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

[Daniel Holth]

On Thu, Dec 6, 2012 at 6:33 AM, Donald Stufft <donald.stufft at gmail.com>wrote:

> On Thursday, December 6, 2012 at 6:28 AM, Vinay Sajip wrote:
>
> Donald Stufft <donald.stufft <at> gmail.com> writes:
>
> Never mind the "Obsoletes" information - even the more useful
> "Requires-Dist"
> information is not exposed via PyPI, even though it appears to be stored
> in the
> database. (Or if it is, please point me to where - I must have missed it.)
>
> Requires-Dist doesn't exist for more than a handful of packages. But PyPI
> exposes
> it via the XMLRPC API, possibly the JSON api as well.
>
>
> Even if this were to be made available, it's presumably obtained from
> PKG-INFO.
> As I understand, this data is not considered reliable - for example, pip
> runs
> egg_info on downloaded packages to get updated information when determining
> dependencies to be downloaded. If the Requires-Dist info in PKG-INFO can't
> be
> relied on, surely less critical information such as Obsoletes can't be
> relied on,
> either?
>
> pip runs egg_info because setuptools does not write out to PKG-INFO what
> the dependencies are (it does write it out to a different text file
> though). But IIRC
> that text file is not guaranteed to exist in the distribution. There's
> also the
> history where pip was trying to preserve as much backwards compat with
> easy_install as it could, and if you used the file that egg_info writes out
> then you'll only get the requirements for the system that the distribution
> was
> packaged on. Any if statements that affect the dependencies won't be
> in effect.
>

It will be Obsoleted-By:. The "drop in replacement" requirement will be
removed. The package manager will say "you are using these obsolete
packages; check out these non-obsolete ones" but will not automatically
pull the replacement without a Requires tag.

I will probably add the unambiguous Conflicts: tag "uninstall this other
package if I am installed".


Many packages (IIRC more than half) have the pre-Metadata-1.2 equivalent of
Requires-Dist: which is the very easy to parse requires.txt. This
information is not reliable because it could depend on conditions in
setup.py. Someone should write a setup.py compiler that determines whether
a package's requirements are conditional or not.


Environment markers (limited Python expressions at the end of Requires-Dist
lines) attempt to make Requires-Dist reliable. You can execute them safely
in your environment to determine whether  a requirement is right for you:

Requires-Dist: pywin32 (>1.0); sys.platform == 'win32'


The wheel implementation makes sure all the metadata (the .dist-info
directory) is at the end of the .zip archive. It's possible to read the
metadata with a single HTTP partial request for the end of the archive
without downloading the entire archive.
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