: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.

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

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 since Python 3.0.

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

.. deprecated:: 3.0
 The keyword argument *buffering* was deprecated and is now ignored.

.. 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`.

Incremental (de)compression

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.

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

.. 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.

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

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