Doc/tutorial/inputoutput.rst - platform/external/python/cpython3 - Gitiles

 .. _tut-io:

 ****************
 Input and Output
 ****************

 There are several ways to present the output of a program; data can be printed
 in a human-readable form, or written to a file for future use. This chapter will
 discuss some of the possibilities.


 .. _tut-formatting:

 Fancier Output Formatting
 =========================

 So far we've encountered two ways of writing values: *expression statements* and
 the :func:`print` function.  (A third way is using the :meth:`write` method
 of file objects; the standard output file can be referenced as ``sys.stdout``.
 See the Library Reference for more information on this.)

 Often you'll want more control over the formatting of your output than simply
 printing space-separated values. There are several ways to format output.

 * To use :ref:`formatted string literals <tut-f-strings>`, begin a string
   with ``f`` or ``F`` before the opening quotation mark or triple quotation mark.
   Inside this string, you can write a Python expression between ``{`` and ``}``
   characters that can refer to variables or literal values.

   ::

      >>> year = 2016
      >>> event = 'Referendum'
      >>> f'Results of the {year} {event}'
      'Results of the 2016 Referendum'

 * The :meth:`str.format` method of strings requires more manual
   effort.  You'll still use ``{`` and ``}`` to mark where a variable
   will be substituted and can provide detailed formatting directives,
   but you'll also need to provide the information to be formatted.

   ::

      >>> yes_votes = 42_572_654
      >>> no_votes = 43_132_495
      >>> percentage = yes_votes / (yes_votes + no_votes)
      >>> '{:-9} YES votes  {:2.2%}'.format(yes_votes, percentage)
      ' 42572654 YES votes  49.67%'

 * Finally, you can do all the string handling yourself by using string slicing and
   concatenation operations to create any layout you can imagine.  The
   string type has some methods that perform useful operations for padding
   strings to a given column width.

 When you don't need fancy output but just want a quick display of some
 variables for debugging purposes, you can convert any value to a string with
 the :func:`repr` or :func:`str` functions.

 The :func:`str` function is meant to return representations of values which are
 fairly human-readable, while :func:`repr` is meant to generate representations
 which can be read by the interpreter (or will force a :exc:`SyntaxError` if
 there is no equivalent syntax).  For objects which don't have a particular
 representation for human consumption, :func:`str` will return the same value as
 :func:`repr`.  Many values, such as numbers or structures like lists and
 dictionaries, have the same representation using either function.  Strings, in
 particular, have two distinct representations.

 Some examples::

    >>> s = 'Hello, world.'
    >>> str(s)
    'Hello, world.'
    >>> repr(s)
    "'Hello, world.'"
    >>> str(1/7)
    '0.14285714285714285'
    >>> x = 10 * 3.25
    >>> y = 200 * 200
    >>> s = 'The value of x is ' + repr(x) + ', and y is ' + repr(y) + '...'
    >>> print(s)
    The value of x is 32.5, and y is 40000...
    >>> # The repr() of a string adds string quotes and backslashes:
    ... hello = 'hello, world\n'
    >>> hellos = repr(hello)
    >>> print(hellos)
    'hello, world\n'
    >>> # The argument to repr() may be any Python object:
    ... repr((x, y, ('spam', 'eggs')))
    "(32.5, 40000, ('spam', 'eggs'))"

 The :mod:`string` module contains a :class:`~string.Template` class that offers
 yet another way to substitute values into strings, using placeholders like
 ``$x`` and replacing them with values from a dictionary, but offers much less
 control of the formatting.


 .. _tut-f-strings:

 Formatted String Literals
 -------------------------

 :ref:`Formatted string literals <f-strings>` (also called f-strings for
 short) let you include the value of Python expressions inside a string by
 prefixing the string with ``f`` or ``F`` and writing expressions as
 ``{expression}``.

 An optional format specifier can follow the expression. This allows greater
 control over how the value is formatted. The following example rounds pi to
 three places after the decimal::

    >>> import math
    >>> print(f'The value of pi is approximately {math.pi:.3f}.')
    The value of pi is approximately 3.142.

