Doc/library/io.rst

:mod:`io` --- Core tools for working with streams
=================================================

.. module:: io
   :synopsis: Core tools for working with streams.
.. moduleauthor:: Guido van Rossum <guido@python.org>
.. moduleauthor:: Mike Verdone <mike.verdone@gmail.com>
.. moduleauthor:: Mark Russell <mark.russell@zen.co.uk>
.. moduleauthor:: Antoine Pitrou <solipsis@pitrou.net>
.. moduleauthor:: Amaury Forgeot d'Arc <amauryfa@gmail.com>
.. moduleauthor:: Benjamin Peterson <benjamin@python.org>
.. sectionauthor:: Benjamin Peterson <benjamin@python.org>

The :mod:`io` module provides the Python interfaces to stream handling.  The
builtin :func:`open` function is defined in this module.

At the top of the I/O hierarchy is the abstract base class :class:`IOBase`.  It
defines the basic interface to a stream.  Note, however, that there is no
separation between reading and writing to streams; implementations are allowed
to throw an :exc:`IOError` if they do not support a given operation.

Extending :class:`IOBase` is :class:`RawIOBase` which deals simply with the
reading and writing of raw bytes to a stream.  :class:`FileIO` subclasses
:class:`RawIOBase` to provide an interface to files in the machine's
file system.

:class:`BufferedIOBase` deals with buffering on a raw byte stream
(:class:`RawIOBase`).  Its subclasses, :class:`BufferedWriter`,
:class:`BufferedReader`, and :class:`BufferedRWPair` buffer streams that are
readable, writable, and both readable and writable.
:class:`BufferedRandom` provides a buffered interface to random access
streams.  :class:`BytesIO` is a simple stream of in-memory bytes.

Another :class:`IOBase` subclass, :class:`TextIOBase`, deals with
streams whose bytes represent text, and handles encoding and decoding
from and to strings. :class:`TextIOWrapper`, which extends it, is a
buffered text interface to a buffered raw stream
(:class:`BufferedIOBase`). Finally, :class:`StringIO` is an in-memory
stream for text.

Argument names are not part of the specification, and only the arguments of
:func:`open` are intended to be used as keyword arguments.


Module Interface
----------------

.. data:: DEFAULT_BUFFER_SIZE

   An int containing the default buffer size used by the module's buffered I/O
   classes.  :func:`open` uses the file's blksize (as obtained by
   :func:`os.stat`) if possible.

.. function:: open(file, mode='r', buffering=None, encoding=None, errors=None, newline=None, closefd=True)

   Open *file* and return a corresponding stream.  If the file cannot be opened,
   an :exc:`IOError` is raised.

   *file* is either a string or bytes object giving the name (and the path if
   the file isn't in the current working directory) of the file to be opened or
   an integer file descriptor of the file to be wrapped.  (If a file descriptor
   is given, it is closed when the returned I/O object is closed, unless
   *closefd* is set to ``False``.)

   *mode* is an optional string that specifies the mode in which the file is
   opened.  It defaults to ``'r'`` which means open for reading in text mode.
   Other common values are ``'w'`` for writing (truncating the file if it
   already exists), and ``'a'`` for appending (which on *some* Unix systems,
   means that *all* writes append to the end of the file regardless of the
   current seek position).  In text mode, if *encoding* is not specified the
   encoding used is platform dependent. (For reading and writing raw bytes use
   binary mode and leave *encoding* unspecified.)  The available modes are:

   ========= ===============================================================
   Character Meaning
   --------- ---------------------------------------------------------------
   ``'r'``   open for reading (default)
   ``'w'``   open for writing, truncating the file first
   ``'a'``   open for writing, appending to the end of the file if it exists
   ``'b'``   binary mode
   ``'t'``   text mode (default)
   ``'+'``   open a disk file for updating (reading and writing)
   ``'U'``   universal newline mode (for backwards compatibility; should
             not be used in new code)
   ========= ===============================================================

   The default mode is ``'rt'`` (open for reading text).  For binary random
   access, the mode ``'w+b'`` opens and truncates the file to 0 bytes, while
   ``'r+b'`` opens the file without truncation.

   Python distinguishes between files opened in binary and text modes, even when
   the underlying operating system doesn't.  Files opened in binary mode
   (including ``'b'`` in the *mode* argument) return contents as ``bytes``
   objects without any decoding.  In text mode (the default, or when ``'t'`` is
   included in the *mode* argument), the contents of the file are returned as
   strings, the bytes having been first decoded using a platform-dependent
   encoding or using the specified *encoding* if given.

