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638 lines
21 KiB
638 lines
21 KiB
.. _tut-informal:
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**********************************
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An Informal Introduction to Python
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**********************************
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In the following examples, input and output are distinguished by the presence or
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absence of prompts (:term:`>>>` and :term:`...`): to repeat the example, you must type
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everything after the prompt, when the prompt appears; lines that do not begin
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with a prompt are output from the interpreter. Note that a secondary prompt on a
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line by itself in an example means you must type a blank line; this is used to
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end a multi-line command.
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Many of the examples in this manual, even those entered at the interactive
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prompt, include comments. Comments in Python start with the hash character,
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``#``, and extend to the end of the physical line. A comment may appear at the
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start of a line or following whitespace or code, but not within a string
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literal. A hash character within a string literal is just a hash character.
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Since comments are to clarify code and are not interpreted by Python, they may
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be omitted when typing in examples.
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Some examples::
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# this is the first comment
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spam = 1 # and this is the second comment
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# ... and now a third!
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text = "# This is not a comment because it's inside quotes."
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.. _tut-calculator:
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Using Python as a Calculator
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============================
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Let's try some simple Python commands. Start the interpreter and wait for the
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primary prompt, ``>>>``. (It shouldn't take long.)
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.. _tut-numbers:
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Numbers
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-------
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The interpreter acts as a simple calculator: you can type an expression at it
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and it will write the value. Expression syntax is straightforward: the
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operators ``+``, ``-``, ``*`` and ``/`` work just like in most other languages
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(for example, Pascal or C); parentheses (``()``) can be used for grouping.
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For example::
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>>> 2 + 2
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4
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>>> 50 - 5*6
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20
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>>> (50 - 5.0*6) / 4
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5.0
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>>> 8 / 5.0
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1.6
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The integer numbers (e.g. ``2``, ``4``, ``20``) have type :class:`int`,
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the ones with a fractional part (e.g. ``5.0``, ``1.6``) have type
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:class:`float`. We will see more about numeric types later in the tutorial.
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The return type of a division (``/``) operation depends on its operands. If
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both operands are of type :class:`int`, :term:`floor division` is performed
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and an :class:`int` is returned. If either operand is a :class:`float`,
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classic division is performed and a :class:`float` is returned. The ``//``
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operator is also provided for doing floor division no matter what the
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operands are. The remainder can be calculated with the ``%`` operator::
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>>> 17 / 3 # int / int -> int
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5
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>>> 17 / 3.0 # int / float -> float
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5.666666666666667
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>>> 17 // 3.0 # explicit floor division discards the fractional part
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5.0
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>>> 17 % 3 # the % operator returns the remainder of the division
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2
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>>> 5 * 3 + 2 # result * divisor + remainder
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17
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With Python, it is possible to use the ``**`` operator to calculate powers [#]_::
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>>> 5 ** 2 # 5 squared
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25
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>>> 2 ** 7 # 2 to the power of 7
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128
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The equal sign (``=``) is used to assign a value to a variable. Afterwards, no
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result is displayed before the next interactive prompt::
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>>> width = 20
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>>> height = 5 * 9
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>>> width * height
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900
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If a variable is not "defined" (assigned a value), trying to use it will
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give you an error::
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>>> n # try to access an undefined variable
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Traceback (most recent call last):
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File "<stdin>", line 1, in <module>
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NameError: name 'n' is not defined
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There is full support for floating point; operators with mixed type operands
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convert the integer operand to floating point::
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>>> 3 * 3.75 / 1.5
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7.5
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>>> 7.0 / 2
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3.5
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In interactive mode, the last printed expression is assigned to the variable
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``_``. This means that when you are using Python as a desk calculator, it is
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somewhat easier to continue calculations, for example::
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>>> tax = 12.5 / 100
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>>> price = 100.50
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>>> price * tax
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12.5625
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>>> price + _
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113.0625
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>>> round(_, 2)
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113.06
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This variable should be treated as read-only by the user. Don't explicitly
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assign a value to it --- you would create an independent local variable with the
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same name masking the built-in variable with its magic behavior.
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In addition to :class:`int` and :class:`float`, Python supports other types of
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numbers, such as :class:`~decimal.Decimal` and :class:`~fractions.Fraction`.
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Python also has built-in support for :ref:`complex numbers <typesnumeric>`,
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and uses the ``j`` or ``J`` suffix to indicate the imaginary part
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(e.g. ``3+5j``).