 Passing an integer after the ``':'`` will cause that field to be a minimum
 number of characters wide.  This is useful for making columns line up. ::

    >>> table = {'Sjoerd': 4127, 'Jack': 4098, 'Dcab': 7678}
    >>> for name, phone in table.items():
    ...     print(f'{name:10} ==> {phone:10d}')
    ...
    Sjoerd     ==>       4127
    Jack       ==>       4098
    Dcab       ==>       7678

 Other modifiers can be used to convert the value before it is formatted.
 ``'!a'`` applies :func:`ascii`, ``'!s'`` applies :func:`str`, and ``'!r'``
 applies :func:`repr`::

    >>> animals = 'eels'
    >>> print(f'My hovercraft is full of {animals}.')
    My hovercraft is full of eels.
    >>> print(f'My hovercraft is full of {animals!r}.')
    My hovercraft is full of 'eels'.

 For a reference on these format specifications, see
 the reference guide for the :ref:`formatspec`.

 .. _tut-string-format:

 The String format() Method
 --------------------------

 Basic usage of the :meth:`str.format` method looks like this::

    >>> print('We are the {} who say "{}!"'.format('knights', 'Ni'))
    We are the knights who say "Ni!"

 The brackets and characters within them (called format fields) are replaced with
 the objects passed into the :meth:`str.format` method.  A number in the
 brackets can be used to refer to the position of the object passed into the
 :meth:`str.format` method. ::

    >>> print('{0} and {1}'.format('spam', 'eggs'))
    spam and eggs
    >>> print('{1} and {0}'.format('spam', 'eggs'))
    eggs and spam

 If keyword arguments are used in the :meth:`str.format` method, their values
 are referred to by using the name of the argument. ::

    >>> print('This {food} is {adjective}.'.format(
    ...       food='spam', adjective='absolutely horrible'))
    This spam is absolutely horrible.

 Positional and keyword arguments can be arbitrarily combined::

    >>> print('The story of {0}, {1}, and {other}.'.format('Bill', 'Manfred',
                                                           other='Georg'))
    The story of Bill, Manfred, and Georg.

 If you have a really long format string that you don't want to split up, it
 would be nice if you could reference the variables to be formatted by name
 instead of by position.  This can be done by simply passing the dict and using
 square brackets ``'[]'`` to access the keys ::

    >>> table = {'Sjoerd': 4127, 'Jack': 4098, 'Dcab': 8637678}
    >>> print('Jack: {0[Jack]:d}; Sjoerd: {0[Sjoerd]:d}; '
    ...       'Dcab: {0[Dcab]:d}'.format(table))
    Jack: 4098; Sjoerd: 4127; Dcab: 8637678

 This could also be done by passing the table as keyword arguments with the '**'
 notation. ::

    >>> table = {'Sjoerd': 4127, 'Jack': 4098, 'Dcab': 8637678}
    >>> print('Jack: {Jack:d}; Sjoerd: {Sjoerd:d}; Dcab: {Dcab:d}'.format(**table))
    Jack: 4098; Sjoerd: 4127; Dcab: 8637678

 This is particularly useful in combination with the built-in function
 :func:`vars`, which returns a dictionary containing all local variables.

 As an example, the following lines produce a tidily-aligned
 set of columns giving integers and their squares and cubes::

    >>> for x in range(1, 11):
    ...     print('{0:2d} {1:3d} {2:4d}'.format(x, x*x, x*x*x))
    ...
     1   1    1
     2   4    8
     3   9   27
     4  16   64
     5  25  125
     6  36  216
     7  49  343
     8  64  512
     9  81  729
    10 100 1000

 For a complete overview of string formatting with :meth:`str.format`, see
 :ref:`formatstrings`.


 Manual String Formatting
 ------------------------

 Here's the same table of squares and cubes, formatted manually::

    >>> for x in range(1, 11):
    ...     print(repr(x).rjust(2), repr(x*x).rjust(3), end=' ')
    ...     # Note use of 'end' on previous line
    ...     print(repr(x*x*x).rjust(4))
    ...
     1   1    1
     2   4    8
     3   9   27
     4  16   64
     5  25  125
     6  36  216
     7  49  343
     8  64  512
     9  81  729
    10 100 1000

 (Note that the one space between each column was added by the
 way :func:`print` works: it always adds spaces between its arguments.)

 The :meth:`str.rjust` method of string objects right-justifies a string in a
 field of a given width by padding it with spaces on the left. There are
 similar methods :meth:`str.ljust` and :meth:`str.center`. These methods do
 not write anything, they just return a new string. If the input string is too
 long, they don't truncate it, but return it unchanged; this will mess up your
 column lay-out but that's usually better than the alternative, which would be
 lying about a value. (If you really want truncation you can always add a
 slice operation, as in ``x.ljust(n)[:n]``.)

