python-xz

python-xz

Pure Python implementation of the XZ file format with random access support

Leveraging the lzma module for fast (de)compression

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📖 Documentation   |   📃 Changelog

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A XZ file can be composed of several streams and blocks. This allows for fast random access when reading, but this is not supported by Python's builtin lzma module (which would read all previous blocks for nothing).

	lzma	lzmaffi	python-xz
module type	builtin	cffi (C extension)	pure Python
📄 read
random access	❌ no1	✔️ yes2	✔️ yes2
several blocks	✔️ yes	✔️✔️ yes3	✔️✔️ yes3
several streams	✔️ yes	✔️ yes	✔️✔️ yes4
stream padding	❌ no5	✔️ yes	✔️ yes
📝 write
w mode	✔️ yes	✔️ yes	✔️ yes
x mode	✔️ yes	❌ no	✔️ yes
a mode	✔️ new stream	✔️ new stream	⏳ planned
r+/w+/… modes	❌ no	❌ no	✔️ yes
several blocks	❌ no	❌ no	✔️ yes
several streams	❌ no6	❌ no6	✔️ yes
stream padding	❌ no	❌ no	⏳ planned

Notes

1. Reading from a position will read the file from the very beginning
2. Reading from a position will read the file from the beginning of the block
3. Block positions available with the block_boundaries attribute
4. Stream positions available with the stream_boundaries attribute
5. Related issue
6. Possible by manually closing and re-opening in append mode

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Install

Install python-xz with pip:

$ python -m pip install python-xz

An unofficial package for conda is also available, see issue #5 for more information.

Usage

The API is similar to lzma: you can use either xz.open or xz.XZFile.

Read mode

>>> with xz.open('example.xz') as fin:
...     fin.read(18)
...     fin.stream_boundaries  # 2 streams
...     fin.block_boundaries   # 4 blocks in first stream, 2 blocks in second stream
...     fin.seek(1000)
...     fin.read(31)
...
b'Hello, world! \xf0\x9f\x91\x8b'
[0, 2000]
[0, 500, 1000, 1500, 2000, 3000]
1000
b'\xe2\x9c\xa8 Random access is fast! \xf0\x9f\x9a\x80'

Opening in text mode works as well, but notice that seek arguments as well as boundaries are still in bytes (just like with lzma.open).

>>> with xz.open('example.xz', 'rt') as fin:
...     fin.read(15)
...     fin.stream_boundaries
...     fin.block_boundaries
...     fin.seek(1000)
...     fin.read(26)
...
'Hello, world! 👋'
[0, 2000]
[0, 500, 1000, 1500, 2000, 3000]
1000
'✨ Random access is fast! 🚀'

Write mode

Writing is only supported from the end of file. It is however possible to truncate the file first. Note that truncating is only supported on block boundaries.

>>> with xz.open('test.xz', 'w') as fout:
...     fout.write(b'Hello, world!\n')
...     fout.write(b'This sentence is still in the previous block\n')
...     fout.change_block()
...     fout.write(b'But this one is in its own!\n')
...
14
45
28

Advanced usage:

• Modes like r+/w+/x+ allow to open for both read and write at the same time; however in the current implementation, a block with writing in progress is automatically closed when reading data from it.
• The check, preset and filters arguments to xz.open and xz.XZFile allow to configure the default values for new streams and blocks.
• Change block with the change_block method (the preset and filters attributes can be changed beforehand to apply to the new block).
• Change stream with the change_stream method (the check attribute can be changed beforehand to apply to the new stream).

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FAQ

How does random-access works?

XZ files are made of a number of streams, and each stream is composed of a number of block. This can be seen with xz --list:

$ xz --list file.xz
Strms  Blocks   Compressed Uncompressed  Ratio  Check   Filename
    1      13     16.8 MiB    297.9 MiB  0.056  CRC64   file.xz

To read data from the middle of the 10th block, we will decompress the 10th block from its start it until we reach the middle (and drop that decompressed data), then returned the decompressed data from that point.

Choosing the good block size is a tradeoff between seeking time during random access and compression ratio.

How can I create XZ files optimized for random-access?

You can open the file for writing and use the change_block method to create several blocks.

Other tools allow to create XZ files with several blocks as well:

• XZ Utils needs to be called with flags:

$ xz -T0 file                          # threading mode
$ xz --block-size 16M file             # same size for all blocks
$ xz --block-list 16M,32M,8M,42M file  # specific size for each block

• PIXZ creates files with several blocks by default:

$ pixz file

Python version support

As a general rule, all Python versions that are both released and still officially supported are supported by python-xz and tested against (both CPython and PyPy implementations).

If you have other use cases or find issues with some Python versions, feel free to open a ticket!