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:

(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).

Added 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).

Added in version 3.3.

fileno()

Return the file descriptor for the underlying file.

Added in version 3.3.

readable()

Return whether the file was opened for reading.

Added in version 3.3.

seekable()

Return whether the file supports seeking.

Added in version 3.3.

writable()

Return whether the file was opened for writing.

Added 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.

Added in version 3.3.

readinto(b)

Read bytes into b.

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

Added 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.

Added 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.

Added 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