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# 13.3. `bz2` — Support for bzip2 compression

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.

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

## 13.3.1. (De)compression of files

`bz2.``open`(filename, mode='r', 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'`, `'a'`, or `'ab'` for binary mode, or `'rt'`, `'wt'`, 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.

class `bz2.``BZ2File`(filename, mode='r', buffering=None, 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, or `'a'` for appending. These can equivalently be given as `'rb'`, `'wb'`, 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.

  `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 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 `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.

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

  Changed in version 3.3: The `fileno()`, `readable()`, `seekable()`, `writable()`, `read1()` and `readinto()` methods were added.

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

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

## 13.3.2. 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 a number 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)

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

    Attempting to decompress data after the end of the current stream is reached raises an `EOFError`. If any data is found after the end of the stream, it is ignored and saved in the `unused_data` attribute.

  `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''`.

## 13.3.3. One-shot (de)compression

`bz2.``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 `BZ2Compressor` instead.

`bz2.``decompress`(data)

  Decompress data.

  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.

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