This program gets a list of packages from repositories files in /var/lib/apt/lists and a list of installed packages. Then, list the packages/versions which not in files with stable in your name.

debian_packages.py

#!/usr/bin/env python
# -*- coding: utf-8 -*-
'''
Author: Junior Polegato
Date: 29 May 2013
Description: This program gets a list of packages from repositories
             files in /var/lib/apt/lists and a list of installed
             packages. Then, list the packages/versions which not in
             files with stable in your name.
'''

import os
import subprocess

DIR = '/var/lib/apt/lists'

def read_packages_list(print_info = True):
    repos_files = [p for p in os.listdir(DIR) if p[-9:] == "_Packages"]
    packages = {}
    for repo_file in repos_files:
        if print_info: print repo_file
        packs = open(DIR + "/" + repo_file).read().strip().split('\n\n')
        repo_packages = 0
        for pack_lines in [p.split('\n') for p in packs]:
            package = {}
            for line in pack_lines:
                if not line: continue
                if line[0] != ' ' and ': ' in line:
                    info, descr = line.split(': ', 1)
                    package[info] = descr
                elif info in package:
                    package[info] += line
            if 'Package' in package:
                repo_packages += 1
                package_name = package['Package']
                if 'Version' in package:
                    package_version = package['Version']
                else:
                    package_version = 'Unknown'
                if package_name in packages:
                    if package_version in packages[package_name]:
                        packages[package_name][
                                      package_version].append(repo_file)
                    else:
                        packages[package_name][
                                          package_version] = [repo_file]
                else:
                    packages[package_name] = {
                                           package_version: [repo_file]}
        if print_info: print '  \-->', repo_packages, 'package(s)'
    return packages

def installed_list():
    dpkg_query = subprocess.Popen(
        "dpkg-query --show"
        " --showformat='${Package} ${Version} ${Status}\n'",
        shell=True, stdout=subprocess.PIPE, stderr=subprocess.STDOUT)
    list_packages = dpkg_query.stdout.read().strip().split('\n')
    retval = dpkg_query.wait()
    return dict(package.split(' ', 2)[:2] for package in list_packages)

if __name__ == "__main__":
    packages = read_packages_list()
    installeds = installed_list()
    for p, v in installeds.items():
        if p in packages:
            if v in packages[p]:
                if 'stable' not in repr(packages[p][v]):
                    print p, v, packages[p][v]
            else:
                print p, v, 'version not in', packages[p].keys()
        else:
            print p, v, 'package not in repos'