doc: guix: add lzma python tip

[jamesob]

I tripped on this error when attempting to do Guix builds for macOS
from a debian host.

[luke-jr]

Why would you have to reinstall Python? Debian doesn't compile it, so shouldn't the package be the same regardless?

[jamesob]

When building the shared libs (--enable-shared), if the lzma header is missing Python apparently just skips it. I build my Python from source so I'm not sure how this might work with Debian packaging.

[laanwj]

I'm kind of confused here. How does installing headers make a Python package appear? Only if you build Python afterward?

I think it would be better to handle this as a required build check/dependency, e.g. through the conventional requirements.txt system.

Specific instructions related to building Python from source is out of scope for this document, i think, as it's definitely not a specific need here. You only need a Python with the lzma package.

[jamesob]

haha okay, well this comment would've probably saved me fifteen minutes but I guess it's out of scope. Will close.

[jamesob]

Just for posterity...

All Python installations ship with the lzma.py file below, but (I guess) if you compile Python without having the lzma headers installed, you don't get the .so file alongside it:

% ls /usr/local/lib/python3.10/*lzma*
/usr/local/lib/python3.10/_lzma.cpython-39-x86_64-linux-gnu.so  /usr/local/lib/python3.10/lzma.py

which then causes the following import to fail:

% grep _lzma /usr/local/lib/python3.10/lzma.py
from _lzma import *
from _lzma import _encode_filter_properties, _decode_filter_properties

I don't know why Python builds don't require the lzma headers if this is part of the standard library, but apparently you can have a "valid" Python installation built from source (as I did) that lacks this shared object file.

[JeremyRubin]

concept ack

if you can figure out the minimal set of optional things required to build the python we need, that would be awesome. there are probably some things you got lucky on!

we could even add a python-check script to ensure we have all the components not erroring.

this is a good first step!