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.


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

.. function:: open(filename, mode='rb', 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.

   .. versionchanged:: 3.6
      Accepts a :term:`path-like object`.


.. class:: BZ2File(filename, mode='r', *, 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'``.

   If *mode* is ``'w'`` or ``'a'``, *compresslevel* can be an integer 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``.

   .. versionchanged:: 3.6
      Accepts a :term:`path-like object`.

   .. versionchanged:: 3.9
      The *buffering* parameter has been removed. It was ignored and deprecated
      since Python 3.0. Pass an open file object to control how the file is
      opened.

      The *compresslevel* parameter became keyword-only.

   .. versionchanged:: 3.10
      This class is thread unsafe in the face of multiple simultaneous
      readers or writers, just like its equivalent classes in :mod:`gzip` and
      :mod:`lzma` have always been.


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 an integer 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*, a :term:`bytes-like object <bytes-like object>`.

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

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


.. function:: decompress(data)

   Decompress *data*, a :term:`bytes-like object <bytes-like object>`.

   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.

.. _bz2-usage-examples:

Examples of usage
-----------------

Below are some examples of typical usage of the :mod:`bz2` module.

Using :func:`compress` and :func:`decompress` to demonstrate round-trip compression:

    >>> import bz2
    >>> data = b"""\
    ... Donec rhoncus quis sapien sit amet molestie. Fusce scelerisque vel augue
    ... nec ullamcorper. Nam rutrum pretium placerat. Aliquam vel tristique lorem,
    ... sit amet cursus ante. In interdum laoreet mi, sit amet ultrices purus
    ... pulvinar a. Nam gravida euismod magna, non varius justo tincidunt feugiat.
    ... Aliquam pharetra lacus non risus vehicula rutrum. Maecenas aliquam leo
    ... felis. Pellentesque semper nunc sit amet nibh ullamcorper, ac elementum
    ... dolor luctus. Curabitur lacinia mi ornare consectetur vestibulum."""
    >>> c = bz2.compress(data)
    >>> len(data) / len(c)  # Data compression ratio
    1.513595166163142
    >>> d = bz2.decompress(c)
    >>> data == d  # Check equality to original object after round-trip
    True

Using :class:`BZ2Compressor` for incremental compression:

    >>> import bz2
    >>> def gen_data(chunks=10, chunksize=1000):
    ...     """Yield incremental blocks of chunksize bytes."""
    ...     for _ in range(chunks):
    ...         yield b"z" * chunksize
    ...
    >>> comp = bz2.BZ2Compressor()
    >>> out = b""
    >>> for chunk in gen_data():
    ...     # Provide data to the compressor object
    ...     out = out + comp.compress(chunk)
    ...
    >>> # Finish the compression process.  Call this once you have
    >>> # finished providing data to the compressor.
    >>> out = out + comp.flush()

The example above uses a very "nonrandom" stream of data
(a stream of `b"z"` chunks).  Random data tends to compress poorly,
while ordered, repetitive data usually yields a high compression ratio.

Writing and reading a bzip2-compressed file in binary mode:

    >>> import bz2
    >>> data = b"""\
    ... Donec rhoncus quis sapien sit amet molestie. Fusce scelerisque vel augue
    ... nec ullamcorper. Nam rutrum pretium placerat. Aliquam vel tristique lorem,
    ... sit amet cursus ante. In interdum laoreet mi, sit amet ultrices purus
    ... pulvinar a. Nam gravida euismod magna, non varius justo tincidunt feugiat.
    ... Aliquam pharetra lacus non risus vehicula rutrum. Maecenas aliquam leo
    ... felis. Pellentesque semper nunc sit amet nibh ullamcorper, ac elementum
    ... dolor luctus. Curabitur lacinia mi ornare consectetur vestibulum."""
    >>> with bz2.open("myfile.bz2", "wb") as f:
    ...     # Write compressed data to file
    ...     unused = f.write(data)
    >>> with bz2.open("myfile.bz2", "rb") as f:
    ...     # Decompress data from file
    ...     content = f.read()
    >>> content == data  # Check equality to original object after round-trip
    True