kbe/src/lib/python/Doc/library/lzma.rst
lzma --- Compression using the LZMA algorithm.. module:: lzma :synopsis: A Python wrapper for the liblzma compression library.
.. moduleauthor:: Nadeem Vawda [email protected] .. sectionauthor:: Nadeem Vawda [email protected]
.. versionadded:: 3.3
Source code: :source:Lib/lzma.py
This module provides classes and convenience functions for compressing and
decompressing data using the LZMA compression algorithm. Also included is a file
interface supporting the .xz and legacy .lzma file formats used by the
:program:xz utility, as well as raw compressed streams.
The interface provided by this module is very similar to that of the :mod:bz2
module. However, note that :class:LZMAFile is not thread-safe, unlike
:class:bz2.BZ2File, so if you need to use a single :class:LZMAFile instance
from multiple threads, it is necessary to protect it with a lock.
.. exception:: LZMAError
This exception is raised when an error occurs during compression or decompression, or while initializing the compressor/decompressor state.
.. function:: open(filename, mode="rb", *, format=None, check=-1, preset=None, filters=None, encoding=None, errors=None, newline=None)
Open an LZMA-compressed file in binary or text mode, returning a :term:file object.
The filename argument can be either an actual file name (given as a
:class:str, :class:bytes or :term:path-like <path-like object> object), in
which case the named file is opened, or it can be 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".
When opening a file for reading, the format and filters arguments have
the same meanings as for :class:LZMADecompressor. In this case, the check
and preset arguments should not be used.
When opening a file for writing, the format, check, preset and
filters arguments have the same meanings as for :class:LZMACompressor.
For binary mode, this function is equivalent to the :class:LZMAFile
constructor: LZMAFile(filename, mode, ...). In this case, the encoding,
errors and newline arguments must not be provided.
For text mode, a :class:LZMAFile object is created, and wrapped in an
:class:io.TextIOWrapper instance with the specified encoding, error
handling behavior, and line ending(s).
.. versionchanged:: 3.4
Added support for the "x", "xb" and "xt" modes.
.. versionchanged:: 3.6
Accepts a :term:path-like object.
.. class:: LZMAFile(filename=None, mode="r", *, format=None, check=-1, preset=None, filters=None)
Open an LZMA-compressed file in binary mode.
An :class:LZMAFile can wrap an already-open :term:file object, or operate
directly on a named file. The filename argument specifies either the file
object to wrap, or the name of the file to open (as a :class:str,
:class:bytes or :term:path-like <path-like object> object). When wrapping an
existing file object, the wrapped file will not be closed when the
:class:LZMAFile is closed.
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".
When opening a file for reading, the input file may be the concatenation of multiple separate compressed streams. These are transparently decoded as a single logical stream.
When opening a file for reading, the format and filters arguments have
the same meanings as for :class:LZMADecompressor. In this case, the check
and preset arguments should not be used.
When opening a file for writing, the format, check, preset and
filters arguments have the same meanings as for :class:LZMACompressor.
:class:LZMAFile supports all the members specified by
:class:io.BufferedIOBase, except for :meth:detach and :meth:truncate.
Iteration and the :keyword:with statement are supported.
The following method is also provided:
.. method:: peek(size=-1)
Return buffered data without advancing the file position. At least one
byte of data will be returned, unless EOF has been reached. The exact
number of bytes returned is unspecified (the *size* argument is ignored).
.. note:: While calling :meth:`peek` does not change the file position of
the :class:`LZMAFile`, it may change the position of the underlying
file object (e.g. if the :class:`LZMAFile` was constructed by passing a
file object for *filename*).
.. versionchanged:: 3.4
Added support for the "x" and "xb" modes.
.. versionchanged:: 3.5
The :meth:~io.BufferedIOBase.read method now accepts an argument of
None.
.. versionchanged:: 3.6
Accepts a :term:path-like object.
.. class:: LZMACompressor(format=FORMAT_XZ, check=-1, preset=None, filters=None)
Create a compressor object, which can be used to compress data incrementally.
For a more convenient way of compressing a single chunk of data, see
:func:compress.
The format argument specifies what container format should be used. Possible values are:
:const:FORMAT_XZ: The .xz container format.
This is the default format.
:const:FORMAT_ALONE: The legacy .lzma container format.
This format is more limited than .xz -- it does not support integrity
checks or multiple filters.