   *buffering* is an optional integer used to set the buffering policy.  By
   default full buffering is on.  Pass 0 to switch buffering off (only allowed
   in binary mode), 1 to set line buffering, and an integer > 1 for full
   buffering.

   *encoding* is the name of the encoding used to decode or encode the file.
   This should only be used in text mode.  The default encoding is platform
   dependent (whatever :func:`locale.getpreferredencoding` returns), but any
   encoding supported by Python can be used.  See the :mod:`codecs` module for
   the list of supported encodings.

   *errors* is an optional string that specifies how encoding and decoding
   errors are to be handled--this cannot be used in binary mode.  Pass
   ``'strict'`` to raise a :exc:`ValueError` exception if there is an encoding
   error (the default of ``None`` has the same effect), or pass ``'ignore'`` to
   ignore errors.  (Note that ignoring encoding errors can lead to data loss.)
   ``'replace'`` causes a replacement marker (such as ``'?'``) to be inserted
   where there is malformed data.  When writing, ``'xmlcharrefreplace'``
   (replace with the appropriate XML character reference) or
   ``'backslashreplace'`` (replace with backslashed escape sequences) can be
   used.  Any other error handling name that has been registered with
   :func:`codecs.register_error` is also valid.

   *newline* controls how universal newlines works (it only applies to text
   mode).  It can be ``None``, ``''``, ``'\n'``, ``'\r'``, and ``'\r\n'``.  It
   works as follows:

   * On input, if *newline* is ``None``, universal newlines mode is enabled.
     Lines in the input can end in ``'\n'``, ``'\r'``, or ``'\r\n'``, and these
     are translated into ``'\n'`` before being returned to the caller.  If it is
     ``''``, universal newline mode is enabled, but line endings are returned to
     the caller untranslated.  If it has any of the other legal values, input
     lines are only terminated by the given string, and the line ending is
     returned to the caller untranslated.

   * On output, if *newline* is ``None``, any ``'\n'`` characters written are
     translated to the system default line separator, :data:`os.linesep`.  If
     *newline* is ``''``, no translation takes place.  If *newline* is any of
     the other legal values, any ``'\n'`` characters written are translated to
     the given string.

   If *closefd* is ``False`` and a file descriptor rather than a filename was
   given, the underlying file descriptor will be kept open when the file is
   closed.  If a filename is given *closefd* has no effect and must be ``True``
   (the default).

   The type of file object returned by the :func:`open` function depends on the
   mode.  When :func:`open` is used to open a file in a text mode (``'w'``,
   ``'r'``, ``'wt'``, ``'rt'``, etc.), it returns a subclass of
   :class:`TextIOBase` (specifically :class:`TextIOWrapper`).  When used to open
   a file in a binary mode with buffering, the returned class is a subclass of
   :class:`BufferedIOBase`.  The exact class varies: in read binary mode, it
   returns a :class:`BufferedReader`; in write binary and append binary modes,
   it returns a :class:`BufferedWriter`, and in read/write mode, it returns a
   :class:`BufferedRandom`.  When buffering is disabled, the raw stream, a
   subclass of :class:`RawIOBase`, :class:`FileIO`, is returned.

   It is also possible to use a string or bytearray as a file for both reading
   and writing.  For strings :class:`StringIO` can be used like a file opened in
   a text mode, and for bytearrays a :class:`BytesIO` can be used like a
   file opened in a binary mode.


.. exception:: BlockingIOError

   Error raised when blocking would occur on a non-blocking stream.  It inherits
   :exc:`IOError`.

   In addition to those of :exc:`IOError`, :exc:`BlockingIOError` has one
   attribute:

   .. attribute:: characters_written

      An integer containing the number of characters written to the stream
      before it blocked.


.. exception:: UnsupportedOperation

   An exception inheriting :exc:`IOError` and :exc:`ValueError` that is raised
   when an unsupported operation is called on a stream.


I/O Base Classes
----------------

.. class:: IOBase

   The abstract base class for all I/O classes, acting on streams of bytes.
   There is no public constructor.

   This class provides empty abstract implementations for many methods
   that derived classes can override selectively; the default
   implementations represent a file that cannot be read, written or
   seeked.

