[Distutils] Q about best practices now (or near future)

[Nick Coghlan]

On 22 Jul 2013 21:29, "Paul Moore" <p.f.moore at gmail.com> wrote:
>
> On 22 July 2013 12:22, Nick Coghlan <ncoghlan at gmail.com> wrote:
>>
>> since extras aren't really a thing in their own right (they're just a
shorthand for referring to an additional set of dependencies)
>
>
> I'm still trying to be clear in my mind about what extras are, and how
they should work. From this description, it occurs to me to ask, what is
the difference between an extra and a (metadata only, empty) second
distribution that depends on the base project as well as the "additional
set of dependencies"? Is it just the admin overhead of registering a second
project?

Sort of. The idea of an extra is "We have installed all the code for this,
but it won't work due runtime failures if these dependencies aren't
available".

With an actual separate distribution, you can't easily tell that the other
distribution contains no code of its own, and naming and versioning gets
more complicated. You also can't do the trick 426 adds where "*" means "all
optional dependencies".

For other package systems like RPM that don't have the notion of extras,
then yes, an extra would probably be mapped to a virtual package (in the
specific case of yum, it copes fairly well with version locked virtual
packages like that).

> Looking at extras this way gives a possible way of generating scripts
only when the extras are present - just add the scripts to the dummy
"extra" distribution.

Partial installs are problematic, since checking for optional dependencies
is supposed to be a runtime thing, so it doesn't matter *how* those
dependencies got there. Optional functionality like that would be better
handled through a script that accepted subcommands, some of which would
report an error if dependencies were missing.

For a truly optional script, then it needs to be a genuinely separate
package.

Cheers,
Nick.

>
> Paul.
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