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.. _tut-strings:
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Strings
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-------
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Besides numbers, Python can also manipulate strings, which can be expressed
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in several ways. They can be enclosed in single quotes (``'...'``) or
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double quotes (``"..."``) with the same result [#]_. ``\`` can be used
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to escape quotes::
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>>> 'spam eggs' # single quotes
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'spam eggs'
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>>> 'doesn\'t' # use \' to escape the single quote...
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"doesn't"
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>>> "doesn't" # ...or use double quotes instead
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"doesn't"
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>>> '"Yes," they said.'
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'"Yes," they said.'
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>>> "\"Yes,\" they said."
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'"Yes," they said.'
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>>> '"Isn\'t," they said.'
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'"Isn\'t," they said.'
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In the interactive interpreter, the output string is enclosed in quotes and
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special characters are escaped with backslashes. While this might sometimes
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look different from the input (the enclosing quotes could change), the two
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strings are equivalent. The string is enclosed in double quotes if
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the string contains a single quote and no double quotes, otherwise it is
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enclosed in single quotes. The :keyword:`print` statement produces a more
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readable output, by omitting the enclosing quotes and by printing escaped
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and special characters::
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>>> '"Isn\'t," they said.'
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'"Isn\'t," they said.'
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>>> print '"Isn\'t," they said.'
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"Isn't," they said.
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>>> s = 'First line.\nSecond line.' # \n means newline
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>>> s # without print, \n is included in the output
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'First line.\nSecond line.'
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>>> print s # with print, \n produces a new line
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First line.
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Second line.
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If you don't want characters prefaced by ``\`` to be interpreted as
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special characters, you can use *raw strings* by adding an ``r`` before
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the first quote::
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>>> print 'C:\some\name' # here \n means newline!
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C:\some
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ame
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>>> print r'C:\some\name' # note the r before the quote
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C:\some\name
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String literals can span multiple lines. One way is using triple-quotes:
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``"""..."""`` or ``'''...'''``. End of lines are automatically
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included in the string, but it's possible to prevent this by adding a ``\`` at
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the end of the line. The following example::
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print """\
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Usage: thingy [OPTIONS]
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-h Display this usage message
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-H hostname Hostname to connect to
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"""
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produces the following output (note that the initial newline is not included):
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.. code-block:: text
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Usage: thingy [OPTIONS]
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-h Display this usage message
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-H hostname Hostname to connect to
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Strings can be concatenated (glued together) with the ``+`` operator, and
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repeated with ``*``::
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>>> # 3 times 'un', followed by 'ium'
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>>> 3 * 'un' + 'ium'
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'unununium'
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Two or more *string literals* (i.e. the ones enclosed between quotes) next
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to each other are automatically concatenated. ::
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>>> 'Py' 'thon'
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'Python'
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This feature is particularly useful when you want to break long strings::
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>>> text = ('Put several strings within parentheses '
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... 'to have them joined together.')
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>>> text
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'Put several strings within parentheses to have them joined together.'
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This only works with two literals though, not with variables or expressions::
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>>> prefix = 'Py'
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>>> prefix 'thon' # can't concatenate a variable and a string literal
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...
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SyntaxError: invalid syntax
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>>> ('un' * 3) 'ium'
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...
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SyntaxError: invalid syntax
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If you want to concatenate variables or a variable and a literal, use ``+``::
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>>> prefix + 'thon'
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'Python'
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Strings can be *indexed* (subscripted), with the first character having index 0.
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There is no separate character type; a character is simply a string of size
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one::
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>>> word = 'Python'
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>>> word[0] # character in position 0
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'P'
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>>> word[5] # character in position 5
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'n'
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Indices may also be negative numbers, to start counting from the right::
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>>> word[-1] # last character
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'n'
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>>> word[-2] # second-last character
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'o'
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>>> word[-6]
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'P'
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Note that since -0 is the same as 0, negative indices start from -1.
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In addition to indexing, *slicing* is also supported. While indexing is used
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to obtain individual characters, *slicing* allows you to obtain a substring::
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>>> word[0:2] # characters from position 0 (included) to 2 (excluded)
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'Py'
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>>> word[2:5] # characters from position 2 (included) to 5 (excluded)
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'tho'
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Note how the start is always included, and the end always excluded. This
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makes sure that ``s[:i] + s[i:]`` is always equal to ``s``::
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>>> word[:2] + word[2:]
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'Python'
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>>> word[:4] + word[4:]
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'Python'
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Slice indices have useful defaults; an omitted first index defaults to zero, an
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omitted second index defaults to the size of the string being sliced. ::
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>>> word[:2] # character from the beginning to position 2 (excluded)
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'Py'
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>>> word[4:] # characters from position 4 (included) to the end
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'on'
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>>> word[-2:] # characters from the second-last (included) to the end
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'on'
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One way to remember how slices work is to think of the indices as pointing
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*between* characters, with the left edge of the first character numbered 0.