:const:FORMAT_RAW: A raw data stream, not using any container format.
This format specifier does not support integrity checks, and requires that
you always specify a custom filter chain (for both compression and
decompression). Additionally, data compressed in this manner cannot be
decompressed using :const:FORMAT_AUTO (see :class:LZMADecompressor).
The check argument specifies the type of integrity check to include in the compressed data. This check is used when decompressing, to ensure that the data has not been corrupted. Possible values are:
:const:CHECK_NONE: No integrity check.
This is the default (and the only acceptable value) for
:const:FORMAT_ALONE and :const:FORMAT_RAW.
:const:CHECK_CRC32: 32-bit Cyclic Redundancy Check.
:const:CHECK_CRC64: 64-bit Cyclic Redundancy Check.
This is the default for :const:FORMAT_XZ.
:const:CHECK_SHA256: 256-bit Secure Hash Algorithm.
If the specified check is not supported, an :class:LZMAError is raised.
The compression settings can be specified either as a preset compression level (with the preset argument), or in detail as a custom filter chain (with the filters argument).
The preset argument (if provided) should be an integer between 0 and
9 (inclusive), optionally OR-ed with the constant
:const:PRESET_EXTREME. If neither preset nor filters are given, the
default behavior is to use :const:PRESET_DEFAULT (preset level 6).
Higher presets produce smaller output, but make the compression process
slower.
.. note::
In addition to being more CPU-intensive, compression with higher presets
also requires much more memory (and produces output that needs more memory
to decompress). With preset ``9`` for example, the overhead for an
:class:`LZMACompressor` object can be as high as 800 MiB. For this reason,
it is generally best to stick with the default preset.
The filters argument (if provided) should be a filter chain specifier.
See :ref:filter-chain-specs for details.
.. method:: compress(data)
Compress *data* (a :class:`bytes` object), returning a :class:`bytes`
object containing compressed data for at least part of the input. Some of
*data* may be buffered internally, for use in later calls to
:meth:`compress` and :meth:`flush`. The returned data should be
concatenated with the output of any previous calls to :meth:`compress`.
.. method:: flush()
Finish the compression process, returning a :class:`bytes` object
containing any data stored in the compressor's internal buffers.
The compressor cannot be used after this method has been called.
.. class:: LZMADecompressor(format=FORMAT_AUTO, memlimit=None, filters=None)
Create a decompressor object, which can be used to decompress data incrementally.
For a more convenient way of decompressing an entire compressed stream at
once, see :func:decompress.
The format argument specifies the container format that should be used. The
default is :const:FORMAT_AUTO, which can decompress both .xz and
.lzma files. Other possible values are :const:FORMAT_XZ,
:const:FORMAT_ALONE, and :const:FORMAT_RAW.
The memlimit argument specifies a limit (in bytes) on the amount of memory
that the decompressor can use. When this argument is used, decompression will
fail with an :class:LZMAError if it is not possible to decompress the input
within the given memory limit.
The filters argument specifies the filter chain that was used to create
the stream being decompressed. This argument is required if format is
:const:FORMAT_RAW, but should not be used for other formats.
See :ref:filter-chain-specs for more information about filter chains.
.. note::
This class does not transparently handle inputs containing multiple
compressed streams, unlike :func:decompress and :class:LZMAFile. To
decompress a multi-stream input with :class:LZMADecompressor, you must
create a new decompressor for each stream.
.. method:: decompress(data, max_length=-1)
Decompress *data* (a :term:`bytes-like object`), returning
uncompressed data as bytes. Some of *data* may be buffered
internally, for use in later calls to :meth:`decompress`. The
returned data should be concatenated with the output of any
previous calls to :meth:`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 :attr:`~.needs_input` attribute will
be set to ``False``. In this case, the next call to
:meth:`~.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 :attr:`~.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 :attr:`~.unused_data` attribute.
.. versionchanged:: 3.5
Added the *max_length* parameter.
.. attribute:: check
The ID of the integrity check used by the input stream. This may be
:const:`CHECK_UNKNOWN` until enough of the input has been decoded to
determine what integrity check it uses.
.. attribute:: eof
``True`` if the end-of-stream marker has been reached.
.. attribute:: unused_data
Data found after the end of the compressed stream.
Before the end of the stream is reached, this will be ``b""``.
.. attribute:: needs_input
``False`` if the :meth:`.decompress` method can provide more
decompressed data before requiring new uncompressed input.
