12.3. bz2 — Compression compatible with bzip2

New in version 2.3.

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:

12.3.1. (De)compression of files

Handling of compressed files is offered by the BZ2File class.

class bz2.BZ2File(filename[, mode[, buffering[, compresslevel]]])

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 in universal newlines mode. 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 2.7: Support for the with statement was added.


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


Return the next line from the file, as a 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 string at EOF.


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.


For backward compatibility. BZ2File objects now include the performance optimizations previously implemented in the xreadlines module.

Deprecated since version 2.3: This exists only for compatibility with the method by this name on file objects, which is deprecated. Use for line in file instead.

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.


Return the current file position, an integer (may be a long integer).


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


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

12.3.2. Sequential (de)compression

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

class bz2.BZ2Compressor([compresslevel])

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.


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.


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.


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])

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.


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