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Then the right edge of the last character of a string of *n* characters has
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index *n*, for example::
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+---+---+---+---+---+---+
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| P | y | t | h | o | n |
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+---+---+---+---+---+---+
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0 1 2 3 4 5 6
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-6 -5 -4 -3 -2 -1
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The first row of numbers gives the position of the indices 0...6 in the string;
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the second row gives the corresponding negative indices. The slice from *i* to
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*j* consists of all characters between the edges labeled *i* and *j*,
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respectively.
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For non-negative indices, the length of a slice is the difference of the
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indices, if both are within bounds. For example, the length of ``word[1:3]`` is
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2.
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Attempting to use an index that is too large will result in an error::
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>>> word[42] # the word only has 6 characters
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Traceback (most recent call last):
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File "<stdin>", line 1, in <module>
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IndexError: string index out of range
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However, out of range slice indexes are handled gracefully when used for
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slicing::
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>>> word[4:42]
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'on'
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>>> word[42:]
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''
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Python strings cannot be changed --- they are :term:`immutable`.
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Therefore, assigning to an indexed position in the string results in an error::
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>>> word[0] = 'J'
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...
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TypeError: 'str' object does not support item assignment
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>>> word[2:] = 'py'
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...
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TypeError: 'str' object does not support item assignment
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If you need a different string, you should create a new one::
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>>> 'J' + word[1:]
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'Jython'
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>>> word[:2] + 'py'
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'Pypy'
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The built-in function :func:`len` returns the length of a string::
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>>> s = 'supercalifragilisticexpialidocious'
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>>> len(s)
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34
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.. seealso::
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:ref:`typesseq`
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Strings, and the Unicode strings described in the next section, are
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examples of *sequence types*, and support the common operations supported
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by such types.
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:ref:`string-methods`
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Both strings and Unicode strings support a large number of methods for
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basic transformations and searching.
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:ref:`formatstrings`
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Information about string formatting with :meth:`str.format`.
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:ref:`string-formatting`
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The old formatting operations invoked when strings and Unicode strings are
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the left operand of the ``%`` operator are described in more detail here.
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.. _tut-unicodestrings:
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Unicode Strings
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---------------
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.. sectionauthor:: Marc-Andre Lemburg <mal@lemburg.com>
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Starting with Python 2.0 a new data type for storing text data is available to
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the programmer: the Unicode object. It can be used to store and manipulate
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Unicode data (see http://www.unicode.org/) and integrates well with the existing
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string objects, providing auto-conversions where necessary.
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Unicode has the advantage of providing one ordinal for every character in every
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script used in modern and ancient texts. Previously, there were only 256
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possible ordinals for script characters. Texts were typically bound to a code
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page which mapped the ordinals to script characters. This lead to very much
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confusion especially with respect to internationalization (usually written as
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``i18n`` --- ``'i'`` + 18 characters + ``'n'``) of software. Unicode solves
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these problems by defining one code page for all scripts.
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Creating Unicode strings in Python is just as simple as creating normal
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strings::
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>>> u'Hello World !'
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u'Hello World !'
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The small ``'u'`` in front of the quote indicates that a Unicode string is
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supposed to be created. If you want to include special characters in the string,
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you can do so by using the Python *Unicode-Escape* encoding. The following
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example shows how::
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>>> u'Hello\u0020World !'
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u'Hello World !'
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The escape sequence ``\u0020`` indicates to insert the Unicode character with
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the ordinal value 0x0020 (the space character) at the given position.
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Other characters are interpreted by using their respective ordinal values
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directly as Unicode ordinals. If you have literal strings in the standard
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Latin-1 encoding that is used in many Western countries, you will find it
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convenient that the lower 256 characters of Unicode are the same as the 256
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characters of Latin-1.
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For experts, there is also a raw mode just like the one for normal strings. You
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have to prefix the opening quote with 'ur' to have Python use the
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*Raw-Unicode-Escape* encoding. It will only apply the above ``\uXXXX``
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conversion if there is an uneven number of backslashes in front of the small
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'u'. ::
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>>> ur'Hello\u0020World !'