 There is another method, :meth:`str.zfill`, which pads a numeric string on the
 left with zeros.  It understands about plus and minus signs::

    >>> '12'.zfill(5)
    '00012'
    >>> '-3.14'.zfill(7)
    '-003.14'
    >>> '3.14159265359'.zfill(5)
    '3.14159265359'


 Old string formatting
 ---------------------

 The ``%`` operator can also be used for string formatting. It interprets the
 left argument much like a :c:func:`sprintf`\ -style format string to be applied
 to the right argument, and returns the string resulting from this formatting
 operation. For example::

    >>> import math
    >>> print('The value of pi is approximately %5.3f.' % math.pi)
    The value of pi is approximately 3.142.

 More information can be found in the :ref:`old-string-formatting` section.


 .. _tut-files:

 Reading and Writing Files
 =========================

 .. index::
    builtin: open
    object: file

 :func:`open` returns a :term:`file object`, and is most commonly used with
 two arguments: ``open(filename, mode)``.

 ::

    >>> f = open('workfile', 'w')

 .. XXX str(f) is <io.TextIOWrapper object at 0x82e8dc4>

    >>> print(f)
    <open file 'workfile', mode 'w' at 80a0960>

 The first argument is a string containing the filename.  The second argument is
 another string containing a few characters describing the way in which the file
 will be used.  *mode* can be ``'r'`` when the file will only be read, ``'w'``
 for only writing (an existing file with the same name will be erased), and
 ``'a'`` opens the file for appending; any data written to the file is
 automatically added to the end.  ``'r+'`` opens the file for both reading and
 writing. The *mode* argument is optional; ``'r'`` will be assumed if it's
 omitted.

 Normally, files are opened in :dfn:`text mode`, that means, you read and write
 strings from and to the file, which are encoded in a specific encoding. If
 encoding is not specified, the default is platform dependent (see
 :func:`open`). ``'b'`` appended to the mode opens the file in
 :dfn:`binary mode`: now the data is read and written in the form of bytes
 objects.  This mode should be used for all files that don't contain text.

 In text mode, the default when reading is to convert platform-specific line
 endings (``\n`` on Unix, ``\r\n`` on Windows) to just ``\n``.  When writing in
 text mode, the default is to convert occurrences of ``\n`` back to
 platform-specific line endings.  This behind-the-scenes modification
 to file data is fine for text files, but will corrupt binary data like that in
 :file:`JPEG` or :file:`EXE` files.  Be very careful to use binary mode when
 reading and writing such files.

 It is good practice to use the :keyword:`with` keyword when dealing
 with file objects.  The advantage is that the file is properly closed
 after its suite finishes, even if an exception is raised at some
 point.  Using :keyword:`with` is also much shorter than writing
 equivalent :keyword:`try`\ -\ :keyword:`finally` blocks::

     >>> with open('workfile') as f:
     ...     read_data = f.read()
     >>> f.closed
     True

 If you're not using the :keyword:`with` keyword, then you should call
 ``f.close()`` to close the file and immediately free up any system
 resources used by it. If you don't explicitly close a file, Python's
 garbage collector will eventually destroy the object and close the
 open file for you, but the file may stay open for a while.  Another
 risk is that different Python implementations will do this clean-up at
 different times.

 After a file object is closed, either by a :keyword:`with` statement
 or by calling ``f.close()``, attempts to use the file object will
 automatically fail. ::

    >>> f.close()
    >>> f.read()
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    ValueError: I/O operation on closed file.


 .. _tut-filemethods:

 Methods of File Objects
 -----------------------

 The rest of the examples in this section will assume that a file object called
 ``f`` has already been created.

 To read a file's contents, call ``f.read(size)``, which reads some quantity of
 data and returns it as a string (in text mode) or bytes object (in binary mode).
 *size* is an optional numeric argument.  When *size* is omitted or negative, the
 entire contents of the file will be read and returned; it's your problem if the
 file is twice as large as your machine's memory. Otherwise, at most *size* bytes
 are read and returned.
 If the end of the file has been reached, ``f.read()`` will return an empty
 string (``''``).  ::

    >>> f.read()
    'This is the entire file.\n'
    >>> f.read()
    ''

 ``f.readline()`` reads a single line from the file; a newline character (``\n``)
 is left at the end of the string, and is only omitted on the last line of the
 file if the file doesn't end in a newline.  This makes the return value
 unambiguous; if ``f.readline()`` returns an empty string, the end of the file
 has been reached, while a blank line is represented by ``'\n'``, a string
 containing only a single newline.  ::

    >>> f.readline()
    'This is the first line of the file.\n'
    >>> f.readline()
    'Second line of the file\n'
    >>> f.readline()
    ''

 For reading lines from a file, you can loop over the file object. This is memory
 efficient, fast, and leads to simple code::

    >>> for line in f:
    ...     print(line, end='')
    ...
    This is the first line of the file.
    Second line of the file

 If you want to read all the lines of a file in a list you can also use
 ``list(f)`` or ``f.readlines()``.

