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  • .. _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
    
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    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
    
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    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.'"
    
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       >>> str(1/7)
    
       >>> x = 10 * 3.25
       >>> y = 200 * 200
       >>> s = 'The value of x is ' + repr(x) + ', and y is ' + repr(y) + '...'
    
       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}')
    
    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}.')
    
    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'))
    
       >>> print('{1} and {0}'.format('spam', 'eggs'))
    
    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',
    
       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`` dictionary 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
    
    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 (modulo) can also be used for string formatting. Given ``'string'
    % values``, instances of ``%`` in ``string`` are replaced with zero or more
    elements of ``values``. This operation is commonly known as string
    interpolation. For example::
    
       >>> 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 positional arguments and one keyword argument:
    ``open(filename, mode, encoding=None)``
    
       >>> f = open('workfile', 'w', encoding="utf-8")
    
    
    .. XXX str(f) is <io.TextIOWrapper object at 0x82e8dc4>
    
    
       <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`).
    Because UTF-8 is the modern de-facto standard, ``encoding="utf-8"`` is
    recommended unless you know that you need to use a different encoding.
    Appending a ``'b'`` to the mode opens the file in :dfn:`binary mode`.
    Binary mode data is read and written as :class:`bytes` objects.
    You can not specify *encoding* when opening file in binary mode.
    
    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', encoding="utf-8") as f:
    
    
        >>> # We can check that the file has been automatically closed.
    
        >>> 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.
    
    .. warning::
       Calling ``f.write()`` without using the :keyword:`!with` keyword or calling
       ``f.close()`` **might** result in the arguments
       of ``f.write()`` not being completely written to the disk, even if the
       program exits successfully.
    
    ..
       See also https://bugs.python.org/issue17852
    
    
    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*
    characters (in text mode) or *size* bytes (in binary mode) 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')
    
    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)
    
    ``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, whence)``.  The position is computed
    
    from adding *offset* to a reference point; the reference point is selected by
    
    the *whence* argument.  A *whence* 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.  *whence* 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
    
       >>> f.read(1)
    
       >>> f.seek(-3, 2)  # Go to the 3rd byte before the end
    
       >>> f.read(1)
    
    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.
    
    Saving structured data with :mod:`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::
    
       >>> x = [1, 'simple', 'list']
       >>> json.dumps(x)
    
    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::
    
    To decode the object again, if ``f`` is a :term:`binary file` or
    :term:`text file` object which has been opened for reading::
    
    .. note::
       JSON files must be encoded in UTF-8. Use ``encoding="utf-8"`` when opening
       JSON file as a :term:`text file` for both of reading and writing.
    
    
    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.