ludo@gnu.org (Ludovic Courtès) skribis:

> An idea I haven’t taken the time to test yet would be to use
> ‘properties’:
>
>   (define python-foobar   ;with Python 3
>     (package
>       (name "foobar")
>       ;; Specify which Python 2 variant to use.
>       (properties `((python2-variant . ,(delay python2-foobar))))))
>
>   (define python2-foobar
>     (package (inherit python-foobar)
>       ;; … stuff beyond the mechanical python 2→3 switch…
>       ))
>
> ‘package-with-python2’ would honor this ‘python2-variant’ property.

Here’s a first stab at this.

As an example, I modified ‘python-matplotlib’ to use that feature, so we
can test that ‘python2-scipy’ is using the right ‘python2-matplotlib’:

--8<---------------cut here---------------start------------->8---
$ ./pre-inst-env guix build python2-matplotlib -d 2>/dev/null
/gnu/store/07zr2pqg61b742czb2aqyisml2i90r4a-python2-matplotlib-1.4.3.drv
$ ./pre-inst-env guix build python2-scipy -d 2>/dev/null
/gnu/store/8yhxdbyjvx2xwynpqqcrj9ilksd2pb01-python2-scipy-0.16.0.drv
$ guix gc --references /gnu/store/8yhxdbyjvx2xwynpqqcrj9ilksd2pb01-python2-scipy-0.16.0.drv | grep python2-matplotlib
/gnu/store/07zr2pqg61b742czb2aqyisml2i90r4a-python2-matplotlib-1.4.3.drv
--8<---------------cut here---------------end--------------->8---

This will trigger rebuilds (but with an identical result) because in
manually-written variants we would use “python2-foo” as the label of
inputs, whereas the automatic transformations keeps the original
“python-foo” label.

What do people think?

I can apply this patch of the approach sounds good to you.  I think we
should probably do one commit per rewrite for clarity.

We should probably start with the lowest level, like python2-pycairo and
python2-pygobject and even python itself, because if we fix them then
some of the higher-level stuff won’t even need their own
‘python2-variant’ property.  For instance, if python, pycairo, and
pygobject have their ‘python2-variant’ set, then we no longer need this:

--8<---------------cut here---------------start------------->8---
(define-public python2-matplotlib
  (let ((matplotlib (package-with-python2 %python-matplotlib)))
    (package (inherit matplotlib)
      ;; Make sure to use special packages for Python 2 instead
      ;; of those automatically rewritten by package-with-python2.
      (propagated-inputs
       `(("python2-pycairo" ,python2-pycairo)
         ("python2-pygobject-2" ,python2-pygobject-2)
         ("python2-tkinter" ,python-2 "tk")
         ,@(fold alist-delete (package-propagated-inputs matplotlib)
                 '("python-pycairo" "python-pygobject" "python-tkinter")))))))
--8<---------------cut here---------------end--------------->8---

… and as a consequence, we don’t need a ‘python2-variant’ in
‘python-matplotlib’.

Does that make sense?  Any takers?  (This can be done incrementally.)

Thanks,
Ludo’.