On Wed, Dec 5, 2012 at 2:46 AM, Donald Stufft <donald.stufft@gmail.com> wrote:Unless the user wrote those things that depend on A, they aren't going
> 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.
to be in a position to do anything about it. (Contrast with a distro,
where dependencies are indirect - the other package will depend on an
abstraction provided by both A and B, rather than directly depending
on A *or* B.)
(Also note that all the user knows at this point is that the author of
B *claims* to obsolete A, not that the authority managing the
repository as a whole has decreed B to obsolete A.)
My point is that this can only work if the "obsoleting" is effectively
> You can automatically uninstall A from B in an automatic dependency
management system
just a rename, in which case the field should be "renames", or better
still, "renamed-to" on the originating package.
As I've mentioned repeatedly, Obsoleted-By handles more use cases than
Obsoletes, and has at least one practical automated use case
(notifying a developer that their project is depending on something
that's obsolete).
Also, the example given as a use case in the PEP (Gorgon to Torqued)
is not just wrong, it's *actively misleading*. Gorgon and Torqued are
transparent renames of Medusa and Twisted, which do not share a common
API and thus cannot be used as the subject of any automated processing
(in the case of Obsoletes) without doing some kind of PyPI metadata
search for every package installed every time a package is installed.
1. It cannot be used to prevent the installation of an obsolete
> I think Obsoletes as is an alright bit of information.
package without a PyPI metadata search, since you must examine every
*other* package on PyPI to find out whether some package obsoletes the
one you're trying to install.
2. Unlike RPM, where metadata is provided by a trusted third party,
Obsoletes can be specified by any random forker (no pun intended),
which makes this information a mere advertisement... and an
advertisement to the wrong audience at that, because they must have
*already* found B in order to discover that it replaces A!
3. Nobody has yet supplied a use case where Obsoletes would not be
strictly improved upon by Obsoleted-By. (Note that "the author of
package X no longer maintains it" does not equal "package Y is
entitled to name itself the successor and enforce this upon all users"
-- this can work in RPM only because it is a third party Z who
declares Y the successor to X, and there is no such party Z in the
Python world.)
http://mail.python.org/pipermail/catalog-sig/2010-October/003368.html
> I don't see this in this thread, could you link it again?
http://mail.python.org/pipermail/catalog-sig/2010-October/003364.html
These posts also address why a "Conflicts" field is *also* unlikely to
be particularly useful in practice, in part for reasons that relate to
differences between RPM-land and Python-land. (For example, RPMs can
conflict over things besides files, due to runtime and configuration
issues that are out-of-scope for a Python installer tool.)
While it's certainly desirable to not invent wheels, it's important to