
``io`` --- Core tools for working with streams
**********************************************

The ``io`` module provides the Python interfaces to stream handling.
The builtin ``open()`` function is defined in this module.

At the top of the I/O hierarchy is the abstract base 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 ``IOError`` if they do not
support a given operation.

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

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

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

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


Module Interface
================

io.DEFAULT_BUFFER_SIZE

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

io.open(file[, mode[, buffering[, encoding[, errors[, newline[, closefd=True]]]]]])

   Open *file* and return a stream.  If the file cannot be opened, an
   ``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
   a file descriptor of the file to be opened.  (If a file descriptor
   is given, for example, from ``os.fdopen()``, 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, but any encoding supported by Python can be
   used.  See the ``codecs`` module for the list of supported
   encodings.

   *errors* is an optional string that specifies how encoding and
   decoding errors are to be handled.  Pass ``'strict'`` to raise a
   ``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 ``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,
     ``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 but must be ``True`` (the default).

   The type of file object returned by the ``open()`` function depends
   on the mode.  When ``open()`` is used to open a file in a text mode
   (``'w'``, ``'r'``, ``'wt'``, ``'rt'``, etc.), it returns a
   ``TextIOWrapper``. When used to open a file in a binary mode, the
   returned class varies: in read binary mode, it returns a
   ``BufferedReader``; in write binary and append binary modes, it
   returns a ``BufferedWriter``, and in read/write mode, it returns a
   ``BufferedRandom``.

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

exception io.BlockingIOError

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

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

   characters_written

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

exception io.UnsupportedOperation

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


I/O Base Classes
================

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

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

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

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

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

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

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

   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 ``IOError``.
      The internal file descriptor isn't closed if *closefd* was
      False.

   closed

      True if the stream is closed.

   fileno()

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

   flush()

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

   isatty()

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

   readable()

      Return ``True`` if the stream can be read from.  If False,
      ``read()`` will raise ``IOError``.

   readline([limit])

      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 ``open()`` can be used to
      select the line terminator(s) recognized.

   readlines([hint])

      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*.

   seek(offset[, whence])

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

      * ``0`` -- start of the stream (the default); *offset* should be
        zero or positive

      * ``1`` -- current stream position; *offset* may be negative

      * ``2`` -- end of the stream; *offset* is usually negative

      Return the new absolute position.

   seekable()

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

   tell()

      Return the current stream position.

   truncate([size])

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

   writable()

      Return ``True`` if the stream supports writing.  If ``False``,
      ``write()`` and ``truncate()`` will raise ``IOError``.

   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 io.RawIOBase

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

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

   read([n])

      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.

   readall()

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

   readinto(b)

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

   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 ``IOError`` will be raised).


Raw File I/O
============

class io.FileIO(name[, mode])

   ``FileIO`` represents a file containing bytes data.  It implements
   the ``RawIOBase`` interface (and therefore the ``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 ``IOBase`` and
   ``RawIOBase``, ``FileIO`` provides the following data attributes
   and methods:

   mode

      The mode as given in the constructor.

   name

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

   read([n])

      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 ``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.)

   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.

   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.

   Note that the inherited ``readinto()`` method should not be used on
   ``FileIO`` objects.


Buffered Streams
================

class io.BufferedIOBase

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

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

   In addition, ``read()``, ``readinto()``, and ``write()`` may raise
   ``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 ``RawIOBase``
   implementation, but wrap one like ``BufferedWriter`` and
   ``BufferedReader``.

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

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

   readinto(b)

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

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

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

   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
      ``IOError`` will be raised).

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

class io.BytesIO([initial_bytes])

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

   The argument *initial_bytes* is an optional initial bytearray.

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

   getvalue()

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

   read1()

      In ``BytesIO``, this is the same as ``read()``.

   truncate([size])

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

class io.BufferedReader(raw[, buffer_size])

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

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

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

   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.

   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.

   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 io.BufferedWriter(raw[, buffer_size[, max_buffer_size]])

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

   The constructor creates a ``BufferedWriter`` for the given
   writeable *raw* stream.  If the *buffer_size* is not given, it
   defaults to ``DEAFULT_BUFFER_SIZE``.  If *max_buffer_size* is
   omitted, it defaults to twice the buffer size.

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

   flush()

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

   write(b)

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

class io.BufferedRWPair(reader, writer[, buffer_size[, max_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
   ``read()``, ``write()``, and their variants.  This is useful for
   sockets and two-way pipes. It inherits ``BufferedIOBase``.

   *reader* and *writer* are ``RawIOBase`` objects that are readable
   and writeable respectively.  If the *buffer_size* is omitted it
   defaults to ``DEFAULT_BUFFER_SIZE``.  The *max_buffer_size* (for
   the buffered writer) defaults to twice the buffer size.

   ``BufferedRWPair`` implements all of ``BufferedIOBase``'s methods.

class io.BufferedRandom(raw[, buffer_size[, max_buffer_size]])

   A buffered interface to random access streams.  It inherits
   ``BufferedReader`` and ``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 ``DEFAULT_BUFFER_SIZE``.  The
   *max_buffer_size* (for the buffered writer) defaults to twice the
   buffer size.

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


Text I/O
========

class io.TextIOBase

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

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

   encoding

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

   newlines

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

   read(n)

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

   readline()

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

   write(s)

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

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

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

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

   *errors* is an optional string that specifies how encoding and
   decoding errors are to be handled.  Pass ``'strict'`` to raise a
   ``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 ``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 seperator, ``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``, ``flush()`` is implied when a call
   to write contains a newline character.

   ``TextIOWrapper`` provides these data attributes in addition to
   those of ``TextIOBase`` and its parents:

   errors

      The encoding and decoding error setting.

   line_buffering

      Whether line buffering is enabled.

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

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

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

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

   getvalue()

      Return a ``str`` containing the entire contents of the buffer at
      any time before the ``StringIO`` object's ``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 io.IncrementalNewlineDecoder

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