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>

.. versionadded:: 2.6

The :mod:`io` module provides the Python interfaces to stream handling.
Under Python 2.x, this is proposed as an alternative to the built-in
:class:`file` object, but in Python 3.x it is the default interface to
access files and streams.

.. note::

   Since this module has been designed primarily for Python 3.x, you have to
   be aware that all uses of "bytes" in this document refer to the
   :class:`str` type (of which :class:`bytes` is an alias), and all uses
   of "text" refer to the :class:`unicode` type.  Furthermore, those two
   types are not interchangeable in the :mod:`io` APIs.

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 raise 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 :class:`unicode` strings.  :class:`TextIOWrapper`, which extends
it, is a buffered text interface to a buffered raw stream
(:class:`BufferedIOBase`). Finally, :class:`~io.StringIO` is an in-memory
stream for unicode 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=-1, 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 giving the pathname (absolute or
   relative to 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 newlines 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 :class:`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
   :class:`unicode` 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.
   Pass 0 to switch buffering off (only allowed in binary mode), 1 to select
   line buffering (only usable in text mode), and an integer > 1 to indicate
   the size of a fixed-size chunk buffer.  When no *buffering* argument is
   given, the default buffering policy works as follows:

   * Binary files are buffered in fixed-size chunks; the size of the buffer
     is chosen using a heuristic trying to determine the underlying device's
     "block size" and falling back on :attr:`DEFAULT_BUFFER_SIZE`.
     On many systems, the buffer will typically be 4096 or 8192 bytes long.

   * "Interactive" text files (files for which :meth:`isatty` returns True)
     use line buffering.  Other text files use the policy described above
     for binary files.

   *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.

   .. index::
      single: universal newlines; open() (in module io)

   *newline* controls how :term:`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 newlines 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 an :class:`unicode` or :class:`bytes` string
   as a file for both reading and writing.  For :class:`unicode` strings
   :class:`~io.StringIO` can be used like a file opened in text mode,
   and for :class:`bytes` 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 an :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` (also known as :class:`str`).  Method arguments may
   also be :class:`bytearray` or :class:`memoryview` of arrays of bytes.
   In some cases, such as :meth:`~RawIOBase.readinto`, a writable object
   such as :class:`bytearray` is required.
   Text I/O classes work with :class:`unicode` data.

   .. versionchanged:: 2.7
      Implementations should support :class:`memoryview` arguments.

   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.
   Lines are defined slightly differently depending on whether the stream is
   a binary stream (yielding :class:`bytes`), or a text stream (yielding
   :class:`unicode` strings).  See :meth:`~IOBase.readline` below.

   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 io.open('spam.txt', 'w') as file:
          file.write(u'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 a :exc:`ValueError`.

      As a convenience, it is allowed to call this method more than once;
      only the first call, however, will have an effect.

   .. 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 *newline* 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*.

      Note that it's already possible to iterate on file objects using ``for
      line in file: ...`` without calling ``file.readlines()``.

   .. method:: seek(offset[, whence])

      Change the stream position to the given byte *offset*.  *offset* is
      interpreted relative to the position indicated by *whence*.  The default
      value for *whence* is :data:`SEEK_SET`.  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:: 2.7
         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)

      Resize the stream to the given *size* in bytes (or the current position
      if *size* is not specified).  The current stream position isn't changed.
      This resizing can extend or reduce the current file size.  In case of
      extension, the contents of the new file area depend on the platform
      (on most systems, additional bytes are zero-filled, on Windows they're
      undetermined).  The new file size is returned.

   .. 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.

   .. method:: __del__()

      Prepare for object destruction. :class:`IOBase` provides a default
      implementation of this method that calls the instance's
      :meth:`~IOBase.close` method.


.. class:: RawIOBase

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

   Raw binary I/O typically provides low-level access to an underlying OS
   device or API, and does not try to encapsulate it in high-level primitives
   (this is left to Buffered I/O and Text I/O, described later in this page).

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

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

      Read up to *n* bytes from the object and return them.  As a convenience,
      if *n* is unspecified or -1, :meth:`readall` is called.  Otherwise,
      only one system call is ever made.  Fewer than *n* bytes may be
      returned if the operating system call returns fewer than *n* bytes.

      If 0 bytes are returned, and *n* was not 0, this indicates end of file.
      If the object is in non-blocking mode and no bytes are available,
      ``None`` is returned.

   .. 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 *b*, and return the number
      of bytes read.  The object *b* should be a pre-allocated, writable
      array of bytes, either :class:`bytearray` or :class:`memoryview`.
      If the object is in non-blocking mode and no
      bytes are available, ``None`` is returned.

   .. method:: write(b)

      Write *b* to the underlying raw stream, and return the
      number of bytes written.  The object *b* should be an array
      of bytes, either :class:`bytes`, :class:`bytearray`, or
      :class:`memoryview`.  The return value can be less than
      ``len(b)``, depending on specifics of the underlying raw stream, and
      especially if it is in non-blocking mode.  ``None`` is returned if the
      raw stream is set not to block and no single byte could be readily
      written to it.  The caller may release or mutate *b* after
      this method returns, so the implementation should only access *b*
      during the method call.


