Bug#733489: python-apt: Improve 'Dependency' and 'BaseDependency' to get target package versions that satisfy dependencies

[Michael Schaller <michael@5challer.de>]

Awesome! My first patch in Debian. ^^
Thanks for all your patience so far...

Next thing on the agenda is the relation handling in BaseDependency. I'm 
wondering if I'm the only one trying to use this but the relation 
handling seems to be totally broken at the moment. There is a huge 
difference between the Debian manual (what I should get) and the actual 
results from python-apt...

According to the Debian Manual [1] the only allowed relations are "<<, 
<=, =, >= and >>". Back in the days there have been "<" which maps to 
"<=" and ">" which maps to ">=". "<" and ">" have been deprecated 
because they "were confusingly used".


python-apt on the other side behaves totally different:
-------------------
#!/usr/bin/python

import apt

cache = apt.cache.Cache()
relations = set()

for pkg in cache:  # apt.package.Package
  ver = pkg.candidate  # apt.package.Version
  or_deps = ver.get_dependencies("PreDepends", "Depends", "Recommends")
  for or_dep in or_deps:  # apt.package.Dependency
    for dep in or_dep:  # apt.package.BaseDependency
      relations.add(dep.relation)

print relations
-------------------
Output: set(['', '>=', '<=', '=', '<', '>'])


Looking a bit deeper this behaviour is already present in apt_pkg:
-------------------
#!/usr/bin/python

import apt

cache = apt.cache.Cache()
relations = set()

for pkg in cache:  # apt.package.Package
  ver = pkg.candidate  # apt.package.Version
  _ver = ver._cand  # apt_pkg.Version
  dep_dict = _ver.depends_list
  pre_deps = dep_dict.get('PreDepends', [])
  deps = dep_dict.get('Depends', [])
  recs = dep_dict.get('Recommends', [])
  for or_dep in pre_deps:
    for dep in or_dep:  # apt_pkg.Dependency
      relations.add(dep.comp_type)
  for or_dep in deps:
    for dep in or_dep:  # apt_pkg.Dependency
      relations.add(dep.comp_type)
  for or_dep in recs:
    for dep in or_dep:  # apt_pkg.Dependency
      relations.add(dep.comp_type)

print relations
-------------------
Output: set(['', '>=', '<=', '=', '<', '>'])


The documentation of apt_pkg.Dependency.comp_type reads like this:
    .. attribute:: comp_type

        The type of comparison (>=, ==, >>, <=), as string.


dpkg seems to be only adhering to what is written in the Debian Manual:
$ egrep -o '\(< |\(<< |\(<= |\(> |\(>> |\(>= |\(= |\(== ' 
/var/lib/dpkg/status | tr -d '(' | sort | uniq
<<
<=
=
>=
>>


I hope you are confused too, so can we fix this in python-apt?
I know that this would break compatibility but the current 
implementation is plain wrong IMHO. Also the question would be where 
this needs fixing... (libapt-pkg, apt_pkg.Dependency or 
apt.Package.BaseDependency)


[1] http://www.debian.org/doc/debian-policy/ch-relationships.html