[Python-Dev] Fwd: PEP 426 is now the draft spec fordistribution metadata 2.0

[Daniel Holth]

On Feb 19, 2013 6:57 AM, <a.cavallo at cavallinux.eu> wrote:

> > I've never seen environment markers being used or supported
> > in the wild.
> >
> > I'm not against modernizing the format, but given that version 1.2
> > has been out for around 8 years now, without much following,
> > I think we need to make the implementation bit a requirement
> > before accepting the PEP.
>
> Elephant in the room? Somethin's been around for so long and hasn't got
> any sort of
> mainstream support is *not* a good candidate for a requirement but maybe
> it's me
> being naive.
>

I think 1.2 was started and then amended over a long period of time to
become what it is today. I wanted to edit it again just to add
Provides-Extra. It turned out to be more practical to make a new PEP. Nick
explains that for complicated reasons the implementation of Metadata 1.2
was not successful. For Metadata 2.0 we had a different strategy: put it
into distribute first. You will find that this aids adoption. distlib will
provide another implementation that is in many ways interchangeable.

We also have three implementations of an installer for the related wheel
format, three implementations of the wheel builder, and two surprisingly
short implementations of environment markers. Yes, the wheel project has
the same name as the wheel format.

Metadata 2.0's Provides-Extra field is there to represent the setuptools
"extras" feature which is necessary for a very large number of
distributions on pypi. For a while the most common environment markers will
look like just "extra == 'foo'". So it should not be surprising that
distribute is the first tool to recognize Provides-Extra. Previously
distribute did not use METADATA / PKG-INFO for dependencies, instead
reading from requires.txt

If you can get around the fact that we've incorporated most of another PEP
386 into v2.0, rewritten and explained everything to death, I would argue
that 2.0 is actually simpler than 1.2. Only four headers are actually
required in a valid Metadata 2.0 file and a number of the fields are
informational and do not have to be understood by the installer.
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