| :mod:`lzma` --- Compression using the LZMA algorithm |
| ==================================================== |
| |
| .. module:: lzma |
| :synopsis: A Python wrapper for the liblzma compression library. |
| .. moduleauthor:: Nadeem Vawda <nadeem.vawda@gmail.com> |
| .. sectionauthor:: Nadeem Vawda <nadeem.vawda@gmail.com> |
| |
| .. versionadded:: 3.3 |
| |
| |
| 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. |
| |
| |
| Reading and writing compressed files |
| ------------------------------------ |
| |
| .. class:: LZMAFile(filename=None, mode="r", \*, fileobj=None, format=None, check=-1, preset=None, filters=None) |
| |
| Open an LZMA-compressed file. |
| |
| An :class:`LZMAFile` can wrap an existing :term:`file object` (given by |
| *fileobj*), or operate directly on a named file (named by *filename*). |
| Exactly one of these two parameters should be provided. If *fileobj* is |
| provided, it is not closed when the :class:`LZMAFile` is closed. |
| |
| The *mode* argument can be either ``"r"`` for reading (default), ``"w"`` for |
| overwriting, or ``"a"`` for appending. If *fileobj* is provided, 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). |
| |
| |
| Compressing and decompressing data in memory |
| -------------------------------------------- |
| |
| .. 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 800MiB. 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) |
| |
| Decompress *data* (a :class:`bytes` object), returning a :class:`bytes` |
| object containing the decompressed data for at least part of the input. |
| 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`. |
| |
| .. 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""``. |
| |
| |
| .. 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. |
| |
| |
| Miscellaneous |
| ------------- |
| |
| .. 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. |
| |
| |
| .. function:: encode_filter_properties(filter) |
| |
| Return a :class:`bytes` object encoding the options (properties) of the |
| filter specified by *filter* (a dictionary). |
| |
| *filter* is interpreted as a filter specifier, as described in |
| :ref:`filter-chain-specs`. |
| |
| The returned data does not include the filter ID itself, only the options. |
| |
| This function is primarily of interest to users implementing custom file |
| formats. |
| |
| |
| .. function:: decode_filter_properties(filter_id, encoded_props) |
| |
| Return a dictionary describing a filter with ID *filter_id*, and options |
| (properties) decoded from the :class:`bytes` object *encoded_props*. |
| |
| The returned dictionary is a filter specifier, as described in |
| :ref:`filter-chain-specs`. |
| |
| This function is primarily of interest to users implementing custom file |
| formats. |
| |
| |
| .. _filter-chain-specs: |
| |
| 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: |
| * :const:`FILTER_LZMA1` (for use with :const:`FORMAT_ALONE`) |
| * :const:`FILTER_LZMA2` (for use with :const:`FORMAT_XZ` and :const:`FORMAT_RAW`) |
| |
| * Delta filter: |
| * :const:`FILTER_DELTA` |
| |
| * Branch-Call-Jump (BCJ) filters: |
| * :const:`FILTER_X86` |
| * :const:`FILTER_IA64` |
| * :const:`FILTER_ARM` |
| * :const:`FILTER_ARMTHUMB` |
| * :const:`FILTER_POWERPC` |
| * :const:`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 4KiB and |
| 1.5GiB (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 only supports a single |
| The delta filter supports only 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. |
| |
| |
| Examples |
| -------- |
| |
| Reading in a compressed file:: |
| |
| import lzma |
| with lzma.LZMAFile("file.xz") as f: |
| file_content = f.read() |
| |
| Creating a compressed file:: |
| |
| import lzma |
| data = b"Insert Data Here" |
| with lzma.LZMAFile("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.LZMAFile(fileobj=f, mode="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.LZMAFile("file.xz", "w", filters=my_filters) as f: |
| f.write(b"blah blah blah") |