On Wed, Dec 5, 2012 at 2:46 AM, Donald Stufft <donald.stu...@gmail.com> wrote: > 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.
Unless the user wrote those things that depend on A, they aren't going 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.) > You can automatically uninstall A from B in an automatic dependency management system My point is that this can only work if the "obsoleting" is effectively 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. > I think Obsoletes as is an alright bit of information. 1. It cannot be used to prevent the installation of an obsolete 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.) > I don't see this in this thread, could you link it again? http://mail.python.org/pipermail/catalog-sig/2010-October/003368.html 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 understand that the Python community does not work the same way as a Linux distribution. We are not a single organization shipping a fully-functional and configured machine, we are hundreds of individual authors shipping our own stuff. Conflict resolution and package replacement (and even deciding what it is that things "provide" or "require") are primarily *human* processes, not technical ones. Relationship and support "contracts", IOW, rather than software contracts. That's why, in the distro world, a package manager can use simple fields to carry out the will of the human organization that made those support and compatibility decisions. For Python, the situation is a bit more complicated, which is why clear thinking is needed. Simply copying fields blindly from other packaging systems just isn't going to cut it. Now, if the will of the community is to turn PyPI into a distro-style repository, that's fine... but even if you completely ignore the human issues, there are still technical ones. Generally, distro-style repositories work by downloading the full metadata set (or at least an index) to a user's machine. And that's the sort of architecture you'd need in order for these type of fields to be technically feasible (e.g., doing an index search for Obsoletes), without grinding the PyPI servers into dust. _______________________________________________ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
