cpython/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>
.. sectionauthor:: Benjamin Peterson

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
seperation 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 OS files.

: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 respectively.  :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 the encoding
and decoding of streams into text.  :class:`TextIOWrapper`, which extends it, is
a buffered text interface to a buffered raw stream (:class:`BufferedIOBase`).
Finally, :class:`StringIO` is a 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[, buffering[, encoding[, errors[, newline[, closefd=True]]]]]])

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

   *file* is either a string 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; unneeded
             for 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
   (appending ``'b'`` to the *mode* argument) return contents as ``bytes``
   objects without any decoding.  In text mode (the default, or when ``'t'`` is
   appended to 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, but any encoding supported by Python can be passed.  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 argument should not 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.  For all possible values, see
   :func:`codecs.register`.

   *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``, the underlying file descriptor will be kept open
   when the file is closed.  This does not work when a file name is given and
   must be ``True`` in that case.

   :func:`open` returns a file object whose type depends on the mode, and
   through which the standard file operations such as reading and writing are
   performed.  When :func:`open` is used to open a file in a text mode (``'w'``,
   ``'r'``, ``'wt'``, ``'rt'``, etc.), it returns a :class:`TextIOWrapper`.
   When used to open a file in a binary mode, the returned 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`.

   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 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 dummy 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`) needed.  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 also supports the :keyword:`with` statement.  In this example, *fp* is
   closed after the suite of the with statment is complete::

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

   :class:`IOBase` provides these methods:

   .. method:: close()

      Flush and close this stream.  This method has no effect if the file is
      already closed.

   .. 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()

      Tell if a stream is interactive (connected to a terminal/tty device).

   .. method:: readable()

      Tell if a stream can be read from.  If False, :meth:`read` will raise
      :exc:`IOError`.

   .. method:: readline([limit])

      Read and return a 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])

      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])

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

      * ``0`` -- start of stream (the default); *pos* should be zero or positive
      * ``1`` -- current stream position; *pos* may be negative
      * ``2`` -- end of stream; *pos* is usually negative

      Return the new absolute position.

   .. method:: seekable()

      Tell if a stream supports random IO access.  If ``False``, :meth:`seek`,
      :meth:`tell` and :meth:`truncate` will raise :exc:`IOError`.

   .. method:: tell()

      Return an integer indicating the current stream position.

   .. method:: truncate([pos])

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

   .. method:: writable()

      Tell if a 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.  The lines will not be altered; they
      must contain line separators.


.. class:: RawIOBase

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

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

   .. method:: read([n])

      Read and return all bytes from the stream until EOF, or if *n* is
      specified, up to *n* bytes.  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 bytes from the stream until EOF, using multiple calls
      to the stream.

   .. 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, *b*, to the underlying raw stream and return the
      number of bytes written (never less than ``len(b)``).


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

.. class:: FileIO(name[, mode])

   :class:`FileIO` represents an OS 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.

   :class:`FileIO` provides or overrides these methods in addition to those from
   :class:`RawIOBase` and :class:`IOBase`:

   .. attribute:: mode

      The mode as given in the constructor.

   .. attribute:: name

      The file name.

   .. method:: read([n])

      Read and return bytes at most *n* bytes.  Only one system call is made, so
      less data than requested may be returned.  In non-blocking mode, ``None``
      is returned when no data is available.

   .. method:: readall()

      Read and return as bytes all the data from the file.  As much as
      immediately available is returned in non-blocking mode.  If the EOF has
      been reached, ``b''`` is returned.

   .. method:: readinto(bytearray)

      This method should not be used on :class:`FileIO` objects.

   .. method:: write(b)

      Write the bytes *b* to the file, and return the number actually written.
      Only one system call is made, so not all of the data may be written.


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

.. 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:: read([n])

      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:: 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, *b*, to the underlying raw stream and return the
      number of bytes written (never less than ``len(b)``).

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


.. 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 the bytes value of the buffer.

   .. method:: read1()

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

   .. method:: truncate([pos])

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


.. class:: BufferedReader(raw[, 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 bytes from a buffer without advancing the position.  The argument
      indicates a desired minimal number of bytes; only one read on the raw
      stream is done to satisfy it.  More than the buffer's size is never
      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[, max_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:`DEAFULT_BUFFER_SIZE`.  If *max_buffer_size* is omitted, it defaults to
   twice the buffer size.

   :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` is be raised if the raw stream blocks.

   .. method:: write(b)

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


.. class:: BufferedRWPair(reader, writer[, buffer_size[, max_buffer_size]])

   A buffered writer and reader object together for a raw stream that can be
   written and read from.  It has and supports both :meth:`read`, :meth:`write`,
   and their variants.  This is useful for such applications such as 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`.  The *max_buffer_size* (for the buffered writer)
   defaults to twice the buffer size.

   :class:`BufferedRWPair` implements all of :class:`BufferedIOBase`\'s methods.


.. class:: BufferedRandom(raw[, buffer_size[, max_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`.  The *max_buffer_size* (for the buffered writer)
   defaults to twice the buffer size.

   :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 methods in addition to those
   from :class:`IOBase`:

   .. attribute:: encoding

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

   .. attribute:: newlines

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

   .. method:: read(n)

      Read and return at most *n* characters from the stream.  If *n* is
      negative or ``None``, read to EOF.

   .. method:: readline()

      Read until newline or EOF and return.  If the stream is already at EOF, an
      empty stream is returned.

   .. method:: write(s)

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


.. class:: TextIOWrapper(buffer[, encoding[, errors[, newline[, line_buffering]]]])

   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.  For all
   possible values see :func:`codecs.register`.

   *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 seperator, :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 these methods in addition to those of
   :class:`TextIOBase` and its parents:

   .. attribute:: errors

      The encoding and decoding error setting.

   .. attribute:: line_buffering

      Whether line buffering is enabled.
   

.. class:: StringIO([initial_value[, encoding[, errors[, newline]]]])

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

   Create a new StringIO stream with an inital value, encoding, error handling,
   and newline setting.  See :class:`TextIOWrapper`\'s constructor for more
   information.

   :class:`StringIO` provides these methods in addition to those from
   :class:`TextIOWrapper` and its parents:

   .. method:: getvalue()

      Return a str representation of the contents of the internal buffer.


.. class:: IncrementalNewlineDecoder

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