On Thu, Sep 05, 2013 at 03:00:27PM +0200, Ludovic Courtès wrote:
> BTW, I haven’t check whether this is the case already, but we need
> something like
>   (define (package-with-explicit-python p python)
>     ;; Return a version of P built for PYTHON.
>     (package (inherit p) ...))
> so we can just write:
>   (define python2-pytz
>     (package-with-explicit-python python-pytz python-2))

The attached patch does almost this, following our offline discussion on
how to create packages recursively on the fly:
- It adds an argument pair "#:python ,python-2", or replaces an already
  present argument "#:python something-else", and this recursively
  for all inputs and native-inputs that use the python build system.
- It rewrites the corresponding names from "python-..." to "python2-...",
  or adds a prefix "python2-".
So one can write
(define python2-pytz
  (package-with-explicit-python python-pytz))
As the second argument would always be python-2, I decided to drop it.

When trying this with python2-babel, the previous code in python.scm
(commented out in the attached patch) and the new one create almost the same
derivations, that differ only in
  /nix/store/4rrnwqsj8c086zzviyypr4fikb7pz49v-python2-babel-0.9.6-guile-builder
vs.
  /nix/store/5prbhg600qa9k8p0bgjs12p4dm682gn8-python2-babel-0.9.6-guile-builder
so that the final hash is also different.

Comments are welcome.

Andreas