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.