cpython/Doc/library/bz2.rst

:mod:`bz2` --- Support for :program:`bzip2` compression
=======================================================

.. module:: bz2
   :synopsis: Interfaces for bzip2 compression and decompression.

.. moduleauthor:: Gustavo Niemeyer <niemeyer@conectiva.com>
.. moduleauthor:: Nadeem Vawda <nadeem.vawda@gmail.com>
.. sectionauthor:: Gustavo Niemeyer <niemeyer@conectiva.com>
.. sectionauthor:: Nadeem Vawda <nadeem.vawda@gmail.com>

**Source code:** :source:`Lib/bz2.py`

--------------

This module provides a comprehensive interface for compressing and
decompressing data using the bzip2 compression algorithm.

The :mod:`bz2` module contains:

* The :func:`.open` function and :class:`BZ2File` class for reading and
  writing compressed files.
* The :class:`BZ2Compressor` and :class:`BZ2Decompressor` classes for
  incremental (de)compression.
* The :func:`compress` and :func:`decompress` functions for one-shot
  (de)compression.

All of the classes in this module may safely be accessed from multiple threads.


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

.. function:: open(filename, mode='r', compresslevel=9, encoding=None, errors=None, newline=None)

   Open a bzip2-compressed file in binary or text mode, returning a :term:`file
   object`.

   As with the constructor for :class:`BZ2File`, the *filename* argument can be
   an actual filename (a :class:`str` or :class:`bytes` object), or an existing
   file object to read from or write to.

   The *mode* argument can be any of ``'r'``, ``'rb'``, ``'w'``, ``'wb'``,
   ``'x'``, ``'xb'``, ``'a'`` or ``'ab'`` for binary mode, or ``'rt'``,
   ``'wt'``, ``'xt'``, or ``'at'`` for text mode. The default is ``'rb'``.

   The *compresslevel* argument is an integer from 1 to 9, as for the
   :class:`BZ2File` constructor.

   For binary mode, this function is equivalent to the :class:`BZ2File`
   constructor: ``BZ2File(filename, mode, compresslevel=compresslevel)``. In
   this case, the *encoding*, *errors* and *newline* arguments must not be
   provided.

   For text mode, a :class:`BZ2File` object is created, and wrapped in an
   :class:`io.TextIOWrapper` instance with the specified encoding, error
   handling behavior, and line ending(s).

   .. versionadded:: 3.3

   .. versionchanged:: 3.4
      The ``'x'`` (exclusive creation) mode was added.


.. class:: BZ2File(filename, mode='r', buffering=None, compresslevel=9)

   Open a bzip2-compressed file in binary mode.

   If *filename* is a :class:`str` or :class:`bytes` object, open the named file
   directly. Otherwise, *filename* should be a :term:`file object`, which will
   be used to read or write the compressed data.

   The *mode* argument can be either ``'r'`` for reading (default), ``'w'`` for
   overwriting, ``'x'`` for exclusive creation, or ``'a'`` for appending. These
   can equivalently be given as ``'rb'``, ``'wb'``, ``'xb'`` and ``'ab'``
   respectively.

   If *filename* is a file object (rather than an actual file name), a mode of
   ``'w'`` does not truncate the file, and is instead equivalent to ``'a'``.

   The *buffering* argument is ignored. Its use is deprecated.

   If *mode* is ``'w'`` or ``'a'``, *compresslevel* can be a number between
   ``1`` and ``9`` specifying the level of compression: ``1`` produces the
   least compression, and ``9`` (default) produces the most compression.

   If *mode* is ``'r'``, the input file may be the concatenation of multiple
   compressed streams.

   :class:`BZ2File` provides all of the members specified by the
   :class:`io.BufferedIOBase`, except for :meth:`detach` and :meth:`truncate`.
   Iteration and the :keyword:`with` statement are supported.

   :class:`BZ2File` also provides the following method:

   .. method:: peek([n])

      Return buffered data without advancing the file position. At least one
      byte of data will be returned (unless at EOF). The exact number of bytes
      returned is unspecified.