.. versionadded:: 3.5
.. function:: compress(data, format=FORMAT_XZ, check=-1, preset=None, filters=None)
Compress data (a :class:bytes object), returning the compressed data as a
:class:bytes object.
See :class:LZMACompressor above for a description of the format, check,
preset and filters arguments.
.. function:: decompress(data, format=FORMAT_AUTO, memlimit=None, filters=None)
Decompress data (a :class:bytes object), returning the uncompressed data
as a :class:bytes object.
If data is the concatenation of multiple distinct compressed streams, decompress all of these streams, and return the concatenation of the results.
See :class:LZMADecompressor above for a description of the format,
memlimit and filters arguments.
.. function:: is_check_supported(check)
Returns true if the given integrity check is supported on this system.
:const:CHECK_NONE and :const:CHECK_CRC32 are always supported.
:const:CHECK_CRC64 and :const:CHECK_SHA256 may be unavailable if you are
using a version of :program:liblzma that was compiled with a limited
feature set.
.. _filter-chain-specs:
A filter chain specifier is a sequence of dictionaries, where each dictionary
contains the ID and options for a single filter. Each dictionary must contain
the key "id", and may contain additional keys to specify filter-dependent
options. Valid filter IDs are as follows:
Compression filters:
FILTER_LZMA1 (for use with :const:FORMAT_ALONE)FILTER_LZMA2 (for use with :const:FORMAT_XZ and :const:FORMAT_RAW)Delta filter:
FILTER_DELTABranch-Call-Jump (BCJ) filters:
FILTER_X86FILTER_IA64FILTER_ARMFILTER_ARMTHUMBFILTER_POWERPCFILTER_SPARCA filter chain can consist of up to 4 filters, and cannot be empty. The last filter in the chain must be a compression filter, and any other filters must be delta or BCJ filters.
Compression filters support the following options (specified as additional entries in the dictionary representing the filter):
preset: A compression preset to use as a source of default values for
options that are not specified explicitly.dict_size: Dictionary size in bytes. This should be between 4 KiB and
1.5 GiB (inclusive).lc: Number of literal context bits.lp: Number of literal position bits. The sum lc + lp must be at
most 4.pb: Number of position bits; must be at most 4.mode: :const:MODE_FAST or :const:MODE_NORMAL.nice_len: What should be considered a "nice length" for a match.
This should be 273 or less.mf: What match finder to use -- :const:MF_HC3, :const:MF_HC4,
:const:MF_BT2, :const:MF_BT3, or :const:MF_BT4.depth: Maximum search depth used by match finder. 0 (default) means to
select automatically based on other filter options.The delta filter stores the differences between bytes, producing more repetitive
input for the compressor in certain circumstances. It supports one option,
dist. This indicates the distance between bytes to be subtracted. The
default is 1, i.e. take the differences between adjacent bytes.
The BCJ filters are intended to be applied to machine code. They convert
relative branches, calls and jumps in the code to use absolute addressing, with
the aim of increasing the redundancy that can be exploited by the compressor.
These filters support one option, start_offset. This specifies the address
that should be mapped to the beginning of the input data. The default is 0.
Reading in a compressed file::
import lzma with lzma.open("file.xz") as f: file_content = f.read()
Creating a compressed file::
import lzma data = b"Insert Data Here" with lzma.open("file.xz", "w") as f: f.write(data)
Compressing data in memory::
import lzma data_in = b"Insert Data Here" data_out = lzma.compress(data_in)
Incremental compression::
import lzma lzc = lzma.LZMACompressor() out1 = lzc.compress(b"Some data\n") out2 = lzc.compress(b"Another piece of data\n") out3 = lzc.compress(b"Even more data\n") out4 = lzc.flush()
result = b"".join([out1, out2, out3, out4])
Writing compressed data to an already-open file::
import lzma with open("file.xz", "wb") as f: f.write(b"This data will not be compressed\n") with lzma.open(f, "w") as lzf: lzf.write(b"This will be compressed\n") f.write(b"Not compressed\n")
Creating a compressed file using a custom filter chain::
import lzma my_filters = [ {"id": lzma.FILTER_DELTA, "dist": 5}, {"id": lzma.FILTER_LZMA2, "preset": 7 | lzma.PRESET_EXTREME}, ] with lzma.open("file.xz", "w", filters=my_filters) as f: f.write(b"blah blah blah")