 ``f.write(string)`` writes the contents of *string* to the file, returning
 the number of characters written. ::

    >>> f.write('This is a test\n')
    15

 Other types of objects need to be converted -- either to a string (in text mode)
 or a bytes object (in binary mode) -- before writing them::

    >>> value = ('the answer', 42)
    >>> s = str(value)  # convert the tuple to string
    >>> f.write(s)
    18

 ``f.tell()`` returns an integer giving the file object's current position in the file
 represented as number of bytes from the beginning of the file when in binary mode and
 an opaque number when in text mode.

 To change the file object's position, use ``f.seek(offset, from_what)``.  The position is computed
 from adding *offset* to a reference point; the reference point is selected by
 the *from_what* argument.  A *from_what* value of 0 measures from the beginning
 of the file, 1 uses the current file position, and 2 uses the end of the file as
 the reference point.  *from_what* can be omitted and defaults to 0, using the
 beginning of the file as the reference point. ::

    >>> f = open('workfile', 'rb+')
    >>> f.write(b'0123456789abcdef')
    16
    >>> f.seek(5)      # Go to the 6th byte in the file
    5
    >>> f.read(1)
    b'5'
    >>> f.seek(-3, 2)  # Go to the 3rd byte before the end
    13
    >>> f.read(1)
    b'd'

 In text files (those opened without a ``b`` in the mode string), only seeks
 relative to the beginning of the file are allowed (the exception being seeking
 to the very file end with ``seek(0, 2)``) and the only valid *offset* values are
 those returned from the ``f.tell()``, or zero. Any other *offset* value produces
 undefined behaviour.

 File objects have some additional methods, such as :meth:`~file.isatty` and
 :meth:`~file.truncate` which are less frequently used; consult the Library
 Reference for a complete guide to file objects.


 .. _tut-json:

 Saving structured data with :mod:`json`
 ---------------------------------------

 .. index:: module: json

 Strings can easily be written to and read from a file.  Numbers take a bit more
 effort, since the :meth:`read` method only returns strings, which will have to
 be passed to a function like :func:`int`, which takes a string like ``'123'``
 and returns its numeric value 123.  When you want to save more complex data
 types like nested lists and dictionaries, parsing and serializing by hand
 becomes complicated.

 Rather than having users constantly writing and debugging code to save
 complicated data types to files, Python allows you to use the popular data
 interchange format called `JSON (JavaScript Object Notation)
 <http://json.org>`_.  The standard module called :mod:`json` can take Python
 data hierarchies, and convert them to string representations; this process is
 called :dfn:`serializing`.  Reconstructing the data from the string representation
 is called :dfn:`deserializing`.  Between serializing and deserializing, the
 string representing the object may have been stored in a file or data, or
 sent over a network connection to some distant machine.

 .. note::
    The JSON format is commonly used by modern applications to allow for data
    exchange.  Many programmers are already familiar with it, which makes
    it a good choice for interoperability.

 If you have an object ``x``, you can view its JSON string representation with a
 simple line of code::

    >>> import json
    >>> json.dumps([1, 'simple', 'list'])
    '[1, "simple", "list"]'

 Another variant of the :func:`~json.dumps` function, called :func:`~json.dump`,
 simply serializes the object to a :term:`text file`.  So if ``f`` is a
 :term:`text file` object opened for writing, we can do this::

    json.dump(x, f)

 To decode the object again, if ``f`` is a :term:`text file` object which has
 been opened for reading::

    x = json.load(f)

 This simple serialization technique can handle lists and dictionaries, but
 serializing arbitrary class instances in JSON requires a bit of extra effort.
 The reference for the :mod:`json` module contains an explanation of this.