   Even though :class:`IOBase` does not declare :meth:`read`, :meth:`readinto`,
   or :meth:`write` because their signatures will vary, implementations and
   clients should consider those methods part of the interface.  Also,
   implementations may raise a :exc:`IOError` when operations they do not
   support are called.

   The basic type used for binary data read from or written to a file is
   :class:`bytes`.  :class:`bytearray`\s are accepted too, and in some cases
   (such as :class:`readinto`) required.  Text I/O classes work with
   :class:`str` data.

   Note that calling any method (even inquiries) on a closed stream is
   undefined.  Implementations may raise :exc:`IOError` in this case.

   IOBase (and its subclasses) support the iterator protocol, meaning that an
   :class:`IOBase` object can be iterated over yielding the lines in a stream.

   IOBase is also a context manager and therefore supports the
   :keyword:`with` statement.  In this example, *file* is closed after the
   :keyword:`with` statement's suite is finished---even if an exception occurs::

      with open('spam.txt', 'w') as file:
          file.write('Spam and eggs!')

   :class:`IOBase` provides these data attributes and methods:

   .. method:: close()

      Flush and close this stream. This method has no effect if the file is
      already closed. Once the file is closed, any operation on the file
      (e.g. reading or writing) will raise an :exc:`IOError`. The internal
      file descriptor isn't closed if *closefd* was False.

   .. attribute:: closed

      True if the stream is closed.

   .. method:: fileno()

      Return the underlying file descriptor (an integer) of the stream if it
      exists.  An :exc:`IOError` is raised if the IO object does not use a file
      descriptor.

   .. method:: flush()

      Flush the write buffers of the stream if applicable.  This does nothing
      for read-only and non-blocking streams.

   .. method:: isatty()

      Return ``True`` if the stream is interactive (i.e., connected to
      a terminal/tty device).

   .. method:: readable()

      Return ``True`` if the stream can be read from.  If False, :meth:`read`
      will raise :exc:`IOError`.

   .. method:: readline(limit=-1)

      Read and return one line from the stream.  If *limit* is specified, at
      most *limit* bytes will be read.

      The line terminator is always ``b'\n'`` for binary files; for text files,
      the *newlines* argument to :func:`open` can be used to select the line
      terminator(s) recognized.

   .. method:: readlines(hint=-1)

      Read and return a list of lines from the stream.  *hint* can be specified
      to control the number of lines read: no more lines will be read if the
      total size (in bytes/characters) of all lines so far exceeds *hint*.

   .. method:: seek(offset, whence=SEEK_SET)

      Change the stream position to the given byte *offset*.  *offset* is
      interpreted relative to the position indicated by *whence*.  Values for
      *whence* are:

      * :data:`SEEK_SET` or ``0`` -- start of the stream (the default);
        *offset* should be zero or positive
      * :data:`SEEK_CUR` or ``1`` -- current stream position; *offset* may
        be negative
      * :data:`SEEK_END` or ``2`` -- end of the stream; *offset* is usually
        negative

      Return the new absolute position.

      .. versionadded:: 3.1
         The ``SEEK_*`` constants

   .. method:: seekable()

      Return ``True`` if the stream supports random access.  If ``False``,
      :meth:`seek`, :meth:`tell` and :meth:`truncate` will raise :exc:`IOError`.

   .. method:: tell()

      Return the current stream position.

   .. method:: truncate(size=None)

      Truncate the file to at most *size* bytes.  *size* defaults to the current
      file position, as returned by :meth:`tell`.

   .. method:: writable()

      Return ``True`` if the stream supports writing.  If ``False``,
      :meth:`write` and :meth:`truncate` will raise :exc:`IOError`.

   .. method:: writelines(lines)

      Write a list of lines to the stream.  Line separators are not added, so it
      is usual for each of the lines provided to have a line separator at the
      end.


.. class:: RawIOBase

   Base class for raw binary I/O.  It inherits :class:`IOBase`.  There is no
   public constructor.

   In addition to the attributes and methods from :class:`IOBase`,
   RawIOBase provides the following methods:

   .. method:: read(n=-1)

      Read and return all the bytes from the stream until EOF, or if *n* is
      specified, up to *n* bytes.  Only one system call is ever made.  An empty
      bytes object is returned on EOF; ``None`` is returned if the object is set
      not to block and has no data to read.

   .. method:: readall()

      Read and return all the bytes from the stream until EOF, using multiple
      calls to the stream if necessary.

   .. method:: readinto(b)

      Read up to len(b) bytes into bytearray *b* and return the number of bytes
      read.

