python_doc_md/Intermediate/library/bz2.md

!bz2{.interpreted-text role="mod"} --- Support for bzip2{.interpreted-text role="program"} compression

::: {.module synopsis="Interfaces for bzip2 compression and decompression."} bz2 :::

Source code: Lib/bz2.py{.interpreted-text role="source"}

---

This module provides a comprehensive interface for compressing and decompressing data using the bzip2 compression algorithm.

The !bz2{.interpreted-text role="mod"} module contains:

• The .open{.interpreted-text role="func"} function and BZ2File{.interpreted-text role="class"} class for reading and writing compressed files.
• The BZ2Compressor{.interpreted-text role="class"} and BZ2Decompressor{.interpreted-text role="class"} classes for incremental (de)compression.
• The compress{.interpreted-text role="func"} and decompress{.interpreted-text role="func"} functions for one-shot (de)compression.

This is an optional module{.interpreted-text role="term"}. If it is missing from your copy of CPython, look for documentation from your distributor (that is, whoever provided Python to you). If you are the distributor, see optional-module-requirements{.interpreted-text role="ref"}.

(De)compression of files

:::::: function 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{.interpreted-text role="term"}.

As with the constructor for BZ2File{.interpreted-text role="class"}, the filename argument can be an actual filename (a str{.interpreted-text role="class"} or bytes{.interpreted-text role="class"} 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{.interpreted-text role="class"} constructor.

For binary mode, this function is equivalent to the BZ2File{.interpreted-text role="class"} constructor: BZ2File(filename, mode, compresslevel=compresslevel). In this case, the encoding, errors and newline arguments must not be provided.

For text mode, a BZ2File{.interpreted-text role="class"} object is created, and wrapped in an io.TextIOWrapper{.interpreted-text role="class"} instance with the specified encoding, error handling behavior, and line ending(s).

::: versionadded 3.3 :::

::: versionchanged 3.4 The 'x' (exclusive creation) mode was added. :::

::: versionchanged 3.6 Accepts a path-like object{.interpreted-text role="term"}. ::: ::::::

:::::::::::::::::::::::::::::: {.BZ2File(filename, .mode='r', .*, .compresslevel=9)} Open a bzip2-compressed file in binary mode.

If filename is a str{.interpreted-text role="class"} or bytes{.interpreted-text role="class"} object, open the named file directly. Otherwise, filename should be a file object{.interpreted-text role="term"}, 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{.interpreted-text role="class"} provides all of the members specified by the io.BufferedIOBase{.interpreted-text role="class"}, except for ~io.BufferedIOBase.detach{.interpreted-text role="meth"} and ~io.IOBase.truncate{.interpreted-text role="meth"}. Iteration and the with{.interpreted-text role="keyword"} statement are supported.

BZ2File{.interpreted-text role="class"} also provides the following methods and attributes:

:::::: 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 ::: title Note :::

While calling peek{.interpreted-text role="meth"} does not change the file position of the BZ2File{.interpreted-text role="class"}, it may change the position of the underlying file object (e.g. if the BZ2File{.interpreted-text role="class"} was constructed by passing a file object for filename). ::::

::: versionadded 3.3 ::: ::::::

:::: method fileno()

Return the file descriptor for the underlying file.

::: versionadded 3.3 ::: ::::

:::: method readable()

Return whether the file was opened for reading.

::: versionadded 3.3 ::: ::::

:::: method seekable()

Return whether the file supports seeking.

::: versionadded 3.3 ::: ::::

:::: method writable()

Return whether the file was opened for writing.

::: versionadded 3.3 ::: ::::

:::: method 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.

::: versionadded 3.3 ::: ::::

:::: method readinto(b)

Read bytes into b.

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

::: versionadded 3.3 ::: ::::

:::: attribute mode

'rb' for reading and 'wb' for writing.

::: versionadded 3.13 ::: ::::

:::: attribute name

The bzip2 file name. Equivalent to the ~io.FileIO.name{.interpreted-text role="attr"} attribute of the underlying file object{.interpreted-text role="term"}.

