[Python-Dev] Accept just PEP-0426

Daniel Holth dholth at gmail.com
Tue Nov 20 15:56:14 CET 2012

On Tue, Nov 20, 2012 at 9:35 AM, Vinay Sajip <vinay_sajip at yahoo.co.uk>wrote:

> Daniel Holth <dholth <at> gmail.com> writes:
> > Mostly it seems a bit silly to have so much conversations about parts of
> the
> > pep that remain unchanged from previously accepted versions...
> I don't agree with the suggestion that we shouldn't discuss it because it
> was
> accepted in a previous version. Perhaps it didn't receive the right
> scrutiny at
> that time, but since it hasn't been implemented, it's reasonable to
> discuss it.
> ISTM that implementing it as suggested in the PEP can lead to certain
> problems,
> since it is a multi-valued field. If it is left in, then something should
> be
> said in the PEP about the potential difficulties and if/how they can be
> resolved.
> The difficulties I am talking about relate to dependency resolution. Given
> the
> current definition of Provides-Dist, it is possible for a package A on
> PyPI to
> "Provide" all of e.g. "A (1.0)", "B (1.2)" and "C (1.5)", and it is also
> possible for packages B and C on PyPI to provide the same (or slightly
> different) versions of logical packages of A, B, and C. This will likely
> lead
> to the need for a sophisticated dependency resolver because the dependency
> graph can get quite convoluted. (Remember, we might need to do this
> resolution
> when removing packages as well as when installing them.) I know there are
> solvers and such, but I'm not sure we need that level of sophistication, or
> whether its complexity cost is outweighed by any benefit. Remember, we are
> managing fine without multi-valued Provides-Dist, and while a case has been
> made for virtual packages and forks (which just require a single-valued
> field),
> no compelling case has been made for bundling packages in general (I
> understand
> that such requirements might sometimes arise in certain corporate
> environments,
> but they don't seem to be a mainstream use case). Hence, no strong case has
> been made for a multi-valued "Provides" field.
> If we have a good index and packaging infrastructure, there is no general
> need
> for packages to bundle other packages, unless those bundled packages are
> changed
> in some way to suit the bundler's needs. In that case, I don't know how you
> could be sure that a bundled "A (1.0)" hasn't diverged from the equivalent
> package on PyPI.
> The "Provides" seems essentially useless in a metadata index, since if,
> when
> asked to install D which has a dependency on A, you would download and
> install
> A to resolve it rather than B or C, and I can't see when you would want to
> query the index to say "who provides A?" and then use some heuristic to
> pick
> e.g. B or C, rather than A.
> distlib currently contains support for the multi-valued "Provides", but I'm
> not confident that will work as expected given pathological cases like the
> example I suggested, without getting "complicated" in the Zen of Python
> sense.
> I'm not convinced that the maintenance burden of a complicated solution is
> worth the heretofore unnecessary ability to bundle stuff in arbitrary ways.

If you don't have Provides-Dist, then distribute must continue to bundle an
extra .egg-info directory to emulate the feature. This is more than enough
justification for me. Name: is essentially an alias for Provides-Dist: (or
vice-versa) so there is no such thing as a single-valued Provides-Dist.
Having two names for a package is just as complicated as having twenty.

You should not implement Provides-Dist by searching for every
Provides-Dist: name on PyPI. You should only use it when deciding whether
to download setuptools when distribute is already installed and a package
depends on setuptools.

The bundling term was bad wording on the part of the PEP. No one should
ever include non-renamed copies of other dists in their dists "import six"
vs. "import django.util.six". I've suggested a new wording in this thread.
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