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Wed Dec 5 16:42:48 CET 2012 · https://mail.python.org/pipermail/python-dev/2012-December/122955.html

On Wed, Dec 05, 2012 at 02:46:11AM -0500, Donald Stufft wrote:
> On Wednesday, December 5, 2012 at 2:13 AM, PJ Eby wrote:
> 
>     On Mon, Dec 3, 2012 at 2:43 PM, Daniel Holth <dholth at gmail.com> wrote:
> 
>         How to use Obsoletes:
> 
>         The author of B decides A is obsolete.
> 
>         A releases an empty version of itself that Requires: B
> 
>         B Obsoletes: A
> 
>         The package manager says "These packages are obsolete: A". Would you
>         like to
>         remove them?
> 
>         User says "OK".
> 
> 
>     Um, no. Even if the the author of A and B are the same person, you
>     can't remove A if there are other things on the user's system using
>     it. The above scenario does not work *at all*, ever, except in the
>     case where B is simply an updated version of A (i.e. identical API) --
>     in which case, why bother? To change the project name? (Then it
>     should be "Formerly-named" or something like that, not "Obsoletes".)
> 
> You can automatically uninstall A from B in an automatic dependency
> management system.  I *think* RPM does this, at the very least

This is correct.

> I believe it refuses to install B if A is already there (and the reverse
> as well).*

I'd have to test this but I believe you are correct about the first.  Not
sure about the reverse.

> There's nothing preventing an installer from, during it's attempt to
> install B, see it Obsoletes A, looking at what depends on A and
> warning the user what is going to happen and prompt it.
> 
In rpm-land, if something depended on A and nothing besides the actual
A package provided A, rpm will refuse to install B.  But rpm is meant to be
used unattended so different package managers could certainly choose to
prompt.  For package renames, package B would have both an Obsoletes: A <=
$OLD_VERSION and a Provides: A = NEW_VERSION

-Toshio
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[Python-Dev] Keyword meanings [was: Accept just PEP-0426]