.. class:: BufferedIOBase

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

   The main difference with :class:`RawIOBase` is that methods :meth:`read`,
   :meth:`readinto` and :meth:`write` will try (respectively) to read as much
   input as requested or to consume all given output, at the expense of
   making perhaps more than one system call.

   In addition, those methods can raise :exc:`BlockingIOError` if the
   underlying raw stream is in non-blocking mode and cannot take or give
   enough data; unlike their :class:`RawIOBase` counterparts, they will
   never return ``None``.

   Besides, the :meth:`read` method does not have a default
   implementation that defers to :meth:`readinto`.

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

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

   .. attribute:: raw

      The underlying raw stream (a :class:`RawIOBase` instance) that
      :class:`BufferedIOBase` deals with.  This is not part of the
      :class:`BufferedIOBase` API and may not exist on some implementations.

   .. 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:: 2.7

   .. 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 is in
      non blocking-mode, and has no data available 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.  This can be useful if you
      are implementing your own buffering on top of a :class:`BufferedIOBase`
      object.

   .. method:: readinto(b)

      Read up to len(b) bytes into *b*, and return the number of bytes read.
      The object *b* should be a pre-allocated, writable array of bytes,
      either :class:`bytearray` or :class:`memoryview`.

      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 is in
      non blocking-mode, and has no data available at the moment.

   .. method:: write(b)

      Write *b*, and return the number of bytes written
      (always equal to ``len(b)``, since if the write fails
      an :exc:`IOError` will be raised).  The object *b* should be
      an array of bytes, either :class:`bytes`, :class:`bytearray`,
      or :class:`memoryview`.  Depending on the actual
      implementation, these bytes may be readily written to the underlying
      stream, or held in a buffer for performance and latency reasons.

      When in non-blocking mode, a :exc:`BlockingIOError` is raised if the
      data needed to be written to the raw stream but it couldn't accept
      all the data without blocking.

      The caller may release or mutate *b* after this method returns,
      so the implementation should only access *b* during the method call.


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

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

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

   The *name* can be one of two things:

   * a string representing the path to the file which will be opened;
   * an integer representing the number of an existing OS-level file descriptor
     to which the resulting :class:`FileIO` object will give access.

   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.

   The :meth:`read` (when called with a positive argument), :meth:`readinto`
   and :meth:`write` methods on this class will only make one system call.

   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.


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

Buffered I/O streams provide a higher-level interface to an I/O device
than raw I/O does.

.. class:: BytesIO([initial_bytes])

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

   The optional argument *initial_bytes* is a :class:`bytes` object that
   contains initial data.

   :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`.


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

   A buffer providing higher-level access to a readable, sequential
   :class:`RawIOBase` object.  It inherits :class:`BufferedIOBase`.
   When reading data from this object, a larger amount of data may be
   requested from the underlying raw stream, and kept in an internal buffer.
   The buffered data can then be returned directly on subsequent reads.

   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 bytes from the stream without advancing the position.  At most one
      single read on the raw stream is done to satisfy the call. The number of
      bytes returned may be less or more than requested.

   .. 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 providing higher-level access to a writeable, sequential
   :class:`RawIOBase` object.  It inherits :class:`BufferedIOBase`.
   When writing to this object, data is normally held into an internal
   buffer.  The buffer will be written out to the underlying :class:`RawIOBase`
   object under various conditions, including:

   * when the buffer gets too small for all pending data;
   * when :meth:`flush()` is called;
   * when a :meth:`seek()` is requested (for :class:`BufferedRandom` objects);
   * when the :class:`BufferedWriter` object is closed or destroyed.

   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 *b*, and return the number of bytes written.
      The object *b* should be an array of bytes, either
      :class:`bytes`, :class:`bytearray`, or :class:`memoryview`.
      When in non-blocking mode, a :exc:`BlockingIOError` is raised
      if the buffer needs to be written out but the raw stream blocks.


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

   A buffered interface to random access streams.  It inherits
   :class:`BufferedReader` and :class:`BufferedWriter`, and further supports
   :meth:`seek` and :meth:`tell` functionality.

   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.


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

   A buffered I/O object combining two unidirectional :class:`RawIOBase`
   objects -- one readable, the other writeable -- into a single bidirectional
   endpoint.  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`.

   .. warning::

      :class:`BufferedRWPair` does not attempt to synchronize accesses to
      its underlying raw streams.  You should not pass it the same object
      as reader and writer; use :class:`BufferedRandom` instead.


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

.. class:: TextIOBase

   Base class for text streams.  This class provides a unicode character
   and line based interface to stream I/O.  There is no :meth:`readinto`
   method because Python's :class:`unicode` 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.  Depending on the implementation and the initial
      constructor flags, this may not be available.

   .. attribute:: buffer

      The underlying binary buffer (a :class:`BufferedIOBase` instance) that
      :class:`TextIOBase` deals with.  This is not part of the
      :class:`TextIOBase` API and may not exist on some implementations.

   .. method:: detach()

      Separate the underlying binary 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:`~io.StringIO`, may not
      have the concept of an underlying buffer and calling this method will
      raise :exc:`UnsupportedOperation`.

