Phil <phil@beadling.co.uk> writes:

> Hi,
>
> It seems standard not to declare python2 or python3 as a dependency on
> python package definitions - however other dependent python libraries are stated.
>
> eg python-scipy will declare dependencies on python-numpy and
> python-matplotlib - but not on a specific version of python package
> required to use it.
>
> I'm guessing this is to avoid tying packages to specific python
> releases, but I'm curious about the mechanics.

Build systems are a mechanic to deduplicate common steps, but also
common inputs between packages, and the python-build-system will include
a default Python as an input.

https://git.savannah.gnu.org/cgit/guix.git/tree/guix/build-system/python.scm#n138

> It looks like 'package-with-python2' might be used to allow us to
> distinguish between python2 and python3, but ignoring the python2 case,
> I have the following python3 questions:
>
> Can we can install python-scipy without installing python3, given python
> isn't an explicit dependency in the package?

Guix packages don't have dependencies like other package managers,
there's propagated-inputs which can act like dependencies where Guix
will attempt to add propagated inputs to the profile when you install a
package. But generally "dependencies" as in other things you should have
when you have a given store item, are just references between store
items:

→ guix gc --references /gnu/store/jj1915czcy6wlf6riajapl1j6dfgwwii-python-scipy-1.3.2
/gnu/store/01b4w3m6mp55y531kyi1g8shh722kwqm-gcc-7.5.0-lib
/gnu/store/741057r2x06zwg6zcmqmdyv51spm6n9i-gfortran-7.5.0-lib
/gnu/store/bs9pl1f805ins80xaf4s3n35a0x2lyq3-openblas-0.3.9
/gnu/store/fa6wj5bxkj5ll1d7292a70knmyl7a0cr-glibc-2.31
/gnu/store/h8jw9qhyfp6fm6nb3cgh4335qhr31wfz-python-wrapper-3.8.2
/gnu/store/jj1915czcy6wlf6riajapl1j6dfgwwii-python-scipy-1.3.2
/gnu/store/jxx8fr78jrcvpid5aplmkplbm1dk6czs-python-3.8.2

→ grep -r jxx8fr78jrcvpid5aplmkplbm1dk6czs /gnu/store/jj1915czcy6wlf6riajapl1j6dfgwwii-python-scipy-1.3.2
Binary file /gnu/store/jj1915czcy6wlf6riajapl1j6dfgwwii-python-scipy-1.3.2/lib/python3.8/site-packages/scipy/optimize/_zeros.cpython-38-x86_64-linux-gnu.so matches
...

So, you can't have this particular python-scipy output in your store
without python as well, as it's referenced by some shared libraries,
which I guess makes sense.

> Which site-packages directory under what python3 version will be used?
> A quick check looks that /path/to/profile/lib/python3.8/site-packages is
> currently used but what makes the decision to put them under python3.8 -
> especially if python3.8 isn't installed in the profile?

The python build system takes care of where files end up in the package
outputs at least:

https://git.savannah.gnu.org/cgit/guix.git/tree/guix/build/python-build-system.scm#n38

> What happens if Guix upgrades python3 from 3.8 -> 3.9?  How are packages
> already installed under the 3.8, moved to the new 3.9 python version, is this seamless?

When the default python version is changed, the build system will change
accordingly.

> If I'm using Guix on top of a foreign distro and don't have python3
> installed as part of Guix, will my python-scipy end-up installed for my
> foreign distro's python install?

Installing Guix things is just ensuring things are in the /gnu/store,
plus maybe some environment variables for the profile. So it won't end
up where your (not Guix) distro installs Python things at least.

Hope that helps!

Chris