On Thu, Dec 6, 2012 at 6:33 AM, Donald Stufft <donald.stu...@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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