .. rubric:: Footnotes
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527
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528 .. [#] Since ``**`` has higher precedence than ``-``, ``-3**2`` will be
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529 interpreted as ``-(3**2)`` and thus result in ``-9``. To avoid this
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530 and get ``9``, you can use ``(-3)**2``.
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531
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532 .. [#] Unlike other languages, special characters such as ``\n`` have the
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533 same meaning with both single (``'...'``) and double (``"..."``) quotes.
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534 The only difference between the two is that within single quotes you don't
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535 need to escape ``"`` (but you have to escape ``\'``) and vice versa.