 .. seealso::

    :mod:`pickle` - the pickle module

    Contrary to :ref:`JSON <tut-json>`, *pickle* is a protocol which allows
    the serialization of arbitrarily complex Python objects.  As such, it is
    specific to Python and cannot be used to communicate with applications
    written in other languages.  It is also insecure by default:
    deserializing pickle data coming from an untrusted source can execute
    arbitrary code, if the data was crafted by a skilled attacker.
	.. _tut-io:

	****************
	Input and Output
	****************

	There are several ways to present the output of a program; data can be printed
	in a human-readable form, or written to a file for future use. This chapter will
	discuss some of the possibilities.


	.. _tut-formatting:

	Fancier Output Formatting
	=========================

	So far we've encountered two ways of writing values: expression statements and
	the :func:`print` function. (A third way is using the :meth:`write` method
	of file objects; the standard output file can be referenced as ``sys.stdout``.
	See the Library Reference for more information on this.)

	Often you'll want more control over the formatting of your output than simply
	printing space-separated values. There are several ways to format output.

	* To use :ref:`formatted string literals <tut-f-strings>`, begin a string
	with ``f`` or ``F`` before the opening quotation mark or triple quotation mark.
	Inside this string, you can write a Python expression between ``{`` and ``}``
	characters that can refer to variables or literal values.

	::

	>>> year = 2016
	>>> event = 'Referendum'
	>>> f'Results of the {year} {event}'
	'Results of the 2016 Referendum'

	* The :meth:`str.format` method of strings requires more manual
	effort. You'll still use ``{`` and ``}`` to mark where a variable
	will be substituted and can provide detailed formatting directives,
	but you'll also need to provide the information to be formatted.

	::

	>>> yes_votes = 42_572_654
	>>> no_votes = 43_132_495
	>>> percentage = yes_votes / (yes_votes + no_votes)
	>>> '{:-9} YES votes {:2.2%}'.format(yes_votes, percentage)
	' 42572654 YES votes 49.67%'

	* Finally, you can do all the string handling yourself by using string slicing and
	concatenation operations to create any layout you can imagine. The
	string type has some methods that perform useful operations for padding
	strings to a given column width.

	When you don't need fancy output but just want a quick display of some
	variables for debugging purposes, you can convert any value to a string with
	the :func:`repr` or :func:`str` functions.

	The :func:`str` function is meant to return representations of values which are
	fairly human-readable, while :func:`repr` is meant to generate representations
	which can be read by the interpreter (or will force a :exc:`SyntaxError` if
	there is no equivalent syntax). For objects which don't have a particular
	representation for human consumption, :func:`str` will return the same value as
	:func:`repr`. Many values, such as numbers or structures like lists and
	dictionaries, have the same representation using either function. Strings, in
	particular, have two distinct representations.

	Some examples::

	>>> s = 'Hello, world.'
	>>> str(s)
	'Hello, world.'
	>>> repr(s)
	"'Hello, world.'"
	>>> str(1/7)
	'0.14285714285714285'
	>>> x = 10 * 3.25
	>>> y = 200 * 200
	>>> s = 'The value of x is ' + repr(x) + ', and y is ' + repr(y) + '...'
	>>> print(s)
	The value of x is 32.5, and y is 40000...
	>>> # The repr() of a string adds string quotes and backslashes:
	... hello = 'hello, world\n'
	>>> hellos = repr(hello)
	>>> print(hellos)
	'hello, world\n'
	>>> # The argument to repr() may be any Python object:
	... repr((x, y, ('spam', 'eggs')))
	"(32.5, 40000, ('spam', 'eggs'))"

	The :mod:`string` module contains a :class:`~string.Template` class that offers
	yet another way to substitute values into strings, using placeholders like
	``$x`` and replacing them with values from a dictionary, but offers much less
	control of the formatting.


	.. _tut-f-strings:

	Formatted String Literals
	-------------------------

	:ref:`Formatted string literals <f-strings>` (also called f-strings for
	short) let you include the value of Python expressions inside a string by
	prefixing the string with ``f`` or ``F`` and writing expressions as
	``{expression}``.

	An optional format specifier can follow the expression. This allows greater
	control over how the value is formatted. The following example rounds pi to
	three places after the decimal::

	>>> import math
	>>> print(f'The value of pi is approximately {math.pi:.3f}.')
	The value of pi is approximately 3.142.

