535 lines
17 KiB
ReStructuredText
535 lines
17 KiB
ReStructuredText
.. _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*6) / 4
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5.0
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>>> 8 / 5 # division always returns a floating point number
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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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Division (``/``) always returns a float. To do :term:`floor division` and
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get an integer result (discarding any fractional result) you can use the ``//``
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operator; to calculate the remainder you can use ``%``::
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>>> 17 / 3 # classic division returns a float
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5.666666666666667
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>>>
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>>> 17 // 3 # floor division discards the fractional part
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5
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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," he said.'
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'"Yes," he said.'
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>>> "\"Yes,\" he said."
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'"Yes," he said.'
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>>> '"Isn\'t," she said.'
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'"Isn\'t," she 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 :func:`print` function 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," she said.'
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'"Isn\'t," she said.'
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>>> print('"Isn\'t," she said.')
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"Isn't," she 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 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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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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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 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:`textseq`
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Strings are examples of *sequence types*, and support the common
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operations supported by such types.
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:ref:`string-methods`
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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:`old-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-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 support 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']
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>>> letters
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['a', 'b', 'c', 'd', 'e', 'f', 'g']
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>>> # replace some values
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>>> letters[2:5] = ['C', 'D', 'E']
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>>> letters
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['a', 'b', 'C', 'D', 'E', 'f', 'g']
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>>> # now remove them
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>>> letters[2:5] = []
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>>> letters
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['a', 'b', 'f', 'g']
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>>> # clear the list by replacing all the elements with an empty list
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>>> letters[:] = []
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>>> letters
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[]
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The built-in function :func:`len` also applies to lists::
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>>> letters = ['a', 'b', 'c', 'd']
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>>> len(letters)
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4
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It is possible to nest lists (create lists containing other lists), for
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example::
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>>> a = ['a', 'b', 'c']
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>>> n = [1, 2, 3]
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>>> x = [a, n]
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>>> x
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[['a', 'b', 'c'], [1, 2, 3]]
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>>> x[0]
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['a', 'b', 'c']
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>>> x[0][1]
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'b'
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.. _tut-firststeps:
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First Steps Towards Programming
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===============================
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Of course, we can use Python for more complicated tasks than adding two and two
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together. For instance, we can write an initial sub-sequence of the *Fibonacci*
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series as follows::
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>>> # Fibonacci series:
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... # the sum of two elements defines the next
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... a, b = 0, 1
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>>> while b < 10:
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... print(b)
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... a, b = b, a+b
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...
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1
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1
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2
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3
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5
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8
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This example introduces several new features.
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* The first line contains a *multiple assignment*: the variables ``a`` and ``b``
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simultaneously get the new values 0 and 1. On the last line this is used again,
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demonstrating that the expressions on the right-hand side are all evaluated
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first before any of the assignments take place. The right-hand side expressions
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are evaluated from the left to the right.
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* The :keyword:`while` loop executes as long as the condition (here: ``b < 10``)
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remains true. In Python, like in C, any non-zero integer value is true; zero is
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false. The condition may also be a string or list value, in fact any sequence;
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anything with a non-zero length is true, empty sequences are false. The test
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used in the example is a simple comparison. The standard comparison operators
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are written the same as in C: ``<`` (less than), ``>`` (greater than), ``==``
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(equal to), ``<=`` (less than or equal to), ``>=`` (greater than or equal to)
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and ``!=`` (not equal to).
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* The *body* of the loop is *indented*: indentation is Python's way of grouping
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statements. At the interactive prompt, you have to type a tab or space(s) for
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each indented line. In practice you will prepare more complicated input
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for Python with a text editor; all decent text editors have an auto-indent
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facility. When a compound statement is entered interactively, it must be
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followed by a blank line to indicate completion (since the parser cannot
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guess when you have typed the last line). Note that each line within a basic
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block must be indented by the same amount.
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* The :func:`print` function writes the value of the argument(s) it is given.
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It differs from just writing the expression you want to write (as we did
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earlier in the calculator examples) in the way it handles multiple arguments,
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floating point quantities, and strings. Strings are printed without quotes,
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and a space is inserted between items, so you can format things nicely, like
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this::
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>>> i = 256*256
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>>> print('The value of i is', i)
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The value of i is 65536
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The keyword argument *end* can be used to avoid the newline after the output,
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or end the output with a different string::
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>>> a, b = 0, 1
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>>> while b < 1000:
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... print(b, end=',')
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... a, b = b, a+b
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...
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1,1,2,3,5,8,13,21,34,55,89,144,233,377,610,987,
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.. rubric:: Footnotes
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.. [#] Since ``**`` has higher precedence than ``-``, ``-3**2`` will be
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interpreted as ``-(3**2)`` and thus result in ``-9``. To avoid this
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and get ``9``, you can use ``(-3)**2``.
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.. [#] Unlike other languages, special characters such as ``\n`` have the
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same meaning with both single (``'...'``) and double (``"..."``) quotes.
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The only difference between the two is that within single quotes you don't
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need to escape ``"`` (but you have to escape ``\'``) and vice versa.
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