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u'Hello World !'
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>>> ur'Hello\\u0020World !'
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u'Hello\\\\u0020World !'
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The raw mode is most useful when you have to enter lots of backslashes, as can
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be necessary in regular expressions.
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Apart from these standard encodings, Python provides a whole set of other ways
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of creating Unicode strings on the basis of a known encoding.
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.. index:: builtin: unicode
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The built-in function :func:`unicode` provides access to all registered Unicode
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codecs (COders and DECoders). Some of the more well known encodings which these
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codecs can convert are *Latin-1*, *ASCII*, *UTF-8*, and *UTF-16*. The latter two
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are variable-length encodings that store each Unicode character in one or more
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bytes. The default encoding is normally set to ASCII, which passes through
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characters in the range 0 to 127 and rejects any other characters with an error.
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When a Unicode string is printed, written to a file, or converted with
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:func:`str`, conversion takes place using this default encoding. ::
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>>> u"abc"
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u'abc'
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>>> str(u"abc")
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'abc'
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>>> u"äöü"
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u'\xe4\xf6\xfc'
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>>> str(u"äöü")
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Traceback (most recent call last):
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File "<stdin>", line 1, in ?
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UnicodeEncodeError: 'ascii' codec can't encode characters in position 0-2: ordinal not in range(128)
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To convert a Unicode string into an 8-bit string using a specific encoding,
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Unicode objects provide an :func:`encode` method that takes one argument, the
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name of the encoding. Lowercase names for encodings are preferred. ::
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>>> u"äöü".encode('utf-8')
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'\xc3\xa4\xc3\xb6\xc3\xbc'
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If you have data in a specific encoding and want to produce a corresponding
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Unicode string from it, you can use the :func:`unicode` function with the
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encoding name as the second argument. ::
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>>> unicode('\xc3\xa4\xc3\xb6\xc3\xbc', 'utf-8')
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u'\xe4\xf6\xfc'
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.. _tut-lists:
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Lists
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-----
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Python knows a number of *compound* data types, used to group together other
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values. The most versatile is the *list*, which can be written as a list of
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comma-separated values (items) between square brackets. Lists might contain
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items of different types, but usually the items all have the same type. ::
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>>> squares = [1, 4, 9, 16, 25]
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>>> squares
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[1, 4, 9, 16, 25]
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Like strings (and all other built-in :term:`sequence` type), lists can be
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indexed and sliced::
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>>> squares[0] # indexing returns the item
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1
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>>> squares[-1]
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25
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>>> squares[-3:] # slicing returns a new list
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[9, 16, 25]
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All slice operations return a new list containing the requested elements. This
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means that the following slice returns a new (shallow) copy of the list::
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>>> squares[:]
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[1, 4, 9, 16, 25]
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Lists also supports operations like concatenation::
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>>> squares + [36, 49, 64, 81, 100]
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[1, 4, 9, 16, 25, 36, 49, 64, 81, 100]
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Unlike strings, which are :term:`immutable`, lists are a :term:`mutable`
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type, i.e. it is possible to change their content::
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>>> cubes = [1, 8, 27, 65, 125] # something's wrong here
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>>> 4 ** 3 # the cube of 4 is 64, not 65!