   .. method:: write(b)

      Write the given bytes or bytearray object, *b*, to the underlying raw
      stream and return the number of bytes written (This is never less than
      ``len(b)``, since if the write fails, an :exc:`IOError` will be raised).


.. class:: BufferedIOBase

   Base class for streams that support buffering.  It inherits :class:`IOBase`.
   There is no public constructor.

   The main difference with :class:`RawIOBase` is that the :meth:`read` method
   supports omitting the *size* argument, and does not have a default
   implementation that defers to :meth:`readinto`.

   In addition, :meth:`read`, :meth:`readinto`, and :meth:`write` may raise
   :exc:`BlockingIOError` if the underlying raw stream is in non-blocking mode
   and not ready; unlike their raw counterparts, they will never return
   ``None``.

   A typical implementation should not inherit from a :class:`RawIOBase`
   implementation, but wrap one like :class:`BufferedWriter` and
   :class:`BufferedReader`.

   :class:`BufferedIOBase` provides or overrides these methods in addition to
   those from :class:`IOBase`:

   .. method:: detach()

      Separate the underlying raw stream from the buffer and return it.

      After the raw stream has been detached, the buffer is in an unusable
      state.

      Some buffers, like :class:`BytesIO`, do not have the concept of a single
      raw stream to return from this method.  They raise
      :exc:`UnsupportedOperation`.

      .. versionadded:: 3.1

   .. method:: read(n=-1)

      Read and return up to *n* bytes.  If the argument is omitted, ``None``, or
      negative, data is read and returned until EOF is reached.  An empty bytes
      object is returned if the stream is already at EOF.

      If the argument is positive, and the underlying raw stream is not
      interactive, multiple raw reads may be issued to satisfy the byte count
      (unless EOF is reached first).  But for interactive raw streams, at most
      one raw read will be issued, and a short result does not imply that EOF is
      imminent.

      A :exc:`BlockingIOError` is raised if the underlying raw stream has no
      data at the moment.

   .. method:: read1(n=-1)

      Read and return up to *n* bytes, with at most one call to the underlying
      raw stream's :meth:`~RawIOBase.read` method.

   .. method:: readinto(b)

      Read up to len(b) bytes into bytearray *b* and return the number of bytes
      read.

      Like :meth:`read`, multiple reads may be issued to the underlying raw
      stream, unless the latter is 'interactive.'

      A :exc:`BlockingIOError` is raised if the underlying raw stream has no
      data at the moment.

   .. method:: write(b)

      Write the given bytes or bytearray object, *b*, to the underlying raw
      stream and return the number of bytes written (never less than ``len(b)``,
      since if the write fails an :exc:`IOError` will be raised).

      A :exc:`BlockingIOError` is raised if the buffer is full, and the
      underlying raw stream cannot accept more data at the moment.


Raw File I/O
------------

.. class:: FileIO(name, mode='r', closefd=True)

   :class:`FileIO` represents a file containing bytes data.  It implements
   the :class:`RawIOBase` interface (and therefore the :class:`IOBase`
   interface, too).

   The *mode* can be ``'r'``, ``'w'`` or ``'a'`` for reading (default), writing,
   or appending.  The file will be created if it doesn't exist when opened for
   writing or appending; it will be truncated when opened for writing.  Add a
   ``'+'`` to the mode to allow simultaneous reading and writing.

   In addition to the attributes and methods from :class:`IOBase` and
   :class:`RawIOBase`, :class:`FileIO` provides the following data
   attributes and methods:

   .. attribute:: mode

      The mode as given in the constructor.

   .. attribute:: name

      The file name.  This is the file descriptor of the file when no name is
      given in the constructor.

   .. method:: read(n=-1)

      Read and return at most *n* bytes.  Only one system call is made, so it is
      possible that less data than was requested is returned.  Use :func:`len`
      on the returned bytes object to see how many bytes were actually returned.
      (In non-blocking mode, ``None`` is returned when no data is available.)

   .. method:: readall()

      Read and return the entire file's contents in a single bytes object.  As
      much as immediately available is returned in non-blocking mode.  If the
      EOF has been reached, ``b''`` is returned.

   .. method:: write(b)

      Write the bytes or bytearray object, *b*, to the file, and return
      the number actually written. Only one system call is made, so it
      is possible that only some of the data is written.


