proper Python inputs (was: gnu: Add python-pytest-pep8.)

[Hartmut Goebel]

Am 25.04.2016 um 20:19 schrieb Leo Famulari:
> This seems like an unusual case, since the package in question appears
> to be a build-time tool. So, it would make sense to have things like
> python-pytest available during python-pytest-pep8's run-time.

> If we have a use case for python-pytest-pep8, it could be worthwhile
> to see if it will work without propagating these anyways; I've noticed
> some Python packages have a way to link to non-propagated-inputs. 

Reading this makes me *totally* confused about how to package Python for
Guix. I think, we should enhance the Guix Python Packaging guide to
clarify thinks for all.

1. Fact: If some Python package lists another package in
"install_requires", then this other Python package is required to run.
In a non Guix-environment, pip would install these other package, too.

2. Until 5 minutes ago I thought, that adding these other packages as
"normal" inputs is enough. But then I found that quite a bunch of
packages are using "propagated-inputs". So I tried to find out, what the
difference is:

  - "normal" inputs get added to PYTHONPATH via the .pth mechanism.

     Example: guix package -i python-execnet puts only
"execnet-1.4.1-py3.4.egg" into site-packages, plus
"python-execnet-1.4.1.pth", which adds the required package via a link
to "/gnu/store/....python-apipkg-1.4/..."

 - "Propagated" inputs are all linked into site-packages

    Example: guix package -i python-zope-interface puts zope-interface
and zope-event into site-packages.

 So this is not much of a difference: In both cases all required inputs
will be available in PYTHONPATH at run-time and can be imported.

3. But anyhow: Trouble is ahead: If some package aaa has a normal input
"address@hidden", while package bbb as normal input "address@hidden" and 
package ccc
as propagated input "address@hidden", we will end up with this in site-packages:

   - aaa.pth adding path to address@hidden
   - bbb.pth adding path to address@hidden
   - zzz-1.3.egg

  Which package "zzz" gets imported is somewhat arbitrary [2].

Is this intended? Did I miss something?

IMHO we should clarify his behavior and then bring all existing Python
packages in line and  document it in the manual.


[1]
<https://python-packaging-user-guide.readthedocs.org/en/latest/distributing/?highlight=install_requires#install-requires>

[2] Not exactly, it depends on the lexical order of the filenames, but
this is implementation-dependent and AFAIK not documented.

-- 
Regards
Hartmut Goebel

| Hartmut Goebel          | address@hidden               |
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