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

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

:::: 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.
:::
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## 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\")
:::