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# 12.3. `bz2` — Compression compatible with bzip2

This module provides a comprehensive interface for the bz2 compression library. It implements a complete file interface, one-shot (de)compression functions, and types for sequential (de)compression.

Here is a summary of the features offered by the bz2 module:

- `BZ2File` class implements a complete file interface, including `readline()`, `readlines()`, `writelines()`, `seek()`, etc;
- `BZ2File` class implements emulated `seek()` support;
- `BZ2File` class implements universal newline support;
- `BZ2File` class offers an optimized line iteration using a readahead algorithm;
- Sequential (de)compression supported by `BZ2Compressor` and `BZ2Decompressor` classes;
- One-shot (de)compression supported by `compress()` and `decompress()` functions;
- Thread safety uses individual locking mechanism.

## 12.3.1. (De)compression of files

Handling of compressed files is offered by the `BZ2File` class.

class `bz2.``BZ2File`(filename, mode='r', buffering=0, compresslevel=9)

  Open a bz2 file. Mode can be either `'r'` or `'w'`, for reading (default) or writing. When opened for writing, the file will be created if it doesn’t exist, and truncated otherwise. If buffering is given, `0` means unbuffered, and larger numbers specify the buffer size; the default is `0`. If compresslevel is given, it must be a number between `1` and `9`; the default is `9`. Add a `'U'` to mode to open the file for input with universal newline support. Any line ending in the input file will be seen as a `'\n'` in Python. Also, a file so opened gains the attribute `newlines`; the value for this attribute is one of `None` (no newline read yet), `'\r'`, `'\n'`, `'\r\n'` or a tuple containing all the newline types seen. Universal newlines are available only when reading. Instances support iteration in the same way as normal `file` instances.

  `BZ2File` supports the `with` statement.

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

  Note

  This class does not support input files containing multiple streams (such as those produced by the pbzip2 tool). When reading such an input file, only the first stream will be accessible. If you require support for multi-stream files, consider using the third-party `bz2file` module (available from PyPI). This module provides a backport of Python 3.3’s `BZ2File` class, which does support multi-stream files.

  `close`()

    Close the file. Sets data attribute `closed` to true. A closed file cannot be used for further I/O operations. `close()` may be called more than once without error.

  `read`([size])

    Read at most size uncompressed bytes, returned as a byte string. If the size argument is negative or omitted, read until EOF is reached.

  `readline`([size])

    Return the next line from the file, as a byte string, retaining newline. A non-negative size argument limits the maximum number of bytes to return (an incomplete line may be returned then). Return an empty byte string at EOF.

  `readlines`([size])

    Return a list of lines read. The optional size argument, if given, is an approximate bound on the total number of bytes in the lines returned.

  `seek`(offset[, whence])

    Move to new file position. Argument offset is a byte count. Optional argument whence defaults to `os.SEEK_SET` or `0` (offset from start of file; offset should be `>= 0`); other values are `os.SEEK_CUR` or `1` (move relative to current position; offset can be positive or negative), and `os.SEEK_END` or `2` (move relative to end of file; offset is usually negative, although many platforms allow seeking beyond the end of a file).

    Note that seeking of bz2 files is emulated, and depending on the parameters the operation may be extremely slow.

  `tell`()

    Return the current file position, an integer.

  `write`(data)

    Write the byte string data to file. Note that due to buffering, `close()` may be needed before the file on disk reflects the data written.

  `writelines`(sequence_of_byte_strings)

    Write the sequence of byte strings to the file. Note that newlines are not added. The sequence can be any iterable object producing byte strings. This is equivalent to calling write() for each byte string.

## 12.3.2. Sequential (de)compression

Sequential compression and decompression is done using the classes `BZ2Compressor` and `BZ2Decompressor`.

class `bz2.``BZ2Compressor`(compresslevel=9)

  Create a new compressor object. This object may be used to compress data sequentially. If you want to compress data in one shot, use the `compress()` function instead. The compresslevel parameter, if given, must be a number between `1` and `9`; the default is `9`.

  `compress`(data)

    Provide more data to the compressor object. It will return chunks of compressed data whenever possible. When you’ve finished providing data to compress, call the `flush()` method to finish the compression process, and return what is left in internal buffers.

  `flush`()

    Finish the compression process and return what is left in internal buffers. You must not use the compressor object after calling this method.

class `bz2.``BZ2Decompressor`

  Create a new decompressor object. This object may be used to decompress data sequentially. If you want to decompress data in one shot, use the `decompress()` function instead.

  `decompress`(data)

    Provide more data to the decompressor object. It will return chunks of decompressed data whenever possible. If you try to decompress data after the end of stream is found, `EOFError` will be raised. If any data was found after the end of stream, it’ll be ignored and saved in `unused_data` attribute.

## 12.3.3. One-shot (de)compression

One-shot compression and decompression is provided through the `compress()` and `decompress()` functions.

`bz2.``compress`(data, compresslevel=9)

  Compress data in one shot. If you want to compress data sequentially, use an instance of `BZ2Compressor` instead. The compresslevel parameter, if given, must be a number between `1` and `9`; the default is `9`.

`bz2.``decompress`(data)

  Decompress data in one shot. If you want to decompress data sequentially, use an instance of `BZ2Decompressor` instead.

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