cpython/Doc/library/bz2.rst

:mod:`bz2` --- Compression compatible with :program:`bzip2`
===========================================================

.. module:: bz2
   :synopsis: Interface to compression and decompression routines
              compatible with bzip2.
.. moduleauthor:: Gustavo Niemeyer <niemeyer@conectiva.com>
.. sectionauthor:: Gustavo Niemeyer <niemeyer@conectiva.com>


This module provides a comprehensive interface for the bz2 compression library.
It implements a complete file interface, one-shot (de)compression functions, and
types for sequential (de)compression.

Here is a summary of the features offered by the bz2 module:

* :class:`BZ2File` class implements a complete file interface, including
  :meth:`~BZ2File.readline`, :meth:`~BZ2File.readlines`,
  :meth:`~BZ2File.writelines`, :meth:`~BZ2File.seek`, etc;

* :class:`BZ2File` class implements emulated :meth:`~BZ2File.seek` support;

* :class:`BZ2File` class implements universal newline support;

* :class:`BZ2File` class offers an optimized line iteration using a readahead
  algorithm;

* Sequential (de)compression supported by :class:`BZ2Compressor` and
  :class:`BZ2Decompressor` classes;

* One-shot (de)compression supported by :func:`compress` and :func:`decompress`
  functions;

* Thread safety uses individual locking mechanism.


(De)compression of files
------------------------

Handling of compressed files is offered by the :class:`BZ2File` class.


.. index::
   single: universal newlines; bz2.BZ2File class

.. class:: BZ2File(filename, mode='r', buffering=0, compresslevel=9)

   Open a bz2 file. Mode can be either ``'r'`` or ``'w'``, for reading (default)
   or writing. When opened for writing, the file will be created if it doesn't
   exist, and truncated otherwise. If *buffering* is given, ``0`` means
   unbuffered, and larger numbers specify the buffer size; the default is
   ``0``. If *compresslevel* is given, it must be a number between ``1`` and
   ``9``; the default is ``9``. Add a ``'U'`` to mode to open the file for input
   in :term:`universal newlines` mode.  Any line ending in the input file will be
   seen as a ``'\n'`` in Python.  Also, a file so opened gains the attribute
   :attr:`newlines`; the value for this attribute is one of ``None`` (no newline
   read yet), ``'\r'``, ``'\n'``, ``'\r\n'`` or a tuple containing all the
   newline types seen. Universal newlines are available only when
   reading. Instances support iteration in the same way as normal :class:`file`
   instances.

   :class:`BZ2File` supports the :keyword:`with` statement.

   .. versionchanged:: 3.1
      Support for the :keyword:`with` statement was added.


   .. note::

      This class does not support input files containing multiple streams (such
      as those produced by the :program:`pbzip2` tool). When reading such an
      input file, only the first stream will be accessible. If you require
      support for multi-stream files, consider using the third-party
      :mod:`bz2file` module (available from
      `PyPI <http://pypi.python.org/pypi/bz2file>`_). This module provides a
      backport of Python 3.3's :class:`BZ2File` class, which does support
      multi-stream files.


   .. method:: close()

      Close the file. Sets data attribute :attr:`closed` to true. A closed file
      cannot be used for further I/O operations. :meth:`close` may be called
      more than once without error.


   .. method:: read([size])

      Read at most *size* uncompressed bytes, returned as a byte string. If the
      *size* argument is negative or omitted, read until EOF is reached.


   .. method:: readline([size])

      Return the next line from the file, as a byte string, retaining newline.
      A non-negative *size* argument limits the maximum number of bytes to
      return (an incomplete line may be returned then). Return an empty byte
      string at EOF.


   .. method:: readlines([size])

      Return a list of lines read. The optional *size* argument, if given, is an
      approximate bound on the total number of bytes in the lines returned.


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

      Move to new file position. Argument *offset* is a byte count. Optional
      argument *whence* defaults to ``os.SEEK_SET`` or ``0`` (offset from start
      of file; offset should be ``>= 0``); other values are ``os.SEEK_CUR`` or
      ``1`` (move relative to current position; offset can be positive or
      negative), and ``os.SEEK_END`` or ``2`` (move relative to end of file;
      offset is usually negative, although many platforms allow seeking beyond
      the end of a file).

      Note that seeking of bz2 files is emulated, and depending on the
      parameters the operation may be extremely slow.


   .. method:: tell()

      Return the current file position, an integer.


   .. method:: write(data)

      Write the byte string *data* to file. Note that due to buffering,
      :meth:`close` may be needed before the file on disk reflects the data
      written.


   .. method:: writelines(sequence_of_byte_strings)

      Write the sequence of byte strings to the file. Note that newlines are not
      added. The sequence can be any iterable object producing byte strings.
      This is equivalent to calling write() for each byte string.


Sequential (de)compression
--------------------------

Sequential compression and decompression is done using the classes
:class:`BZ2Compressor` and :class:`BZ2Decompressor`.


.. class:: BZ2Compressor(compresslevel=9)

   Create a new compressor object. This object may be used to compress data
   sequentially. If you want to compress data in one shot, use the
   :func:`compress` function instead. The *compresslevel* parameter, if given,
   must be a number between ``1`` and ``9``; the default is ``9``.

   .. method:: compress(data)

      Provide more data to the compressor object. It will return chunks of
      compressed data whenever possible. When you've finished providing data to
      compress, call the :meth:`flush` method to finish the compression process,
      and return what is left in internal buffers.


   .. method:: flush()

      Finish the compression process and return what is left in internal
      buffers. You must not use the compressor object after calling this method.


.. class:: BZ2Decompressor()

   Create a new decompressor object. This object may be used to decompress data
   sequentially. If you want to decompress data in one shot, use the
   :func:`decompress` function instead.

   .. method:: decompress(data)

      Provide more data to the decompressor object. It will return chunks of
      decompressed data whenever possible. If you try to decompress data after
      the end of stream is found, :exc:`EOFError` will be raised. If any data
      was found after the end of stream, it'll be ignored and saved in
      :attr:`unused_data` attribute.


One-shot (de)compression
------------------------

One-shot compression and decompression is provided through the :func:`compress`
and :func:`decompress` functions.


.. function:: compress(data, compresslevel=9)

   Compress *data* in one shot. If you want to compress data sequentially, use
   an instance of :class:`BZ2Compressor` instead. The *compresslevel* parameter,
   if given, must be a number between ``1`` and ``9``; the default is ``9``.


.. function:: decompress(data)

   Decompress *data* in one shot. If you want to decompress data sequentially,
   use an instance of :class:`BZ2Decompressor` instead.