Hi Guix,

I've noticed that a fair number of packages in gnu/packages/python.scm
using the python-build-system declare native-inputs.  I suspect that in
every case, these should actually just be inputs.  I also suspect that
this is benign, except perhaps for the fact that it may confuse
Pythonistas who (like myself) initially started out by looking at these
packages as examples of how to get started defining packages in Guix.

The python-build-system's "lower" procedure (in
guix/build-system/python.scm) explicitly forbids cross-compilation:

--8<---------------cut here---------------start------------->8---
(define* (lower name
                #:key source inputs native-inputs outputs system target
                (python (default-python))
                #:allow-other-keys
                #:rest arguments)
  "Return a bag for NAME."
  (define private-keywords
    '(#:source #:target #:python #:inputs #:native-inputs))

  (and (not target)                               ;XXX: no cross-compilation
       (bag
         (name name)
         (system system)
         (host-inputs `(,@(if source
                              `(("source" ,source))
                              '())
                        ,@inputs

                        ;; Keep the standard inputs of 'gnu-build-system'.
                        ,@(standard-packages)))
         (build-inputs `(("python" ,python)
                         ,@native-inputs))
         (outputs outputs)
         (build python-build)
         (arguments (strip-keyword-arguments private-keywords arguments)))))
--8<---------------cut here---------------end--------------->8---

As for the native-inputs, they get stored in the bag's build-inputs,
which eventually find their way to the "inputs" keyword argument used on
the build side by the various build phases.  In fact, the inputs,
propagated-inputs, and native-inputs of any package that uses the
python-build-system are all put into this "inputs" keyword argument.

With this in mind, I have two questions:

* Should we change these native-inputs to inputs to prevent confusion?
  I can personally vouch for the fact that the presence of native-inputs
  in python-build-system packages confused the heck out of me at first!

* Are there any circumstances under which it actually WOULD make sense
  to cross-compile a Python package?

For now, I think the answers to these questions are "sure" and "probably
not", respectively.  I'm very curious to hear your thoughts about the
second question, in particular!

-- 
Chris