python-xz 0.1.2

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

python-xz

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

   

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📖 Documentation   |   📃 Changelog

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A XZ file can be composed of several streams and blocks. This allows for 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	❌ no	✔️ yes	✔️ yes
📝 write
w mode	✔️ yes	✔️ yes	⏳ planned
x mode	✔️ yes	❌ no	⏳ planned
a mode	✔️ new stream	✔️ new stream	⏳ planned
r+w mode	❌ no	❌ no	⏳ planned
several blocks	❌ no	❌ no	⏳ planned
several streams	❌ no5	❌ no5	⏳ planned
stream padding	❌ no6	✔️ yes	⏳ planned

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. Possible by manually closing and re-opening in append mode
6. Related issue

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Usage

Read mode

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

>>> 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

This mode is not available yet.

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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?

XZ Utils can create XZ files with several blocks:

$ 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