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

[Ludovic Courtès]

Hi Maxim,

Maxim Cournoyer <maxim.cournoyer@gmail.com> skribis:

>>> +(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.

Sure, I definitely agree.

I’m advocating for a versioned environment variable too, but I’m
suggesting that versioning should not be part of the variable name.

In the ‘GUIX_LOCPATH’ example I gave, locale data is looked up not
directly in the search path entries, but instead in those entries
prefixed by the libc version.  See ‘glibc-versioned-locpath.patch’.

If possible, the proposed ‘GUIX_PYTHONPATH’ variable could work
similarly: “/MAJOR.MINOR” would automatically be appended to each search
path entry.

That way, both Python 2.7 and 3.8 can populate the same ‘GUIX_LOCPATH’
variable, and eventually 2.7 only picks 2.7 packages and 3.8 only picks
3.8 packages.

Does that make more sense?

Thanks,
Ludo’.