13.4. lzma --- Compression using the LZMA algorithm

3.3 新版功能.

Source code: 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 xz utility, as well as raw compressed streams.

The interface provided by this module is very similar to that of the bz2 module. However, note that LZMAFile is not thread-safe, unlike bz2.BZ2File, so if you need to use a single LZMAFile instance from multiple threads, it is necessary to protect it with a lock.

exception lzma.LZMAError

This exception is raised when an error occurs during compression or decompression, or while initializing the compressor/decompressor state.

13.4.1. Reading and writing compressed files

lzma.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 file object.

The filename argument can be either an actual file name (given as a str, bytes or path-like 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 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 LZMACompressor.

For binary mode, this function is equivalent to the LZMAFile constructor: LZMAFile(filename, mode, ...). In this case, the encoding, errors and newline arguments must not be provided.

For text mode, a LZMAFile object is created, and wrapped in an io.TextIOWrapper instance with the specified encoding, error handling behavior, and line ending(s).

在 3.4 版更改: Added support for the "x", "xb" and "xt" modes.

在 3.6 版更改: Accepts a path-like object.

class lzma.LZMAFile(filename=None, mode="r", *, format=None, check=-1, preset=None, filters=None)

Open an LZMA-compressed file in binary mode.

An LZMAFile can wrap an already-open 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 str, bytes or path-like object). When wrapping an existing file object, the wrapped file will not be closed when the 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 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 LZMACompressor.

LZMAFile supports all the members specified by io.BufferedIOBase, except for detach() and truncate(). Iteration and the with statement are supported.

The following method is also provided:

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).

注解

While calling peek() does not change the file position of the LZMAFile, it may change the position of the underlying file object (e.g. if the LZMAFile was constructed by passing a file object for filename).

在 3.4 版更改: Added support for the "x" and "xb" modes.

在 3.5 版更改: The read() method now accepts an argument of None.

在 3.6 版更改: Accepts a path-like object.

13.4.2. Compressing and decompressing data in memory

class lzma.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 compress().

The format argument specifies what container format should be used. Possible values are:

  • FORMAT_XZ: The .xz container format.
    This is the default format.
  • FORMAT_ALONE: The legacy .lzma container format.
    This format is more limited than .xz -- it does not support integrity checks or multiple filters.
  • 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 FORMAT_AUTO (see 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:

  • CHECK_NONE: No integrity check. This is the default (and the only acceptable value) for FORMAT_ALONE and FORMAT_RAW.
  • CHECK_CRC32: 32-bit Cyclic Redundancy Check.
  • CHECK_CRC64: 64-bit Cyclic Redundancy Check. This is the default for FORMAT_XZ.
  • CHECK_SHA256: 256-bit Secure Hash Algorithm.

If the specified check is not supported, an 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 PRESET_EXTREME. If neither preset nor filters are given, the default behavior is to use PRESET_DEFAULT (preset level 6). Higher presets produce smaller output, but make the compression process slower.

注解

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 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 Specifying custom filter chains for details.

compress(data)

Compress data (a bytes object), returning a 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 compress() and flush(). The returned data should be concatenated with the output of any previous calls to compress().

flush()

Finish the compression process, returning a bytes object containing any data stored in the compressor's internal buffers.

The compressor cannot be used after this method has been called.

class lzma.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 decompress().

The format argument specifies the container format that should be used. The default is FORMAT_AUTO, which can decompress both .xz and .lzma files. Other possible values are FORMAT_XZ, FORMAT_ALONE, and 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 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 FORMAT_RAW, but should not be used for other formats. See Specifying custom filter chains for more information about filter chains.

注解

This class does not transparently handle inputs containing multiple compressed streams, unlike decompress() and LZMAFile. To decompress a multi-stream input with LZMADecompressor, you must create 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.

在 3.5 版更改: Added the max_length parameter.

check

The ID of the integrity check used by the input stream. This may be CHECK_UNKNOWN until enough of the input has been decoded to determine what integrity check it uses.

eof

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

unused_data

Data found after the end of the compressed stream.

Before the end of the stream is reached, this will be b"".

needs_input

False if the decompress() method can provide more decompressed data before requiring new uncompressed input.

3.5 新版功能.

lzma.compress(data, format=FORMAT_XZ, check=-1, preset=None, filters=None)

Compress data (a bytes object), returning the compressed data as a bytes object.

See LZMACompressor above for a description of the format, check, preset and filters arguments.

lzma.decompress(data, format=FORMAT_AUTO, memlimit=None, filters=None)

Decompress data (a bytes object), returning the uncompressed data as a 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 LZMADecompressor above for a description of the format, memlimit and filters arguments.

13.4.3. Miscellaneous

lzma.is_check_supported(check)

Returns true if the given integrity check is supported on this system.

CHECK_NONE and CHECK_CRC32 are always supported. CHECK_CRC64 and CHECK_SHA256 may be unavailable if you are using a version of liblzma that was compiled with a limited feature set.

13.4.4. Specifying custom filter chains

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 FORMAT_ALONE)
    • FILTER_LZMA2 (for use with FORMAT_XZ and FORMAT_RAW)
  • Delta filter:
    • FILTER_DELTA
  • Branch-Call-Jump (BCJ) filters:
    • FILTER_X86
    • FILTER_IA64
    • FILTER_ARM
    • FILTER_ARMTHUMB
    • FILTER_POWERPC
    • FILTER_SPARC

A 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: MODE_FAST or 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 -- MF_HC3, MF_HC4, MF_BT2, MF_BT3, or 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.

13.4.5. Examples

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()
# Concatenate all the partial results:
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")