      .. versionadded:: 2.7

   .. method:: read(n)

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

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

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

      If *limit* is specified, at most *limit* characters will be read.

   .. method:: seek(offset[, whence])

      Change the stream position to the given *offset*.  Behaviour depends on
      the *whence* parameter.  The default value for *whence* is
      :data:`SEEK_SET`.

      * :data:`SEEK_SET` or ``0``: seek from the start of the stream
        (the default); *offset* must either be a number returned by
        :meth:`TextIOBase.tell`, or zero.  Any other *offset* value
        produces undefined behaviour.
      * :data:`SEEK_CUR` or ``1``: "seek" to the current position;
        *offset* must be zero, which is a no-operation (all other values
        are unsupported).
      * :data:`SEEK_END` or ``2``: seek to the end of the stream;
        *offset* must be zero (all other values are unsupported).

      Return the new absolute position as an opaque number.

      .. versionadded:: 2.7
         The ``SEEK_*`` constants.

   .. method:: tell()

      Return the current stream position as an opaque number.  The number
      does not usually represent a number of bytes in the underlying
      binary storage.

   .. method:: write(s)

      Write the :class:`unicode` 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` binary stream.
   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.

   .. index::
      single: universal newlines; io.TextIOWrapper class

   *newline* controls how line endings are handled.  It can be ``None``,
   ``''``, ``'\n'``, ``'\r'``, and ``'\r\n'``.  It works as follows:

   * On input, if *newline* is ``None``, :term:`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 newlines 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 *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=u'', newline=u'\\n')

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

   The initial value of the buffer can be set by providing *initial_value*.
   If newline translation is enabled, newlines will be encoded as if by
   :meth:`~TextIOBase.write`.  The stream is positioned at the start of
   the buffer.

   The *newline* argument works like that of :class:`TextIOWrapper`.
   The default is to consider only ``\n`` characters as ends of lines and
   to do no newline translation.  If *newline* is set to ``None``,
   newlines are written as ``\n`` on all platforms, but universal
   newline decoding is still performed when reading.

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

   .. method:: getvalue()

      Return a ``unicode`` containing the entire contents of the buffer at any
      time before the :class:`~io.StringIO` object's :meth:`close` method is
      called.  Newlines are decoded as if by :meth:`~TextIOBase.read`,
      although the stream position is not changed.

   Example usage::

      import io

      output = io.StringIO()
      output.write(u'First line.\n')
      output.write(u'Second line.\n')

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

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


.. index::
   single: universal newlines; io.IncrementalNewlineDecoder class

.. class:: IncrementalNewlineDecoder

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


Advanced topics
---------------

Here we will discuss several advanced topics pertaining to the concrete
I/O implementations described above.

Performance
^^^^^^^^^^^

Binary I/O
""""""""""

By reading and writing only large chunks of data even when the user asks
for a single byte, buffered I/O is designed to hide any inefficiency in
calling and executing the operating system's unbuffered I/O routines.  The
gain will vary very much depending on the OS and the kind of I/O which is
performed (for example, on some contemporary OSes such as Linux, unbuffered
disk I/O can be as fast as buffered I/O).  The bottom line, however, is
that buffered I/O will offer you predictable performance regardless of the
platform and the backing device.  Therefore, it is most always preferable to
use buffered I/O rather than unbuffered I/O.

Text I/O
""""""""

Text I/O over a binary storage (such as a file) is significantly slower than
binary I/O over the same storage, because it implies conversions from
unicode to binary data using a character codec.  This can become noticeable
if you handle huge amounts of text data (for example very large log files).
Also, :meth:`TextIOWrapper.tell` and :meth:`TextIOWrapper.seek` are both
quite slow due to the reconstruction algorithm used.

:class:`~io.StringIO`, however, is a native in-memory unicode container and will
exhibit similar speed to :class:`BytesIO`.

Multi-threading
^^^^^^^^^^^^^^^

:class:`FileIO` objects are thread-safe to the extent that the operating
system calls (such as ``read(2)`` under Unix) they are wrapping are thread-safe
too.

Binary buffered objects (instances of :class:`BufferedReader`,
:class:`BufferedWriter`, :class:`BufferedRandom` and :class:`BufferedRWPair`)
protect their internal structures using a lock; it is therefore safe to call
them from multiple threads at once.

:class:`TextIOWrapper` objects are not thread-safe.

Reentrancy
^^^^^^^^^^

Binary buffered objects (instances of :class:`BufferedReader`,
:class:`BufferedWriter`, :class:`BufferedRandom` and :class:`BufferedRWPair`)
are not reentrant.  While reentrant calls will not happen in normal situations,
they can arise if you are doing I/O in a :mod:`signal` handler.  If it is
attempted to enter a buffered object again while already being accessed
*from the same thread*, then a :exc:`RuntimeError` is raised.

The above implicitly extends to text files, since the :func:`open()`
function will wrap a buffered object inside a :class:`TextIOWrapper`.  This
includes standard streams and therefore affects the built-in function
:func:`print()` as well.