::: versionadded 3.13 ::: ::::

::: versionchanged 3.1 Support for the with{.interpreted-text role="keyword"} statement was added. :::

::: versionchanged 3.3 Support was added for filename being a file object{.interpreted-text role="term"} instead of an actual filename.

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

::: versionchanged 3.4 The 'x' (exclusive creation) mode was added. :::

::: versionchanged 3.5 The ~io.BufferedIOBase.read{.interpreted-text role="meth"} method now accepts an argument of None. :::

::: versionchanged 3.6 Accepts a path-like object{.interpreted-text role="term"}. :::

::: versionchanged 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. :::

::: versionchanged 3.10 This class is thread unsafe in the face of multiple simultaneous readers or writers, just like its equivalent classes in gzip{.interpreted-text role="mod"} and lzma{.interpreted-text role="mod"} have always been. ::: ::::::::::::::::::::::::::::::

Incremental (de)compression

::::: BZ2Compressor(compresslevel=9) Create a new compressor object. This object may be used to compress data incrementally. For one-shot compression, use the compress{.interpreted-text role="func"} function instead.

compresslevel, if given, must be an integer between 1 and 9. The default is 9.

::: method 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{.interpreted-text role="meth"} method to finish the compression process. :::

::: method 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. ::: :::::

:::::::::::: BZ2Decompressor() Create a new decompressor object. This object may be used to decompress data incrementally. For one-shot compression, use the decompress{.interpreted-text role="func"} function instead.

:::: note ::: title Note :::

This class does not transparently handle inputs containing multiple compressed streams, unlike decompress{.interpreted-text role="func"} and BZ2File{.interpreted-text role="class"}. If you need to decompress a multi-stream input with BZ2Decompressor{.interpreted-text role="class"}, you must use a new decompressor for each stream. ::::

:::: method decompress(data, max_length=-1)

Decompress data (a bytes-like object{.interpreted-text role="term"}), returning uncompressed data as bytes. Some of data may be buffered internally, for use in later calls to decompress{.interpreted-text role="meth"}. The returned data should be concatenated with the output of any previous calls to decompress{.interpreted-text role="meth"}.

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{.interpreted-text role="attr"} attribute will be set to False. In this case, the next call to ~.decompress{.interpreted-text role="meth"} 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{.interpreted-text role="attr"} attribute will be set to True.

Attempting to decompress data after the end of stream is reached raises an EOFError{.interpreted-text role="exc"}. Any data found after the end of the stream is ignored and saved in the ~.unused_data{.interpreted-text role="attr"} attribute.

::: versionchanged 3.5 Added the max_length parameter. ::: ::::

:::: attribute eof

True if the end-of-stream marker has been reached.

::: versionadded 3.3 ::: ::::

::: attribute 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''. :::

:::: attribute needs_input

False if the .decompress{.interpreted-text role="meth"} method can provide more decompressed data before requiring new uncompressed input.

::: versionadded 3.5 ::: :::: ::::::::::::

One-shot (de)compression

::: function compress(data, compresslevel=9)

Compress data, a bytes-like object <bytes-like object>{.interpreted-text role="term"}.

compresslevel, if given, must be an integer between 1 and 9. The default is 9.

For incremental compression, use a BZ2Compressor{.interpreted-text role="class"} instead. :::

:::: function decompress(data)

Decompress data, a bytes-like object <bytes-like object>{.interpreted-text role="term"}.

If data is the concatenation of multiple compressed streams, decompress all of the streams.

For incremental decompression, use a BZ2Decompressor{.interpreted-text role="class"} instead.

::: versionchanged 3.3 Support for multi-stream inputs was added. ::: ::::

Examples of usage {#bz2-usage-examples}

Below are some examples of typical usage of the !bz2{.interpreted-text role="mod"} module.

Using compress{.interpreted-text role="func"} and decompress{.interpreted-text role="func"} 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{.interpreted-text role="class"} 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

::: testcleanup import os os.remove("myfile.bz2") :::