Buffered Streams
----------------

.. class:: BytesIO([initial_bytes])

   A stream implementation using an in-memory bytes buffer.  It inherits
   :class:`BufferedIOBase`.

   The argument *initial_bytes* is an optional initial bytearray.

   :class:`BytesIO` provides or overrides these methods in addition to those
   from :class:`BufferedIOBase` and :class:`IOBase`:

   .. method:: getvalue()

      Return ``bytes`` containing the entire contents of the buffer.

   .. method:: read1()

      In :class:`BytesIO`, this is the same as :meth:`read`.

   .. method:: truncate([size])

      Truncate the buffer to at most *size* bytes.  *size* defaults to the
      current stream position, as returned by :meth:`tell`.


.. class:: BufferedReader(raw, buffer_size=DEFAULT_BUFFER_SIZE)

   A buffer for a readable, sequential :class:`RawIOBase` object.  It inherits
   :class:`BufferedIOBase`.

   The constructor creates a :class:`BufferedReader` for the given readable
   *raw* stream and *buffer_size*.  If *buffer_size* is omitted,
   :data:`DEFAULT_BUFFER_SIZE` is used.

   :class:`BufferedReader` provides or overrides these methods in addition to
   those from :class:`BufferedIOBase` and :class:`IOBase`:

   .. method:: peek([n])

      Return 1 (or *n* if specified) bytes from a buffer without advancing the
      position.  Only a single read on the raw stream is done to satisfy the
      call. The number of bytes returned may be less than requested since at
      most all the buffer's bytes from the current position to the end are
      returned.

   .. method:: read([n])

      Read and return *n* bytes, or if *n* is not given or negative, until EOF
      or if the read call would block in non-blocking mode.

   .. method:: read1(n)

      Read and return up to *n* bytes with only one call on the raw stream.  If
      at least one byte is buffered, only buffered bytes are returned.
      Otherwise, one raw stream read call is made.


.. class:: BufferedWriter(raw, buffer_size=DEFAULT_BUFFER_SIZE)

   A buffer for a writeable sequential RawIO object.  It inherits
   :class:`BufferedIOBase`.

   The constructor creates a :class:`BufferedWriter` for the given writeable
   *raw* stream.  If the *buffer_size* is not given, it defaults to
   :data:`DEFAULT_BUFFER_SIZE`.

   A third argument, *max_buffer_size*, is supported, but unused and deprecated.

   :class:`BufferedWriter` provides or overrides these methods in addition to
   those from :class:`BufferedIOBase` and :class:`IOBase`:

   .. method:: flush()

      Force bytes held in the buffer into the raw stream.  A
      :exc:`BlockingIOError` should be raised if the raw stream blocks.

   .. method:: write(b)

      Write the bytes or bytearray object, *b*, onto the raw stream and return
      the number of bytes written.  A :exc:`BlockingIOError` is raised when the
      raw stream blocks.


.. class:: BufferedRWPair(reader, writer, buffer_size, max_buffer_size=DEFAULT_BUFFER_SIZE)

   A combined buffered writer and reader object for a raw stream that can be
   written to and read from.  It has and supports both :meth:`read`, :meth:`write`,
   and their variants.  This is useful for sockets and two-way pipes.
   It inherits :class:`BufferedIOBase`.

   *reader* and *writer* are :class:`RawIOBase` objects that are readable and
   writeable respectively.  If the *buffer_size* is omitted it defaults to
   :data:`DEFAULT_BUFFER_SIZE`.

   A fourth argument, *max_buffer_size*, is supported, but unused and
   deprecated.

   :class:`BufferedRWPair` implements all of :class:`BufferedIOBase`\'s methods
   except for :meth:`~BufferedIOBase.detach`, which raises
   :exc:`UnsupportedOperation`.


.. class:: BufferedRandom(raw, buffer_size=DEFAULT_BUFFER_SIZE)

   A buffered interface to random access streams.  It inherits
   :class:`BufferedReader` and :class:`BufferedWriter`.

   The constructor creates a reader and writer for a seekable raw stream, given
   in the first argument.  If the *buffer_size* is omitted it defaults to
   :data:`DEFAULT_BUFFER_SIZE`.

   A third argument, *max_buffer_size*, is supported, but unused and deprecated.

   :class:`BufferedRandom` is capable of anything :class:`BufferedReader` or
   :class:`BufferedWriter` can do.


Text I/O
--------

.. class:: TextIOBase

   Base class for text streams.  This class provides a character and line based
   interface to stream I/O.  There is no :meth:`readinto` method because
   Python's character strings are immutable.  It inherits :class:`IOBase`.
   There is no public constructor.

