Re: 03/163: build/python: Add a new guix-pythonpath procedure.

[Maxim Cournoyer <maxim.cournoyer@gmail.com>]

Hi Ludovic!

Ludovic Courtès <ludo@gnu.org> writes:

> Hi!
>
> guix-commits@gnu.org skribis:
>
>> apteryx pushed a commit to branch cu/farewell-to-pythonpath
>> in repository guix.
>
> Woow!
>
>> commit c3c5bdd2b0ea4588ecba424fdbf6df101d967d3d
>> Author: Maxim Cournoyer <maxim.cournoyer@gmail.com>
>> AuthorDate: Fri Jan 22 09:34:33 2021 -0500
>>
>>     build/python: Add a new guix-pythonpath procedure.
>>     
>>     This procedure will be used in place where the static "PYTHONPATH" string used
>>     to be.
>>     
>>     * guix/build/python-build-system.scm (guix-pythonpath): New procedure.
>
> [...]
>
>> +(define (guix-pythonpath inputs)
>> +  "Derive the GUIX_PYTHONPATH_X_Y string , with the major and minor version
>> +numbers substituted for X and Y, respectively."
>> +  (let ((python (assoc-ref inputs "python")))
>> +    (string-append "GUIX_PYTHONPATH_"
>> +                   (string-replace-substring (python-version python) "." "_"))))
>
> One pattern used elsewhere, notably for ‘GUIX_LOCPATH’, is to still keep
> a single environment variable (that’d be ‘GUIX_PYTHONPATH’ here) but to
> ensure that only MAJOR.MINOR sub-directories are taken into
> consideration.
>
> IOW, instead of having GUIX_PYTHONPATH_3_8 and GUIX_PYTHONPATH_2_7,
> you’d have just GUIX_PYTHONPATH but Python 3.8 would only care about
> 3.8/ sub-directories while 2.7 would only care about 2.7/
> sub-directories.
>
> That’s perhaps easier to document and to deal with generally (you only
> need to look at one specific environment variable).
>
> Does that make sense?  I don’t know if it’s applicable in this case
> though.

One of the limitations lifted by this change is the limitation of
running a single version of Python per profile.  As mentioned in past
discussions, it's not trivial ensuring that packages installed won't end
up pulling various versions of Python in a same profile, so it's better
to prepared for it :-).

Using a non-versioned GUIX_PYTHONPATH would not allow for this, unless
I'm missing something.

Thanks!

Maxim