Re: [PATCH 2/2] gnu: Add gnome-tweak-tool.

[Leo Famulari <leo@famulari.name>]

On Sun, Apr 24, 2016 at 01:19:35PM +0200, Jan Nieuwenhuizen wrote:
> Leo Famulari writes:
> > I think we need to wrap gnome-tweak-tool's executable, which the
> > python-build-system normally does automatically. AIUI, that's why all of
> > the python-build-system packages don't require Python itself to be
> > propagated.
> 
> Sorry, I still don't understand.  Can you explain why you want to remove
> python2 from the propagated inputs?

Propagated-inputs are silently installed into the user's profile
alongside the package that propagates them. In this case, installing
gnome-tweak-tool would also install python2 into the user's profile.

Propagation becomes attractive when the software provided by a package
does not have a good mechanism for finding its dependencies. For
example, some software may *only* be able to find a dependency by
looking on PATH. Gnome-tweak-tool, as far as we know, has this
limitation for python2.

The problem with propagating inputs is that only one version of a given
package may be installed into a user's profile. This is in contrast to
"regular" inputs, which are not installed into a user's profile. Indeed,
every package that you install into your profile could refer to a
different version of, say, libfoo, by linking directly to the various
libfoos' directories in the store.

So, letting gnome-tweak-tool propagate python2 would prevent a Python
programmer from choosing which version of python2 they want in their
profile; they'd be forced to choose between gnome-tweak-tool or their
desired python2.

Does that make sense?

> And also...say we wrap gnome-tweak-tool.  If we remove python2 from the
> propagated inputs, AIUI it python2 will not be installed and it even
> could just be that python2 is not downloaded at all under /gnu.
> 
> How is wrapping gnome-tweak-tool going to help if we fail to make sure
> that python2 is present?  Sorry for my ignorance.
 
An alternative to propagated-inputs is to use a wrapper. Actually, all
of our packages using the python-build-system are wrapped automatically
[0]. The wrapper makes the dependent packages available in the run-time
environment without polluting the user's profile, while introducing a
reference to the dependencies into the store directory, which makes sure
that the garbage collector works correctly.

Does that make sense?

Hopefully, I've got that all right — I'll be happy if somebody clarifies
or corrects me!

[0] If gnome-tweak-tool did not break convention by using the Autotools
to build Python software, this discussion would not be happening ;)