   :class:`TextIOBase` provides or overrides these data attributes and
   methods in addition to those from :class:`IOBase`:

   .. attribute:: encoding

      The name of the encoding used to decode the stream's bytes into
      strings, and to encode strings into bytes.

   .. attribute:: errors

      The error setting of the decoder or encoder.

   .. attribute:: newlines

      A string, a tuple of strings, or ``None``, indicating the newlines
      translated so far.

   .. method:: detach()

      Separate the underlying buffer from the :class:`TextIOBase` and return it.

      After the underlying buffer has been detached, the :class:`TextIOBase` is
      in an unusable state.

      Some :class:`TextIOBase` implementations, like :class:`StringIO`, may not
      have the concept of an underlying buffer and calling this method will
      raise :exc:`UnsupportedOperation`.

      .. versionadded:: 3.1

   .. method:: read(n)

      Read and return at most *n* characters from the stream as a single
      :class:`str`.  If *n* is negative or ``None``, reads to EOF.

   .. method:: readline()

      Read until newline or EOF and return a single ``str``.  If the stream is
      already at EOF, an empty string is returned.

   .. method:: write(s)

      Write the string *s* to the stream and return the number of characters
      written.


.. class:: TextIOWrapper(buffer, encoding=None, errors=None, newline=None, line_buffering=False)

   A buffered text stream over a :class:`BufferedIOBase` raw stream, *buffer*.
   It inherits :class:`TextIOBase`.

   *encoding* gives the name of the encoding that the stream will be decoded or
   encoded with.  It defaults to :func:`locale.getpreferredencoding`.

   *errors* is an optional string that specifies how encoding and decoding
   errors are to be handled.  Pass ``'strict'`` to raise a :exc:`ValueError`
   exception if there is an encoding error (the default of ``None`` has the same
   effect), or pass ``'ignore'`` to ignore errors.  (Note that ignoring encoding
   errors can lead to data loss.)  ``'replace'`` causes a replacement marker
   (such as ``'?'``) to be inserted where there is malformed data.  When
   writing, ``'xmlcharrefreplace'`` (replace with the appropriate XML character
   reference) or ``'backslashreplace'`` (replace with backslashed escape
   sequences) can be used.  Any other error handling name that has been
   registered with :func:`codecs.register_error` is also valid.

   *newline* can be ``None``, ``''``, ``'\n'``, ``'\r'``, or ``'\r\n'``.  It
   controls the handling of line endings.  If it is ``None``, universal newlines
   is enabled.  With this enabled, on input, the lines endings ``'\n'``,
   ``'\r'``, or ``'\r\n'`` are translated to ``'\n'`` before being returned to
   the caller.  Conversely, on output, ``'\n'`` is translated to the system
   default line separator, :data:`os.linesep`.  If *newline* is any other of its
   legal values, that newline becomes the newline when the file is read and it
   is returned untranslated.  On output, ``'\n'`` is converted to the *newline*.

   If *line_buffering* is ``True``, :meth:`flush` is implied when a call to
   write contains a newline character.

   :class:`TextIOWrapper` provides one attribute in addition to those of
   :class:`TextIOBase` and its parents:

   .. attribute:: line_buffering

      Whether line buffering is enabled.


.. class:: StringIO(initial_value='', newline=None)

   An in-memory stream for text.  It inherits :class:`TextIOWrapper`.

   The initial value of the buffer (an empty string by default) can be set by
   providing *initial_value*.  The *newline* argument works like that of
   :class:`TextIOWrapper`.  The default is to do no newline translation.

   :class:`StringIO` provides this method in addition to those from
   :class:`TextIOWrapper` and its parents:

   .. method:: getvalue()

      Return a ``str`` containing the entire contents of the buffer at any
      time before the :class:`StringIO` object's :meth:`close` method is
      called.

   Example usage::

      import io

      output = io.StringIO()
      output.write('First line.\n')
      print('Second line.', file=output)

      # Retrieve file contents -- this will be
      # 'First line.\nSecond line.\n'
      contents = output.getvalue()

      # Close object and discard memory buffer --
      # .getvalue() will now raise an exception.
      output.close()

.. class:: IncrementalNewlineDecoder

   A helper codec that decodes newlines for universal newlines mode.  It
   inherits :class:`codecs.IncrementalDecoder`.