Re: No Explicit Python Version Dependency In Package Definitions?

[Phil]

Thanks for the reply.

Christopher Baines writes:

> 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

Got it - so the version is set behind the scenes depending on the
current python package definition.

> 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.

Yep - so even tho python 3.8 is not installed by installing eg
python-scipy, the package is made available in the store as it's referenced.

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

Yep makes sense - so the python package will now reference the 3.9
package here instead of 3.8:

;; Current 3.x version.
(define-public python-3 python-3.8)

I'm guessing my local Guix would stay on 3.8 until I did a guix
upgrade.  At this point the new version of python-3 would force any
python packages I had to reinstall against 3.9.  I assume my local 3.8 system
would be left untouched, such that I could rollback both the python
version and my packages if I wanted?

What would happen if I installed a new python package after pulling the
latest package definitions tho?

So I have a system say with python3.8 and python-scipy, and I decide I
want to then install python-pandas (for example).  Will it not then
build this for python 3.9 (due to the new definition), if the version of
python has incremented between the installs of python-scipy and
python-pandas?  Would I then have to manually reinstall python-scipy to
have it under 3.9 (as well as 3.8) (or do a guix upgrade)?

Last question, if today I wanted to create a profile that installed
python-scipy against the python 3.9 package definition (which already
exists in Guix it's just not the default).  Do I have to change the
python-3 definition as per above, or is there another way of saying "use
python3.9 just for this profile".

I suspect I could use a manifest to install 3.9 rather than the default,
but won't any packages still depend on 3.8 unless I switch the python-3
definition?

Thanks again!