# `bz2` — Support for bzip2 compression

Source code: Lib/bz2.py

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

The `bz2` module contains:

- The `open()` function and `BZ2File` class for reading and writing compressed files.

- The `BZ2Compressor` and `BZ2Decompressor` classes for incremental (de)compression.

- The `compress()` and `decompress()` functions for one-shot (de)compression.

## (De)compression of files

bz2.open(filename, mode='rb', compresslevel=9, encoding=None, errors=None, newline=None)

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

  As with the constructor for `BZ2File`, the filename argument can be an actual filename (a `str` or `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 `BZ2File` constructor.

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

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

  New in version 3.3.

  Changed in version 3.4: The `'x'` (exclusive creation) mode was added.

  Changed in version 3.6: Accepts a path-like object.

class bz2.BZ2File(filename, mode='r', *, compresslevel=9)

  Open a bzip2-compressed file in binary mode.

  If filename is a `str` or `bytes` object, open the named file directly. Otherwise, filename should be a 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.

  `BZ2File` provides all of the members specified by the `io.BufferedIOBase`, except for `detach()` and `truncate()`. Iteration and the `with` statement are supported.

  `BZ2File` also provides the following methods:

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

    New in version 3.3.

  fileno()

    Return the file descriptor for the underlying file.

    New in version 3.3.

  readable()

    Return whether the file was opened for reading.

    New in version 3.3.

  seekable()

    Return whether the file supports seeking.

    New in version 3.3.

  writable()

    Return whether the file was opened for writing.

    New in version 3.3.

  read1(size=-1)

    Read up to size uncompressed bytes, while trying to avoid making multiple reads from the underlying stream. Reads up to a buffer’s worth of data if size is negative.

    Returns `b''` if the file is at EOF.

    New in version 3.3.

  readinto(b)

    Read bytes into b.

    Returns the number of bytes read (0 for EOF).

    New in version 3.3.

  Changed in version 3.1: Support for the `with` statement was added.

  Changed in version 3.3: Support was added for filename being a file object instead of an actual filename.

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

  Changed in version 3.4: The `'x'` (exclusive creation) mode was added.

  Changed in version 3.5: The `read()` method now accepts an argument of `None`.

  Changed in version 3.6: Accepts a path-like object.

  Changed in version 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.

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

## Incremental (de)compression

class bz2.BZ2Compressor(compresslevel=9)

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

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

  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 `flush()` method to finish the compression process.

  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 bz2.BZ2Decompressor

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

  Note

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

  decompress(data, max_length=-1)

    Decompress data (a bytes-like object), returning uncompressed data as bytes. Some of data may be buffered internally, for use in later calls to `decompress()`. The returned data should be concatenated with the output of any previous calls to `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 `needs_input` attribute will be set to `False`. In this case, the next call to `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 `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 `unused_data` attribute.

    Changed in version 3.5: Added the max_length parameter.

  eof

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

    New in version 3.3.

  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''`.

  needs_input

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

    New in version 3.5.

## One-shot (de)compression

bz2.compress(data, compresslevel=9)

  Compress data, a bytes-like object.

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

  For incremental compression, use a `BZ2Compressor` instead.

bz2.decompress(data)

  Decompress data, a bytes-like object.

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

  For incremental decompression, use a `BZ2Decompressor` instead.

  Changed in version 3.3: Support for multi-stream inputs was added.

## Examples of usage

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

Using `compress()` and `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 `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