      .. note:: While calling :meth:`peek` does not change the file position of
         the :class:`BZ2File`, it may change the position of the underlying file
         object (e.g. if the :class:`BZ2File` was constructed by passing a file
         object for *filename*).

      .. versionadded:: 3.3

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

   .. versionchanged:: 3.3
      The :meth:`fileno`, :meth:`readable`, :meth:`seekable`, :meth:`writable`,
      :meth:`read1` and :meth:`readinto` methods were added.

   .. versionchanged:: 3.3
      Support was added for *filename* being a :term:`file object` instead of an
      actual filename.

   .. versionchanged:: 3.3
      The ``'a'`` (append) mode was added, along with support for reading
      multi-stream files.

   .. versionchanged:: 3.4
      The ``'x'`` (exclusive creation) mode was added.

   .. versionchanged:: 3.5
      The :meth:`~io.BufferedIOBase.read` method now accepts an argument of
      ``None``.


Incremental (de)compression
---------------------------

.. class:: BZ2Compressor(compresslevel=9)

   Create a new compressor object. This object may be used to compress data
   incrementally. For one-shot compression, use the :func:`compress` function
   instead.

   *compresslevel*, if given, must be a number between ``1`` and ``9``. The
   default is ``9``.

   .. method:: compress(data)

      Provide data to the compressor object. Returns a chunk of compressed data
      if possible, or an empty byte string otherwise.

      When you have finished providing data to the compressor, call the
      :meth:`flush` method to finish the compression process.


   .. method:: flush()

      Finish the compression process. Returns the compressed data left in
      internal buffers.

      The compressor object may not be used after this method has been called.


.. class:: BZ2Decompressor()

   Create a new decompressor object. This object may be used to decompress data
   incrementally. For one-shot compression, use the :func:`decompress` function
   instead.

   .. note::
      This class does not transparently handle inputs containing multiple
      compressed streams, unlike :func:`decompress` and :class:`BZ2File`. If
      you need to decompress a multi-stream input with :class:`BZ2Decompressor`,
      you must use a new decompressor for each stream.

   .. method:: decompress(data, max_length=-1)

      Decompress *data* (a :term:`bytes-like object`), returning
      uncompressed data as bytes. Some of *data* may be buffered
      internally, for use in later calls to :meth:`decompress`. The
      returned data should be concatenated with the output of any
      previous calls to :meth:`decompress`.

      If *max_length* is nonnegative, returns at most *max_length*
      bytes of decompressed data. If this limit is reached and further
      output can be produced, the :attr:`~.needs_input` attribute will
      be set to ``False``. In this case, the next call to
      :meth:`~.decompress` may provide *data* as ``b''`` to obtain
      more of the output.

      If all of the input data was decompressed and returned (either
      because this was less than *max_length* bytes, or because
      *max_length* was negative), the :attr:`~.needs_input` attribute
      will be set to ``True``.

      Attempting to decompress data after the end of stream is reached
      raises an `EOFError`.  Any data found after the end of the
      stream is ignored and saved in the :attr:`~.unused_data` attribute.

      .. versionchanged:: 3.5
         Added the *max_length* parameter.

   .. attribute:: eof

      ``True`` if the end-of-stream marker has been reached.

      .. versionadded:: 3.3


   .. attribute:: unused_data

      Data found after the end of the compressed stream.

      If this attribute is accessed before the end of the stream has been
      reached, its value will be ``b''``.

   .. attribute:: needs_input

      ``False`` if the :meth:`.decompress` method can provide more
      decompressed data before requiring new uncompressed input.

      .. versionadded:: 3.5


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

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

   Compress *data*.

   *compresslevel*, if given, must be a number between ``1`` and ``9``. The
   default is ``9``.

   For incremental compression, use a :class:`BZ2Compressor` instead.


.. function:: decompress(data)

   Decompress *data*.

   If *data* is the concatenation of multiple compressed streams, decompress
   all of the streams.

   For incremental decompression, use a :class:`BZ2Decompressor` instead.

   .. versionchanged:: 3.3
      Support for multi-stream inputs was added.