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 a file
   descriptor of the file to be opened.  (If a file descriptor is given,
   for example, from :func:`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.
   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, 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.  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 but 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
   :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 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

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])

   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])

   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])

   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.

.. 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])

   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.

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])

   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).

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 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` 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])

   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.

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

Buffered Streams

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`.

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.

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` 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.

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`. The max_buffer_size (for the buffered writer) defaults to twice the buffer size.

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

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

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:: newlines

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

.. 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.

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 these data attributes 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.

An in-memory stream for text. It 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 this method in addition to those from :class:`TextIOWrapper` and its parents:

.. method:: getvalue()

   Return a ``str`` containing the entire contents of the buffer.

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