	Passing an integer after the ``':'`` will cause that field to be a minimum
	number of characters wide. This is useful for making columns line up. ::

	>>> table = {'Sjoerd': 4127, 'Jack': 4098, 'Dcab': 7678}
	>>> for name, phone in table.items():
	... print(f'{name:10} ==> {phone:10d}')
	...
	Sjoerd ==> 4127
	Jack ==> 4098
	Dcab ==> 7678

	Other modifiers can be used to convert the value before it is formatted.
	``'!a'`` applies :func:`ascii`, ``'!s'`` applies :func:`str`, and ``'!r'``
	applies :func:`repr`::

	>>> animals = 'eels'
	>>> print(f'My hovercraft is full of {animals}.')
	My hovercraft is full of eels.
	>>> print(f'My hovercraft is full of {animals!r}.')
	My hovercraft is full of 'eels'.

	For a reference on these format specifications, see
	the reference guide for the :ref:`formatspec`.

	.. _tut-string-format:

	The String format() Method
	--------------------------

	Basic usage of the :meth:`str.format` method looks like this::

	>>> print('We are the {} who say "{}!"'.format('knights', 'Ni'))
	We are the knights who say "Ni!"

	The brackets and characters within them (called format fields) are replaced with
	the objects passed into the :meth:`str.format` method. A number in the
	brackets can be used to refer to the position of the object passed into the
	:meth:`str.format` method. ::

	>>> print('{0} and {1}'.format('spam', 'eggs'))
	spam and eggs
	>>> print('{1} and {0}'.format('spam', 'eggs'))
	eggs and spam

	If keyword arguments are used in the :meth:`str.format` method, their values
	are referred to by using the name of the argument. ::

	>>> print('This {food} is {adjective}.'.format(
	... food='spam', adjective='absolutely horrible'))
	This spam is absolutely horrible.

	Positional and keyword arguments can be arbitrarily combined::

	>>> print('The story of {0}, {1}, and {other}.'.format('Bill', 'Manfred',
	other='Georg'))
	The story of Bill, Manfred, and Georg.

	If you have a really long format string that you don't want to split up, it
	would be nice if you could reference the variables to be formatted by name
	instead of by position. This can be done by simply passing the dict and using
	square brackets ``'[]'`` to access the keys ::

	>>> table = {'Sjoerd': 4127, 'Jack': 4098, 'Dcab': 8637678}
	>>> print('Jack: {0[Jack]:d}; Sjoerd: {0[Sjoerd]:d}; '
	... 'Dcab: {0[Dcab]:d}'.format(table))
	Jack: 4098; Sjoerd: 4127; Dcab: 8637678

	This could also be done by passing the table as keyword arguments with the '**'
	notation. ::

	>>> table = {'Sjoerd': 4127, 'Jack': 4098, 'Dcab': 8637678}
	>>> print('Jack: {Jack:d}; Sjoerd: {Sjoerd:d}; Dcab: {Dcab:d}'.format(**table))
	Jack: 4098; Sjoerd: 4127; Dcab: 8637678

	This is particularly useful in combination with the built-in function
	:func:`vars`, which returns a dictionary containing all local variables.

	As an example, the following lines produce a tidily-aligned
	set of columns giving integers and their squares and cubes::

	>>> for x in range(1, 11):
	... print('{0:2d} {1:3d} {2:4d}'.format(x, xx, xx*x))
	...
	1 1 1
	2 4 8
	3 9 27
	4 16 64
	5 25 125
	6 36 216
	7 49 343
	8 64 512
	9 81 729
	10 100 1000

	For a complete overview of string formatting with :meth:`str.format`, see
	:ref:`formatstrings`.


	Manual String Formatting
	------------------------

	Here's the same table of squares and cubes, formatted manually::

	>>> for x in range(1, 11):
	... print(repr(x).rjust(2), repr(x*x).rjust(3), end=' ')
	... # Note use of 'end' on previous line
	... print(repr(xxx).rjust(4))
	...
	1 1 1
	2 4 8
	3 9 27
	4 16 64
	5 25 125
	6 36 216
	7 49 343
	8 64 512
	9 81 729
	10 100 1000

	(Note that the one space between each column was added by the
	way :func:`print` works: it always adds spaces between its arguments.)

	The :meth:`str.rjust` method of string objects right-justifies a string in a
	field of a given width by padding it with spaces on the left. There are
	similar methods :meth:`str.ljust` and :meth:`str.center`. These methods do
	not write anything, they just return a new string. If the input string is too
	long, they don't truncate it, but return it unchanged; this will mess up your
	column lay-out but that's usually better than the alternative, which would be
	lying about a value. (If you really want truncation you can always add a
	slice operation, as in ``x.ljust(n)[:n]``.)

