13.3. 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.
For other archive formats, see the gzip, zipfile, and
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 the readahead
algorithm borrowed from file objects;
- Sequential (de)compression supported by BZ2Compressor and
- One-shot (de)compression supported by compress() and decompress()
- Thread safety uses individual locking mechanism.
13.3.1. (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
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
- 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
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
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.
13.3.2. Sequential (de)compression
Sequential compression and decompression is done using the classes
BZ2Compressor and BZ2Decompressor.
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.
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
13.3.3. One-shot (de)compression
One-shot compression and decompression is provided through the compress()
and decompress() functions.
- 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.