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 two ways to format your output; the
 first way is to do all the string handling yourself; using string slicing and
 concatenation operations you can create any layout you can imagine.  The
 string type has some methods that perform useful operations for padding
 strings to a given column width; these will be discussed shortly.  The second
 way is to use the :meth:`str.format` method.

 The :mod:`string` module contains a :class:`~string.Template` class which offers
 yet another way to substitute values into strings.

 One question remains, of course: how do you convert values to strings? Luckily,
 Python has ways to convert any value to a string: pass it to 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'))"

 Here are two ways to write a table of squares and cubes::

    >>> 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

    >>> 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

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

 This example demonstrates the :meth:`str.rjust` method of string
 objects, which 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'

 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.

 ``'!a'`` (apply :func:`ascii`), ``'!s'`` (apply :func:`str`) and ``'!r'``
 (apply :func:`repr`) can be used to convert the value before it is formatted::

    >>> import math
    >>> print('The value of PI is approximately {}.'.format(math.pi))
    The value of PI is approximately 3.14159265359.
    >>> print('The value of PI is approximately {!r}.'.format(math.pi))
    The value of PI is approximately 3.141592653589793.

 An optional ``':'`` and format specifier can follow the field name. This allows
 greater control over how the value is formatted.  The following example
 rounds Pi to three places after the decimal.

    >>> import math
    >>> print('The value of PI is approximately {0:.3f}.'.format(math.pi))
    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 tables pretty. ::

    >>> table = {'Sjoerd': 4127, 'Jack': 4098, 'Dcab': 7678}
    >>> for name, phone in table.items():
    ...     print('{0:10} ==> {1:10d}'.format(name, phone))
    ...
    Jack       ==>       4098
    Dcab       ==>       7678
    Sjoerd     ==>       4127

 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.

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


 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 (the
 default being UTF-8).  ``'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.


 .. _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 or bytes object.  *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

 To write something other than a string, it needs to be converted to a string
 first::

    >>> value = ('the answer', 42)
    >>> s = str(value)
    >>> 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.


 When you're done with a file, call ``f.close()`` to close it and free up any
 system resources taken up by the open file.  After 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 ?
    ValueError: I/O operation on closed file

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

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

 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::

    >>> 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 two ways to format your output; the
	first way is to do all the string handling yourself; using string slicing and
	concatenation operations you can create any layout you can imagine. The
	string type has some methods that perform useful operations for padding
	strings to a given column width; these will be discussed shortly. The second
	way is to use the :meth:`str.format` method.

	The :mod:`string` module contains a :class:`~string.Template` class which offers
	yet another way to substitute values into strings.

	One question remains, of course: how do you convert values to strings? Luckily,
	Python has ways to convert any value to a string: pass it to 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'))"

	Here are two ways to write a table of squares and cubes::

	>>> 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

	>>> 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

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

	This example demonstrates the :meth:`str.rjust` method of string
	objects, which 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'

	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.

	``'!a'`` (apply :func:`ascii`), ``'!s'`` (apply :func:`str`) and ``'!r'``
	(apply :func:`repr`) can be used to convert the value before it is formatted::

	>>> import math
	>>> print('The value of PI is approximately {}.'.format(math.pi))
	The value of PI is approximately 3.14159265359.
	>>> print('The value of PI is approximately {!r}.'.format(math.pi))
	The value of PI is approximately 3.141592653589793.

	An optional ``':'`` and format specifier can follow the field name. This allows
	greater control over how the value is formatted. The following example
	rounds Pi to three places after the decimal.

	>>> import math
	>>> print('The value of PI is approximately {0:.3f}.'.format(math.pi))
	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 tables pretty. ::

	>>> table = {'Sjoerd': 4127, 'Jack': 4098, 'Dcab': 7678}
	>>> for name, phone in table.items():
	... print('{0:10} ==> {1:10d}'.format(name, phone))
	...
	Jack ==> 4098
	Dcab ==> 7678
	Sjoerd ==> 4127

	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.

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


	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 (the
	default being UTF-8). ``'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.


	.. _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 or bytes object. 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

	To write something other than a string, it needs to be converted to a string
	first::

	>>> value = ('the answer', 42)
	>>> s = str(value)
	>>> 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.


	When you're done with a file, call ``f.close()`` to close it and free up any
	system resources taken up by the open file. After 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 ?
	ValueError: I/O operation on closed file

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

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

	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::

	>>> 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.