[Catalog-sig] parsing setuptools style requires.txt

[Jim Fulton]

On Mon, Sep 12, 2011 at 4:43 AM, Dylan Jay <djay at pretaweb.com> wrote:
...
> My interest in this is the idea that we could get distutils2 and/or
> zc.buildout to be able to download regular updates of metadata including
> dependencies, then perhaps those tools could avoid certain kinds of conflict
> errors which are a pain to debug without that information. For instance, the
> current design of zc.buildout means that:
>
> Download Bob. Bob 1.0 requires Fred >= 2.0.
> Download Fred 3.0
> Download Marry. Marry 1.0 requires Fred < 2.5
> Conflict error. Marry 1.0 requires Fred < 2.5 but we already have Fred 3.0.

> If instead we knew in advance of this conflict we could have chosen to
> download Fred 2.4 or at least warned the user there was a potential conflict
> and they should pick a compatible version. In the case of preinstalled
> packages, it could offer to downgrade Fred from 3.0 to 2.4.

The issue here isn't that buildout has to download packages
to get meta data. The problem is that it doesn't backtrack.  This is
compounded by fact that when computing and fetching
requirements, it uses a depth-first strategy and doesn't use
information about requirements of other packages in the working
set until too late.  There's definitely room for improvement in the
strategy it uses, even short of backtracking.

In any case, allowing buildout to get requirements info before
downloading packages would be a useful optimization, but wouldn't
solve the conflict problem.

Jim

-- 
Jim Fulton
http://www.linkedin.com/in/jimfulton