	There is another method, :meth:`str.zfill`, which pads a numeric string on the
	left with zeros. It understands about plus and minus signs::

	>>> '12'.zfill(5)
	'00012'
	>>> '-3.14'.zfill(7)
	'-003.14'
	>>> '3.14159265359'.zfill(5)
	'3.14159265359'


	Old string formatting
	---------------------

	The ``%`` operator can also be used for string formatting. It interprets the
	left argument much like a :c:func:`sprintf`\ -style format string to be applied
	to the right argument, and returns the string resulting from this formatting
	operation. For example::

	>>> import math
	>>> print('The value of pi is approximately %5.3f.' % math.pi)
	The value of pi is approximately 3.142.

	More information can be found in the :ref:`old-string-formatting` section.


	.. _tut-files:

	Reading and Writing Files
	=========================

	.. index::
	builtin: open
	object: file

	:func:`open` returns a :term:`file object`, and is most commonly used with
	two arguments: ``open(filename, mode)``.

	::

	>>> f = open('workfile', 'w')

	.. XXX str(f) is <io.TextIOWrapper object at 0x82e8dc4>

	>>> print(f)
	<open file 'workfile', mode 'w' at 80a0960>

	The first argument is a string containing the filename. The second argument is
	another string containing a few characters describing the way in which the file
	will be used. mode can be ``'r'`` when the file will only be read, ``'w'``
	for only writing (an existing file with the same name will be erased), and
	``'a'`` opens the file for appending; any data written to the file is
	automatically added to the end. ``'r+'`` opens the file for both reading and
	writing. The mode argument is optional; ``'r'`` will be assumed if it's
	omitted.

	Normally, files are opened in :dfn:`text mode`, that means, you read and write
	strings from and to the file, which are encoded in a specific encoding. If
	encoding is not specified, the default is platform dependent (see
	:func:`open`). ``'b'`` appended to the mode opens the file in
	:dfn:`binary mode`: now the data is read and written in the form of bytes
	objects. This mode should be used for all files that don't contain text.

	In text mode, the default when reading is to convert platform-specific line
	endings (``\n`` on Unix, ``\r\n`` on Windows) to just ``\n``. When writing in
	text mode, the default is to convert occurrences of ``\n`` back to
	platform-specific line endings. This behind-the-scenes modification
	to file data is fine for text files, but will corrupt binary data like that in
	:file:`JPEG` or :file:`EXE` files. Be very careful to use binary mode when
	reading and writing such files.

	It is good practice to use the :keyword:`with` keyword when dealing
	with file objects. The advantage is that the file is properly closed
	after its suite finishes, even if an exception is raised at some
	point. Using :keyword:`with` is also much shorter than writing
	equivalent :keyword:`try`\ -\ :keyword:`finally` blocks::

	>>> with open('workfile') as f:
	... read_data = f.read()
	>>> f.closed
	True

	If you're not using the :keyword:`with` keyword, then you should call
	``f.close()`` to close the file and immediately free up any system
	resources used by it. If you don't explicitly close a file, Python's
	garbage collector will eventually destroy the object and close the
	open file for you, but the file may stay open for a while. Another
	risk is that different Python implementations will do this clean-up at
	different times.

	After a file object is closed, either by a :keyword:`with` statement
	or by calling ``f.close()``, attempts to use the file object will
	automatically fail. ::

	>>> f.close()
	>>> f.read()
	Traceback (most recent call last):
	File "<stdin>", line 1, in <module>
	ValueError: I/O operation on closed file.


	.. _tut-filemethods:

	Methods of File Objects
	-----------------------

	The rest of the examples in this section will assume that a file object called
	``f`` has already been created.

	To read a file's contents, call ``f.read(size)``, which reads some quantity of
	data and returns it as a string (in text mode) or bytes object (in binary mode).
	size is an optional numeric argument. When size is omitted or negative, the
	entire contents of the file will be read and returned; it's your problem if the
	file is twice as large as your machine's memory. Otherwise, at most size bytes
	are read and returned.
	If the end of the file has been reached, ``f.read()`` will return an empty
	string (``''``). ::

	>>> f.read()
	'This is the entire file.\n'
	>>> f.read()
	''

	``f.readline()`` reads a single line from the file; a newline character (``\n``)
	is left at the end of the string, and is only omitted on the last line of the
	file if the file doesn't end in a newline. This makes the return value
	unambiguous; if ``f.readline()`` returns an empty string, the end of the file
	has been reached, while a blank line is represented by ``'\n'``, a string
	containing only a single newline. ::

	>>> f.readline()
	'This is the first line of the file.\n'
	>>> f.readline()
	'Second line of the file\n'
	>>> f.readline()
	''

	For reading lines from a file, you can loop over the file object. This is memory
	efficient, fast, and leads to simple code::

	>>> for line in f:
	... print(line, end='')
	...
	This is the first line of the file.
	Second line of the file

	If you want to read all the lines of a file in a list you can also use
	``list(f)`` or ``f.readlines()``.

