bug#22013: python2 versions of packages failing as inputs

[Efraim Flashner <efraim@flashner.co.il>]

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On Thu, 26 Nov 2015 09:59:12 +0100
ludo@gnu.org (Ludovic Courtès) wrote:

> Ricardo Wurmus <rekado@elephly.net> skribis:
> 
> > Efraim Flashner <efraim@flashner.co.il> writes:
> >  
> >> Guix build python2-cryptography builds fine, guix build python2-oauthlib
> >> rebuilds python2-cryptography, and fails to build. Python2-cryptography has
> >> an additional input of python2-ipaddress which doesn't get carried along when
> >> python2-cryptography is an input and not the final build.  
> >
> > I think that’s pretty normal.  We have the same situation in
> > ‘python2-openssl’ where we have to explicitly add ‘python2-cryptography’
> > and remove ‘python-cryptography’.  
> 
> Yes, and this is due to the fact that ‘python2-cryptography’ is not just
> an automatically-translated package:
> 
>   (define-public python2-cryptography
>     (let ((crypto (package-with-python2 python-cryptography)))
>       (package (inherit crypto)
>         (propagated-inputs
>          `(("python2-ipaddress" ,python2-ipaddress)
>            ,@(package-propagated-inputs crypto))))))
> 
> If it were simply equal to (package-with-python2 python-cryptography),
> there would be no need for the manual tweak in ‘python-pyopenssl’.
> 
> Efraim, could you apply the same strategy as for ‘python2-pyopenssl’ in
> ‘python2-oauthlib’?

Ok, I understand now. I thought I had to edit python2-cryptography to fix
python2-oauthlib, now I realize everytime I use python2-cryptography I need
to make sure it pulls the right one.

> 
> > It would, of course, be nice if package-with-python2 could handle this
> > automatically.  
> 
> I think we would need a way to register “translation” results.
> Currently ‘package-with-python2’ internally uses a hash table to memoize
> conversion results.  We could imagine adding something like:
> 
>   (register-python2-variant PY3VARIANT PY2VARIANT)
> 
> to explicitly add a pair to that hash table.
> 
> The downside of this approach is that this would break referential
> transparency.
> 
> WDYT?
> 
> Thanks,
> Ludo’.

We already have the python- and python2- variants as the same packages, just
with the occasional additional input to make it build. Mapping the two
together might speed up package-with-python2.

I'm going to keep on looking at guix/build-systems/python.scm to see if I can
come up with something to not need to specifically declare python2-foo every
time python-foo ends up being used.

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Efraim Flashner   <efraim@flashner.co.il>   אפרים פלשנר
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