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64
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>>> cubes[3] = 64 # replace the wrong value
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>>> cubes
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[1, 8, 27, 64, 125]
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You can also add new items at the end of the list, by using
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the :meth:`~list.append` *method* (we will see more about methods later)::
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>>> cubes.append(216) # add the cube of 6
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>>> cubes.append(7 ** 3) # and the cube of 7
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>>> cubes
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[1, 8, 27, 64, 125, 216, 343]
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Assignment to slices is also possible, and this can even change the size of the
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list or clear it entirely::
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|
>>> letters = ['a', 'b', 'c', 'd', 'e', 'f', 'g']
|
|
>>> letters
|
|
['a', 'b', 'c', 'd', 'e', 'f', 'g']
|
|
>>> # replace some values
|
|
>>> letters[2:5] = ['C', 'D', 'E']
|
|
>>> letters
|
|
['a', 'b', 'C', 'D', 'E', 'f', 'g']
|
|
>>> # now remove them
|
|
>>> letters[2:5] = []
|
|
>>> letters
|
|
['a', 'b', 'f', 'g']
|
|
>>> # clear the list by replacing all the elements with an empty list
|
|
>>> letters[:] = []
|
|
>>> letters
|
|
[]
|
|
|
|
The built-in function :func:`len` also applies to lists::
|
|
|
|
>>> letters = ['a', 'b', 'c', 'd']
|
|
>>> len(letters)
|
|
4
|
|
|
|
It is possible to nest lists (create lists containing other lists), for
|
|
example::
|
|
|
|
>>> a = ['a', 'b', 'c']
|
|
>>> n = [1, 2, 3]
|
|
>>> x = [a, n]
|
|
>>> x
|
|
[['a', 'b', 'c'], [1, 2, 3]]
|
|
>>> x[0]
|
|
['a', 'b', 'c']
|
|
>>> x[0][1]
|
|
'b'
|
|
|
|
.. _tut-firststeps:
|
|
|
|
First Steps Towards Programming
|
|
===============================
|
|
|
|
Of course, we can use Python for more complicated tasks than adding two and two
|
|
together. For instance, we can write an initial sub-sequence of the *Fibonacci*
|
|
series as follows::
|
|
|
|
>>> # Fibonacci series:
|
|
... # the sum of two elements defines the next
|
|
... a, b = 0, 1
|
|
>>> while b < 10:
|
|
... print b
|
|
... a, b = b, a+b
|
|
...
|
|
1
|
|
1
|
|
2
|
|
3
|
|
5
|
|
8
|
|
|
|
This example introduces several new features.
|
|
|
|
* The first line contains a *multiple assignment*: the variables ``a`` and ``b``
|
|
simultaneously get the new values 0 and 1. On the last line this is used again,
|
|
demonstrating that the expressions on the right-hand side are all evaluated
|
|
first before any of the assignments take place. The right-hand side expressions
|
|
are evaluated from the left to the right.
|
|
|
|
* The :keyword:`while` loop executes as long as the condition (here: ``b < 10``)
|
|
remains true. In Python, like in C, any non-zero integer value is true; zero is
|
|
false. The condition may also be a string or list value, in fact any sequence;
|
|
anything with a non-zero length is true, empty sequences are false. The test
|
|
used in the example is a simple comparison. The standard comparison operators
|
|
are written the same as in C: ``<`` (less than), ``>`` (greater than), ``==``
|
|
(equal to), ``<=`` (less than or equal to), ``>=`` (greater than or equal to)
|
|
and ``!=`` (not equal to).
|
|
|
|
* The *body* of the loop is *indented*: indentation is Python's way of grouping
|
|
statements. At the interactive prompt, you have to type a tab or space(s) for
|
|
each indented line. In practice you will prepare more complicated input
|
|
for Python with a text editor; all decent text editors have an auto-indent
|
|
facility. When a compound statement is entered interactively, it must be
|
|
followed by a blank line to indicate completion (since the parser cannot
|
|
guess when you have typed the last line). Note that each line within a basic
|
|
block must be indented by the same amount.
|
|
|
|
* The :keyword:`print` statement writes the value of the expression(s) it is
|
|
given. It differs from just writing the expression you want to write (as we did
|
|
earlier in the calculator examples) in the way it handles multiple expressions
|
|
and strings. Strings are printed without quotes, and a space is inserted
|
|
between items, so you can format things nicely, like this::
|
|
|
|
>>> i = 256*256
|
|
>>> print 'The value of i is', i
|
|
The value of i is 65536
|
|
|
|
A trailing comma avoids the newline after the output::
|
|
|
|
>>> a, b = 0, 1
|
|
>>> while b < 1000:
|
|
... print b,
|
|
... a, b = b, a+b
|
|
...
|
|
1 1 2 3 5 8 13 21 34 55 89 144 233 377 610 987
|
|
|
|
Note that the interpreter inserts a newline before it prints the next prompt if
|
|
the last line was not completed.
|
|
|
|
.. rubric:: Footnotes
|
|
|
|
.. [#] Since ``**`` has higher precedence than ``-``, ``-3**2`` will be
|
|
interpreted as ``-(3**2)`` and thus result in ``-9``. To avoid this
|
|
and get ``9``, you can use ``(-3)**2``.
|
|
|
|
.. [#] Unlike other languages, special characters such as ``\n`` have the
|
|
same meaning with both single (``'...'``) and double (``"..."``) quotes.
|
|
The only difference between the two is that within single quotes you don't
|
|
need to escape ``"`` (but you have to escape ``\'``) and vice versa.
|