	``f.write(string)`` writes the contents of string to the file, returning
	the number of characters written. ::

	>>> f.write('This is a test\n')
	15

	Other types of objects need to be converted -- either to a string (in text mode)
	or a bytes object (in binary mode) -- before writing them::

	>>> value = ('the answer', 42)
	>>> s = str(value) # convert the tuple to string
	>>> f.write(s)
	18

	``f.tell()`` returns an integer giving the file object's current position in the file
	represented as number of bytes from the beginning of the file when in binary mode and
	an opaque number when in text mode.

	To change the file object's position, use ``f.seek(offset, from_what)``. The position is computed
	from adding offset to a reference point; the reference point is selected by
	the from_what argument. A from_what value of 0 measures from the beginning
	of the file, 1 uses the current file position, and 2 uses the end of the file as
	the reference point. from_what can be omitted and defaults to 0, using the
	beginning of the file as the reference point. ::

	>>> f = open('workfile', 'rb+')
	>>> f.write(b'0123456789abcdef')
	16
	>>> f.seek(5) # Go to the 6th byte in the file
	5
	>>> f.read(1)
	b'5'
	>>> f.seek(-3, 2) # Go to the 3rd byte before the end
	13
	>>> f.read(1)
	b'd'

	In text files (those opened without a ``b`` in the mode string), only seeks
	relative to the beginning of the file are allowed (the exception being seeking
	to the very file end with ``seek(0, 2)``) and the only valid offset values are
	those returned from the ``f.tell()``, or zero. Any other offset value produces
	undefined behaviour.

	File objects have some additional methods, such as :meth:`~file.isatty` and
	:meth:`~file.truncate` which are less frequently used; consult the Library
	Reference for a complete guide to file objects.


	.. _tut-json:

	Saving structured data with :mod:`json`
	---------------------------------------

	.. index:: module: json

	Strings can easily be written to and read from a file. Numbers take a bit more
	effort, since the :meth:`read` method only returns strings, which will have to
	be passed to a function like :func:`int`, which takes a string like ``'123'``
	and returns its numeric value 123. When you want to save more complex data
	types like nested lists and dictionaries, parsing and serializing by hand
	becomes complicated.

	Rather than having users constantly writing and debugging code to save
	complicated data types to files, Python allows you to use the popular data
	interchange format called `JSON (JavaScript Object Notation)
	<http://json.org>`_. The standard module called :mod:`json` can take Python
	data hierarchies, and convert them to string representations; this process is
	called :dfn:`serializing`. Reconstructing the data from the string representation
	is called :dfn:`deserializing`. Between serializing and deserializing, the
	string representing the object may have been stored in a file or data, or
	sent over a network connection to some distant machine.

	.. note::
	The JSON format is commonly used by modern applications to allow for data
	exchange. Many programmers are already familiar with it, which makes
	it a good choice for interoperability.

	If you have an object ``x``, you can view its JSON string representation with a
	simple line of code::

	>>> import json
	>>> json.dumps([1, 'simple', 'list'])
	'[1, "simple", "list"]'

	Another variant of the :func:`~json.dumps` function, called :func:`~json.dump`,
	simply serializes the object to a :term:`text file`. So if ``f`` is a
	:term:`text file` object opened for writing, we can do this::

	json.dump(x, f)

	To decode the object again, if ``f`` is a :term:`text file` object which has
	been opened for reading::

	x = json.load(f)

	This simple serialization technique can handle lists and dictionaries, but
	serializing arbitrary class instances in JSON requires a bit of extra effort.
	The reference for the :mod:`json` module contains an explanation of this.

	.. seealso::

	:mod:`pickle` - the pickle module

	Contrary to :ref:`JSON <tut-json>`, pickle is a protocol which allows
	the serialization of arbitrarily complex Python objects. As such, it is
	specific to Python and cannot be used to communicate with applications
	written in other languages. It is also insecure by default:
	deserializing pickle data coming from an untrusted source can execute
	arbitrary code, if the data was crafted by a skilled attacker.