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

[Hartmut Goebel <h.goebel@crazy-compilers.com>]

Hi Maxim,

many thanks for picking up this issue.
> Indeed, I thought about the possibility to filter the GUIX_PYTHONPATH
> entries based on their version at runtime after I wrote my initial
> reply.  It makes life easier.  I've updated the
> cu/farewell-to-pythonpath branch with this new way of doing things.

I had a look at the first changes (only) and have some remarks:

1) Did I understand this correctly: `sitecustomize.py` is filtering 
GUIX_PYTHONPATH for all parts containing "/lib/pythonX.Y/site-packages" 
(with X.Y being Major.Minor)?

2) This does not remove duplicates and does not honor .pth files in the 
respective directories - which might still be used. Thus 
site.addsitedir() should be called for adding the paths. This also takes 
care about duplicates.

3) Empty part (…::…) are not handled.

4) Since PYTHONPATH is evaluated prior to importing sitecustomize, any 
sitecustominze.py in the user's path will overwrite our file, thus 
inhibiting our paths to be added. Not sure this is what we want in Guix.

5) When implementing the logic into site.py, the code could be 
simplified, Eg. You could patch a "getguixsitepackages" (based on 
getsitepackages) and a "addguixsitepackages" (based on 
addguixsitepackages) into site.py. Downside is that maybe we need 
different patches for different Python versions. Benefit is simpler code 
- no need to search the correct position in the list.

6) Please add some more comments to the code explaining the idea.


Some nitpicking:

 > python_root = os.path.realpath(sys.executable).split('/bin/')[0]

There already is sys.prefix, which is also the base for the entry in 
sys.path (see top of site.py

 > major_minor = '{}.{}'.format(sys.version_info[0], sys.version_info[1])

major_minor = '{}.{}'.format(*sys.version_info)

 >sys.path = sys.path[:index] + matching_sites + sys.path[index:]

sys.path[index:index] = matching_sites


I suggest using os.path.join(), os.path.split(), os.pathsep, etc. to be 
forward-compatible. Imagine we want to port Guix to another platform 
with different separators.

-- 
Regards
Hartmut Goebel

| Hartmut Goebel          | h.goebel@crazy-compilers.com               |
| www.crazy-compilers.com | compilers which you thought are impossible |